Plant Anatomy Physiology Design by Owen Borville December 8, 2024 BIO 30
Plant Physiology
Plant Organ Systems
In plants, similar cells work together to form tissues just like in animals. Tissues work together to form an organ and organs work together to form an organ system. Vascular plants have two distinct organ systems: a shoot system and a root system. The shoot system contains two parts: the vegetative non-reproductive parts of the plant such as the leaves and stems, and the reproductive parts like the flowers and fruits. The shoot system generally grows above ground, as it absorbs light needed for photosynthesis processes. The root system supports the plant by absorbing water and minerals underground.
Plants are multicellular eukaryote organisms with various tissue systems. Two types of plant tissue systems are meristematic tissue and permanent tissue. Meristematic tissue is found in meristems, or plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated and they continue to divide and contribute to growth of the plant. Permanent tissue consists of plant cells that are no longer actively dying.
Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (where the leaf attaches to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.
Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue. Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps store water and sugars.
Secondary tissues are either simple (similar cell types) or complex (different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes three different cell types: vessel elements and tracheids (both of which conduct water), and xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells (which conduct photosynthesis), companion cells, phloem parenchyma, and phloem fibers. Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other. In stems, the xylem and phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.
Stems are part of the shoot system of the plant and their main function is to provide support to the plant, holding leaves, flowers, and buds. Sometimes, stems store food for the plant. The stem may be unbranched or branched and connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.
On plant stems, nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode. The stalk that extends from the stem to the base is called the petiole. An auxiliary bud is usually found in the axil, the area between the base of the leaf and the stem, where it can give rise to a branch or a flower. The apex or tip of the shoot contains the apical meristem within the apical bud.
Parenchyma cells are the most common plant cells and are found in the stem, root, inside of the leaf, and pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.
Collenchyma cells are elongated cells with unevenly thickened walls. They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis.
Sclerenchyma cells also provide support to the plant, but many of them are dead at maturity. Two types of sclerenchyma cells are fibers and sclereids. Both have secondary cell walls that are thickened with deposits of lignin, an organic compound in wood. Fibers are long, slender cells, sclereids are smaller sized.
The stem has three tissue systems: dermal, vascular, and ground tissue.
The dermal tissue of the stem consists primarily of epidermis, a singular layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark for further protection. Epidermal cells are the most numerous and least differentiated cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, where gas exchange takes place. Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface that help to reduce transpiration (loss of water), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.
Vascular tissue in the stem is made of xylem and phloem and are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. In the stem cross section, vascular bundles of dicot stems are arranged in a ring. In stems that live for more than one year, the individual bundles grow together and produce growth rings. In monocot stems, the vascular bundles are randomly scattered in the ground tissue.
Xylem tissue has three types of cells: Xylem parenchyma, tracheids, and vessel elements. Tracheids are xylem cells with thick secondary walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls shorter than the tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.
Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells or elements are arranged at the end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.
Ground tissue is made up of mainly parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.
Growth in stems occurs as primary growth as a result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. Some plant parts continue to grow throughout the life of the plant, called intermediate growth. Other plant parts like leaves and flowers stop growing when reaching a certain size, which is called determinate growth.
Most primary growth occurs in the apices or tips, stems, and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the tips of the shoot and roots. Cell elongation also contributes to primary growth. The influence of the apical bud on overall plant growth is known as apical dominance.
Secondary growth causes an increase in stem thickness due to the activity of lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and cork cambium in woody plants. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem inside and secondary phloem on the outside.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells or bark with a waxy substance called suberin that can repel water and protect the plant. The cork cambium produces another layer called the phelloderm inward from the cambium. The cork cambium, cork cells, and phelloderm are called the periderm and acts as an epidermis. In some plants, the periderm has openings called lenticels that allow for gas exchange with the atmosphere, including oxygen.
Annual rings are caused by the activity of the vascular cambium. During the spring growing season, the cells of the secondary xylem have a large internal diameter and their cell walls are not extensively thickened. In the fall season, the cell walls thicken in the secondary xylem, which is denser. This seasonal cycle of a decrease in vessel elements and increase in the number of tracheids results in the formation of annual growth rings.
Stem variations include rhizomes, a stem that grows horizontally underground with nodes and internodes, and sometimes vertical shoots. Corms are like rhizomes, but more rounded and fleshy. Stolons are stems that run parallel to the ground, where new plants can grow at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at interval nodes. Tubers are modified stems that can store starch and are swollen ends of stolons, along with unusual buds. A bulb works as an underground storage unit, a stem feature that appears to be fleshy leaves emerging from the stem.
Above ground, tendrils are slender, twirling strands that enable a plant to climb other surfaces like a vine or pumpkin. Thorns are sharp outgrowths that help protect the plant, as in roses.
Roots of seeded plants have three major functions: anchoring the plant in the soil, absorbing water and minerals and transporting them upward, and storing the products of photosynthesis. Most roots are underground, but adventitious roots exist above ground.
Two types of root systems include the tap root system that has a main root that grows down vertically, and from which many smaller lateral roots rise and penetrates deep into the soil. The other root system is fibrous root systems, which are located closer to the soil surface and forms a dense network of roots that helps prevent soil erosion. Some plants have a combination of these two types of root systems. Plants in dry places are more likely to have deep root systems in comparison to plants in wet places that likely have shallow root systems.
Root growth begins with seed germination. The tip of the root is protected by the root cap, which is continuously replaced because it gets damaged easily in the soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation. The zone of cell division is closest to the root tip and it is made of the actively dividing cells of the root meristem. The zone of elongation is where the newly formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three of these zones are within the first centimeter of the root tip.
The root has an outer layer of cells called the epidermis, which surrounds areas of ground tissue and vascular tissue. The epidermis provides protection and helps in absorption. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, which greatly enhances absorption of water and minerals. Inside the root, the ground tissue forms two regions: the cortex and the pith. Compared to stems, roots have lots of cortex and little pith. Both regions include cells that store photosynthetic products. The cortex is between the epidermis and the vascular tissue, whereas the pith is between the vascular tissue and the center of the root.
The vascular tissue in the root is arranged in the inner portion of the root, called the stele. A layer of cells known as the endodermis separates the stele from the ground tissue in the outer portion of the root. The endodermis is exclusive to the roots, and serves as a checkpoint for materials entering the root's vascular system. A waxy substance called suberin is present in the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root's vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem of the stele are arranged alternatively in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.
Root structures can have unique features designed for certain purposes. Some roots are bulbous and store starch. Aerial roots and prop roots are two forms of above ground roots that provide additional support to anchor the plant. Tap roots, such as carrots, turnips, and beets are examples of roots that are modified for food storage. Epiphytic roots enable a plant to grow on another plant, such as with the banyan tree and the screwpine.
Leaves are the main site of photosynthesis, the process in which plants synthesize food from sunlight. Most leaves are green color due to the chlorophyl in the leaf cells. However, leaves can be other colors that come from other pigments in the cells. The thickness, size, and shape of leaves are designed for their environmental location. Leaves in tropical environments have larger surface areas than those that grow in dry or cold environments. Smaller surface area of the leaves would minimize water loss.
Leaf structure commonly includes a leaf blade called the lamina, which is the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves without a petiole that are attached directly to the stem are called sessile leaves. Small green appendages usually found at the base of the petiole are are known as stipules. Most leaves have a midrib, which extends the length of the leaf and branches on each side to produce veins of vascular tissue. The edge of the leaf is called the margin.
The vascular tissue of the leaf forms veins and the arrangement of veins is called the venation pattern, which can vary in leaves. Monocots and dicots differ in their patterns of venation. Monocots have a parallel venation, as the veins run in straight lines parallel to each other across the leaf without converging. In dicots, the veins of leaves have a net-like appearance, forming a pattern known as reticulate venation. Dichotomous venation, as in Ginkgo biloba, features veins that fork.
Leaf arrangement on a stem is known as phyllotaxy and this arrangement can vary with species. Leaves are classified as alternate, spiral, or opposite arrangement. Alternate arrangement is where leaves alternate on each side of the stem in a flat plane. Spiral arrangement is where leaves are arranged in a spiral pattern along the stem. In opposite arrangement, leaves arise at the same point and connect on each side of the stem or branch. With three or more leaves connected at a node, it is called whorled arrangement.
Leaves may also be simple or compound in form. In a simple leaf, the blade is undivided or has lobes but the separation does not reach the midrib. In a compound leaf, the leaf blade is completely divided, forming leaflets. Each leaflet may have its own stalk, but it is attached to the rachis. A palmately compound leaf resembles the palm of a hand, with leaflets radiating outward from a point. Pinnately compound leaves have a feather-like appearance and the leaflets are arranged along the midrib.
In leaf structure, the outermost layer of the leaf is the epidermis, which is present on both sides of the leaf. The epidermis helps in the regulation of gas exchange, because it contains stomata, which are small openings in the leaf where gas exchange takes place. The stomata can open and close, regulated by two guard cells. The epidermis is usually one cell layer thick, but in extreme climates, the epidermis may be several layers thick to protect against heat and water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves have small hairs called trichomes on the surface that help prevent insect-eating plant movements. Trichomes also can store toxic or bad-tasting compounds to deter predators. Trichomes can also reduce transpiration by preventing air flow across the leaf surface.
Below the epidermis of dicot leaves are the layers of cells known as mesophyll, or middle leaf. The mesophyll of most leaves usually contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma. The palisade parenchyma (or palisade mesophyll) has a column-shaped, tightly-packed cells and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found in between the spongy parenchyma allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.
Just like the stem, the leaf contains vascular bundles composed of xylem and phloem. The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.
Leaf design in coniferous plants that live in cold environments have leaves that are reduced in size and needle-like in appearance, like spruce, fur, and pine. These needle-like leaves have sunken stomata and a smaller surface area, which help in reducing water loss. In hot climates, cactus plants have leaves that are spines, which help conserve water along with their thick stem trunks. Many aquatic plants have leaves with wide lamina that can float on the water surface, and a thick, waxy cuticle on the leaf surface that repels water.
Transport of Water and Solutes in Plants
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants.
Plants are designed with hydraulic engineering and can transport water throughout their physiology to the top of a tree over 100 meters tall. Hydraulic forces also enable plants enough force to break rocks and ceramic materials through water potential.
Water potential is a measure of potential energy in water. Water potential is recognized by the Greek letter psi (Ψ) and is expressed in units of pressure called megapascals. Water potential of pure water is given a value of zero and water potential values in different parts of the plant are given in relation to pure water.
Water potential (Ψ) is expressed in mathematical form as:
Ψ system = Ψ total = Ψs + Ψp + Ψg + Ψm
Ψs = water potential of the solute
Ψp = water potential of pressure
Ψg = water potential of gravity
Ψm = matric potential of water
The water potential system can refer to the water potential of soil water, root water, stem water, leaf water, or water in the atmosphere. Water moves from higher water potential to lower water potential. In order for water to move from the soil through the plant to the atmosphere, or transpiration, water potential of the soil must be greater than the root, which must be greater than the stem, which must be greater than the leaf, which must be greater than the atmosphere.
A plant can control water movement by controlling individual components of the water potential.
Solute potential (osmotic potential) is related to the solute concentration in molarity. This relationship is expressed by the van't Hoff equation, which is:
Ψs = -MiRT
M= molar concentration
i = van't Hoff factor, the ratio of the amount of particles in the solution to amount of formula units dissolved.
R = Ideal gas constant
T = temperature in Kelvin degrees
Therefore, Ψs decreases with increased solute concentration, and a decrease in Ψs leads to a decrease in Ψ total. The internal water potential of a plant cell is more negative than pure water because of increased solute concentration and because of the water potential difference, water can move from the soil through the roots, stem, and leaves of a plant and into the atmosphere by osmosis, or osmotic potential. Intelligent Design at work. Plant cells can regulate the amount of solute by adding and removing solute molecules and therefore have control over total water potential.
Pressure potential, Ψp (or turgor potential) may be positive or negative and positive Ψp will increase Ψtotal while negative Ψp will decrease Ψtotal. Positive turgor pressure is produced inside plant cells by the cell wall. A plant can manipulate Ψp by manipulating Ψs and the process of osmosis. Increasing the cytoplasmic solute concentration will reduce Ψs and Ψtotal, which will cause osmosis to occur and increase Ψp. The opening and closing of the stomata also affect the Ψp. Stomata opening causes water to evaporate from the leaf, reducing Ψp and Ψtotal, and allowing water to flow from the petiole or leaf stem into the leaf.
Gravity potential Ψg is always negative to zero in a plant with no height and always removes or consumes potential energy from the system. The force of gravity pulls water downwards toward the soil, reducing the total amount of potential energy in the water in the plant Ψtotal. The taller the plant, the more influential Ψg becomes and in short plants, Ψg is negligible. In tall trees, gravity potential is very strong and must be overcome to enable water to rise to the length of the tree by osmosis as water is pulled from the soil into the roots, trunk, leaves, and out through the stomata. Plants are unable to manipulate Ψg.
Matrix potential, Ψm, is always negative to zero and is lowest or most negative in dry systems like seeds and dry soil and zero in water-saturated systems. The binding of water to a matrix or solid surface always removes or consumes potential energy from the system and is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophillic solute molecules, whereas in Ψm, the other components are insoluble, hydrophillic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophillic, producing a matrix for adhesion of water. The matrix potential is very large or negative for dry tissues like seeds or dry soils. However, the matrix potential becomes zero when saturated with water. Matrix potential cannot be manipulated by plants.
Solutes, pressure, gravity and matric potential affect the transport of water in plants as water moves from a higher total water potential location to a lower total water potential location. The water potential is also directly related to Gibbs Free Energy, so higher water potential will have higher Gibbs Free Energy. Gibbs Free Energy is the energy associated with a chemical reaction that can be used to do work and is expressed as ΔΨ.
Transpiration is the loss of water from the plant through evaporation at the leaf surface and is the main cause of water movement in the xylem tissue. Transpiration causes a negative pressure at the leaf surface which is highest in dry conditions and water from the roots is pulled up because of this pressure or tension. When stomata holes close and transpiration stops, water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem tissue, and the cohesion of water molecules to each other.
At the cellular level, water saturates the cell walls and oxygen-carbon dioxide exchange occurs through intercellular air spaces as a requirement for photosynthesis. The water on the wet cell wall and cell surface evaporates through the air spaces in the leaf cells. The evaporation causes greater tension on the water in the cells and increases the pull on the water through the xylem tissue vessels. Small perforations between vessel tissue elements reduce the number and size of gas bubbles by a cavitation process. The gas bubbles in the xylem tissue interrupt the stream of water through the plant, or embolism. Taller trees have more cavitation events.
Transpiration is a passive process and does not require ATP or metabolic processes for water movement. Only the energy change from the pressure potential is needed to move water upward through the plant and this process is controlled. The atmosphere around the leaf drives transpiration and cause a large amount of water loss, up to 90 percent of water from the root can be taken by transpiration.
Leaves are covered by a waxy cuticle on their surface that prevents water loss and regulation of transpiration is accomplished by the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized guard cells that open and close in response to environmental conditions of light, water, and carbon dioxide gas. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. However, when stomata are open, water vapor is lost to the environment through transpiration. Plants must maintain balance between photosynthesis and water loss.
Plants that live in dry environments are designed to have a thicker waxy cuticle than those plants in wetter environments to control water loss. Other plants have a thick covering of trichome appendages or outgrowths that secrete oily substances that prevent air flow on the surface and reduce transpiration. Other plants have stomata sunken below the leaf's surface that help prevent water loss. Some plants also have multiple epidermal layers. These design features help reduce water loss by transpiration.
Products of photosynthesis are called photosynthates commonly in the form of sugars like sucrose. Structures that produce photosynthates for the growing plant are called sources. Sugars produced in sources such as leaves, need to be delivered to growing parts of the plant through the phloem in a process called translocation. The points of sugar delivery, such as roots, shoots, and seeds are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the stage of development and the season. The pattern of flow of photosynthates in the phloem can be in one direction or both directions through the plant, while in the xylem flow is only in one direction. The pattern of photosynthate flow will change as the plant grows and develops and photosynthates are sent where they are needed.
Translocation transport from source to sink occurs as photosynthates such as sucrose are produced in the mesophyll cells of photosynthesizing leaves and translocated through the phloem where they are used or stored. Photosynthates move through cytoplasmic channels called plasmodesmata across mesophyll cells. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles and the photosynthates are loaded onto these STEs. The sucrose is transported against the concentration gradient using ATP into the phloem cells using electrochemical potential of the proton gradient. In addition, a carrier protein called sucrose-H+ symporter carries the sucrose. Phloem STEs have reduced cytoplasmic contents and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells help assist the STEs with metabolic activities and produce energy for the STEs.
Once in the phloem, the photosynthates are translocated to the nearest sink. Phloem sap's high sugar content decreases solute potential, decreasing total water potential and causing water to move by osmosis from the xylem to phloem tubes and therefore increasing pressure. Increase in total water potential causes the bulk flow of phloem from source to sink. Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth or converted to starch or other polymers. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from high to low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.
Plant Sensory Systems and Responses
Responses to Light: Photomorphogenesis is the growth and development of plants in response to light and allows plants to optimize their use of light and space. Photoperiodism is the ability to use light to track time so that plants can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Phototrophism is a directional response that allows plants to grow towards or away from light. The very important response of plants to light is mediated by different photoreceptors, which are comprised of a protein covalently bonded to a light-absorbing pigment called a chromophore. The protein and pigment together are known as a chromoprotein. The different regions of the visible light spectrum including red and blue violet are responsible for structural development in plants. Sensory photoreceptors absorb light in these regions of the visible light spectrum because of the quality of light available in the daylight spectrum.
The phytochromes are a family of chromoprotein photoreceptors with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-inconvertible forms: Pr and Pfr. Pr absorbs red light and is immediately converted to Pfr. Pfr absorbs far-red light and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change in the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of the protein and therefore exposure to red light yields physiological activity. Pr is the inactive form of the protein. Exposure to far-red light inhibits phytochrome activity. Pr and Pfr represent the phytochrome system.
The phytochrome system acts as a biological light switch and it monitors the level, intensity, duration, and color of environmental light. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or in the nucleus it can directly activate or repress specific gene expression. In seeds, the phytochrome system is not used to determine direction and quality of light, but is instead used to determine if there is any light at all. In the dark, the phytochrome is in the Pr or inactive form and will not germinate unless exposed to light at the surface of the soil, where Pr is converted to Pfr and germination continues.
Plants also use the phytochrome system to sense the change of season. Photoperiodism is a biological response to the timing and duration of day and night and it controls flowering, setting of winter buds, and vegetative growth. Detection of seasonal changes is crucial to plant survival and day length is a better indicator of season than temperature and light intensity, which can vary. During the day, phytochrome molecules convert to active Prf form and back to Pf form in the evening. During long nights in the winter, all of the Prf form reverts, but during the short summer nights, some of the Prf remains at sunrise. The plant can sense the Pr/Prf ratio at dawn and can determine the length of the day/night cycle. This information can also be stored for several days and compared to determine the time of season so that the plant can modify its physiology accordingly.
Phototropism is the directional bending of a plant toward or away from a light source and is a response to blue wavelengths of light. Positive phototropism is bending toward light while negative phototropism is bending away from light. Phototropins are protein-based photoreceptors responsible for mediating the phototropic response. Phototropins contain a protein portion and a light-absorbing portion, called the chromophore. In phototropins, the chromophore is a covalently-bound molecule of flavin and phototropins are included in a class of proteins called flavoproteins.
Other responses under the control of phototropins are leaf opening and closing, chloroplast movement, and the opening of stomata. Scientists today know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem.
Cryptochromes are another class of blue-light absorbing photoreceptors that also contain a flavin-based chromophore. Cryptochromes initiate the 24-hour activity cycle of the plant, which is also known as a circadian rhythm, using blue light clues.
Plant responses to gravity occur as shoots usually sprout upward and roots grow downward into the ground. Gravitropism enables roots to grow into the ground and shoots to grow upward toward sunlight. Growth of the shoot upward is called negative gravitropism and growth of roots downward is positive gravitropism.
Amyloplasts (statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in the shoots and specialized cells of the root cap. When a plant is tilted, the amyloplasts drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.
The mechanism of gravitropism occurs when amyloplasts settle to the bottom of gravity sensing cells in the root or shoot and contact endoplasmic reticulum (ER), which releases calcium ions that signals the cells to initiate polar transport of the hormone IAA to the bottom of the cell. In roots, high IAA causes cell elongation. This mechanism causes growth to slow at the root tip or lower root and grow normally at the upper root near the stem. In shoots, the mechanism is the opposite where higher concentration at the lower side causes growth upward. After vertical growth begins, the amyloplasts return to their normal position.
Plant sensory responses depend on hormones that act as chemical messengers. Plant hormones affect all aspects of plant life and morphogenesis and almost every cell in a plant can produce hormones and transport them throughout the plant.
Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. Auxins also control the differentiation of meristem into vascular tissue, and promote leaf development and arrangement. IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance or the inhibition of lateral bud formation, is triggered by auxins in the apical meristem. Flowering, fruit setting and ripening, and abscission (leaf falling) are other plant responses under the control of auxins. Auxins are also associated with the effects of blue light and red/fared light responses. Commercial use of artificial auxins is in agriculture as a growth hormone.
Cytokinins are hormones that produces cytokinesis (cell division) and up to 200 natural and artificial cytokinins are known. Cytokinins are most abundant in growing tissues where cell division is occurring.
Gibberellins (GAs) are a group of some 125 plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation.
Abscisic acid (ABA) is a plant hormone that accumulates as a response to stressful environmental conditions, such as dehydration, cold temperature, or short day lengths. ABAs counter the work of GAs and auxins by inhibiting growth and causing dormancy. The purpose of inhibiting growth is to protect the plant during extreme conditions. ABA also promotes the growth of winter buds or dormant buds. ABA in dry conditions can cause stomata to close to reduce water loss.
Ethylene is a plant hormone associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas, C2H4. Aging tissues produce ethylene. In fruit ripening, ethylene stimulates the conversion of starch and acids to sugars. Ethylene also causes leaf and fruit abscission, flower fading and dropping, and promotes germination in some plant grains. Ethylene is widely used in agriculture.
Recently discovered hormones that influence plant development include jasmonates, which work in defense responses to herbivory, when a plant is wounded by a predator. Volatile chemicals are released from the plant that attract predators of the herbivore.
Oligosaccharins are hormones that play a role in plant defense against bacterial and fungal infections. Strigolactones promote seed germination in some plants and inhibit lateral apical development in the absence of auxins in other plants. Strigolactones play a role in mycorrhizae, the symbiotic relationship of plant roots and fungi. Brassinosteroids promote development and physiological processes.
Thigmotrophism is the movement of a plant subjected to constant directional pressure such as touching or wind. Tendrils are threadlike plant parts that are touch sensitive and cause coiling in addition to helping the plant attach itself to an object for physical support.
Thigmonastic response is a touch response independent of direction of stimulus, such as in the Venus fly trap, where trigger hairs inside the trap cause the leaves to close quickly. Glands on the leaf surface secrete enzymes that slowly digest the prey and the resulting nutrients are absorbed by the plant and the leaves open again.
Thigmomorphogenesis is a slow development change in shape of a plant subjected to continuous mechanical stress, such as when trees are bent by strong winds. Strengthening tissue is produced to add stiffness and thickness to the tree trunk to resist force of the strong winds. Plant hormones such as ethylene and jasmonate are thought to be involved in this phenomenon.
The two main predators of plants are herbivores and pathogens. Herbivores are animals that chew on plants for their own food. Pathogens such as bacteria, fungi, and nematodes are microscopic or small organisms that cause diseases inside of plant cells and tissues.
Defense mechanisms in plants against predators include an impenetrable barrier, such as bark and waxy cuticles. Thorns and spines on plants are also used to protect from predators.
Secondary metabolites are chemicals produced by plants that are not associated with photosynthesis, respiration, or plant growth. Many metabolites are toxic and can be deadly. Some metabolites are alkaloids that have bad odors or tastes. Some are stimulants.
Mechanical wounds from predators initiate defense mechanisms inside the plant to stop the infection around the wound. Plants can also initiate abscission to remove a portion of its body that is damaged beyond repair.
Jasomonate hormones can create compounds that are toxic to predators or attract predators of pests of plants, including insects and parasites.
Plant Physiology
Plant Organ Systems
In plants, similar cells work together to form tissues just like in animals. Tissues work together to form an organ and organs work together to form an organ system. Vascular plants have two distinct organ systems: a shoot system and a root system. The shoot system contains two parts: the vegetative non-reproductive parts of the plant such as the leaves and stems, and the reproductive parts like the flowers and fruits. The shoot system generally grows above ground, as it absorbs light needed for photosynthesis processes. The root system supports the plant by absorbing water and minerals underground.
Plants are multicellular eukaryote organisms with various tissue systems. Two types of plant tissue systems are meristematic tissue and permanent tissue. Meristematic tissue is found in meristems, or plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated and they continue to divide and contribute to growth of the plant. Permanent tissue consists of plant cells that are no longer actively dying.
Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (where the leaf attaches to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.
Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue. Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps store water and sugars.
Secondary tissues are either simple (similar cell types) or complex (different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes three different cell types: vessel elements and tracheids (both of which conduct water), and xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells (which conduct photosynthesis), companion cells, phloem parenchyma, and phloem fibers. Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other. In stems, the xylem and phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.
Stems are part of the shoot system of the plant and their main function is to provide support to the plant, holding leaves, flowers, and buds. Sometimes, stems store food for the plant. The stem may be unbranched or branched and connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.
On plant stems, nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode. The stalk that extends from the stem to the base is called the petiole. An auxiliary bud is usually found in the axil, the area between the base of the leaf and the stem, where it can give rise to a branch or a flower. The apex or tip of the shoot contains the apical meristem within the apical bud.
Parenchyma cells are the most common plant cells and are found in the stem, root, inside of the leaf, and pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.
Collenchyma cells are elongated cells with unevenly thickened walls. They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis.
Sclerenchyma cells also provide support to the plant, but many of them are dead at maturity. Two types of sclerenchyma cells are fibers and sclereids. Both have secondary cell walls that are thickened with deposits of lignin, an organic compound in wood. Fibers are long, slender cells, sclereids are smaller sized.
The stem has three tissue systems: dermal, vascular, and ground tissue.
The dermal tissue of the stem consists primarily of epidermis, a singular layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark for further protection. Epidermal cells are the most numerous and least differentiated cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, where gas exchange takes place. Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface that help to reduce transpiration (loss of water), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.
Vascular tissue in the stem is made of xylem and phloem and are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. In the stem cross section, vascular bundles of dicot stems are arranged in a ring. In stems that live for more than one year, the individual bundles grow together and produce growth rings. In monocot stems, the vascular bundles are randomly scattered in the ground tissue.
Xylem tissue has three types of cells: Xylem parenchyma, tracheids, and vessel elements. Tracheids are xylem cells with thick secondary walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls shorter than the tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.
Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells or elements are arranged at the end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.
Ground tissue is made up of mainly parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.
Growth in stems occurs as primary growth as a result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. Some plant parts continue to grow throughout the life of the plant, called intermediate growth. Other plant parts like leaves and flowers stop growing when reaching a certain size, which is called determinate growth.
Most primary growth occurs in the apices or tips, stems, and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the tips of the shoot and roots. Cell elongation also contributes to primary growth. The influence of the apical bud on overall plant growth is known as apical dominance.
Secondary growth causes an increase in stem thickness due to the activity of lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and cork cambium in woody plants. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem inside and secondary phloem on the outside.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells or bark with a waxy substance called suberin that can repel water and protect the plant. The cork cambium produces another layer called the phelloderm inward from the cambium. The cork cambium, cork cells, and phelloderm are called the periderm and acts as an epidermis. In some plants, the periderm has openings called lenticels that allow for gas exchange with the atmosphere, including oxygen.
Annual rings are caused by the activity of the vascular cambium. During the spring growing season, the cells of the secondary xylem have a large internal diameter and their cell walls are not extensively thickened. In the fall season, the cell walls thicken in the secondary xylem, which is denser. This seasonal cycle of a decrease in vessel elements and increase in the number of tracheids results in the formation of annual growth rings.
Stem variations include rhizomes, a stem that grows horizontally underground with nodes and internodes, and sometimes vertical shoots. Corms are like rhizomes, but more rounded and fleshy. Stolons are stems that run parallel to the ground, where new plants can grow at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at interval nodes. Tubers are modified stems that can store starch and are swollen ends of stolons, along with unusual buds. A bulb works as an underground storage unit, a stem feature that appears to be fleshy leaves emerging from the stem.
Above ground, tendrils are slender, twirling strands that enable a plant to climb other surfaces like a vine or pumpkin. Thorns are sharp outgrowths that help protect the plant, as in roses.
Roots of seeded plants have three major functions: anchoring the plant in the soil, absorbing water and minerals and transporting them upward, and storing the products of photosynthesis. Most roots are underground, but adventitious roots exist above ground.
Two types of root systems include the tap root system that has a main root that grows down vertically, and from which many smaller lateral roots rise and penetrates deep into the soil. The other root system is fibrous root systems, which are located closer to the soil surface and forms a dense network of roots that helps prevent soil erosion. Some plants have a combination of these two types of root systems. Plants in dry places are more likely to have deep root systems in comparison to plants in wet places that likely have shallow root systems.
Root growth begins with seed germination. The tip of the root is protected by the root cap, which is continuously replaced because it gets damaged easily in the soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation. The zone of cell division is closest to the root tip and it is made of the actively dividing cells of the root meristem. The zone of elongation is where the newly formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three of these zones are within the first centimeter of the root tip.
The root has an outer layer of cells called the epidermis, which surrounds areas of ground tissue and vascular tissue. The epidermis provides protection and helps in absorption. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, which greatly enhances absorption of water and minerals. Inside the root, the ground tissue forms two regions: the cortex and the pith. Compared to stems, roots have lots of cortex and little pith. Both regions include cells that store photosynthetic products. The cortex is between the epidermis and the vascular tissue, whereas the pith is between the vascular tissue and the center of the root.
The vascular tissue in the root is arranged in the inner portion of the root, called the stele. A layer of cells known as the endodermis separates the stele from the ground tissue in the outer portion of the root. The endodermis is exclusive to the roots, and serves as a checkpoint for materials entering the root's vascular system. A waxy substance called suberin is present in the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root's vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem of the stele are arranged alternatively in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.
Root structures can have unique features designed for certain purposes. Some roots are bulbous and store starch. Aerial roots and prop roots are two forms of above ground roots that provide additional support to anchor the plant. Tap roots, such as carrots, turnips, and beets are examples of roots that are modified for food storage. Epiphytic roots enable a plant to grow on another plant, such as with the banyan tree and the screwpine.
Leaves are the main site of photosynthesis, the process in which plants synthesize food from sunlight. Most leaves are green color due to the chlorophyl in the leaf cells. However, leaves can be other colors that come from other pigments in the cells. The thickness, size, and shape of leaves are designed for their environmental location. Leaves in tropical environments have larger surface areas than those that grow in dry or cold environments. Smaller surface area of the leaves would minimize water loss.
Leaf structure commonly includes a leaf blade called the lamina, which is the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves without a petiole that are attached directly to the stem are called sessile leaves. Small green appendages usually found at the base of the petiole are are known as stipules. Most leaves have a midrib, which extends the length of the leaf and branches on each side to produce veins of vascular tissue. The edge of the leaf is called the margin.
The vascular tissue of the leaf forms veins and the arrangement of veins is called the venation pattern, which can vary in leaves. Monocots and dicots differ in their patterns of venation. Monocots have a parallel venation, as the veins run in straight lines parallel to each other across the leaf without converging. In dicots, the veins of leaves have a net-like appearance, forming a pattern known as reticulate venation. Dichotomous venation, as in Ginkgo biloba, features veins that fork.
Leaf arrangement on a stem is known as phyllotaxy and this arrangement can vary with species. Leaves are classified as alternate, spiral, or opposite arrangement. Alternate arrangement is where leaves alternate on each side of the stem in a flat plane. Spiral arrangement is where leaves are arranged in a spiral pattern along the stem. In opposite arrangement, leaves arise at the same point and connect on each side of the stem or branch. With three or more leaves connected at a node, it is called whorled arrangement.
Leaves may also be simple or compound in form. In a simple leaf, the blade is undivided or has lobes but the separation does not reach the midrib. In a compound leaf, the leaf blade is completely divided, forming leaflets. Each leaflet may have its own stalk, but it is attached to the rachis. A palmately compound leaf resembles the palm of a hand, with leaflets radiating outward from a point. Pinnately compound leaves have a feather-like appearance and the leaflets are arranged along the midrib.
In leaf structure, the outermost layer of the leaf is the epidermis, which is present on both sides of the leaf. The epidermis helps in the regulation of gas exchange, because it contains stomata, which are small openings in the leaf where gas exchange takes place. The stomata can open and close, regulated by two guard cells. The epidermis is usually one cell layer thick, but in extreme climates, the epidermis may be several layers thick to protect against heat and water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves have small hairs called trichomes on the surface that help prevent insect-eating plant movements. Trichomes also can store toxic or bad-tasting compounds to deter predators. Trichomes can also reduce transpiration by preventing air flow across the leaf surface.
Below the epidermis of dicot leaves are the layers of cells known as mesophyll, or middle leaf. The mesophyll of most leaves usually contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma. The palisade parenchyma (or palisade mesophyll) has a column-shaped, tightly-packed cells and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found in between the spongy parenchyma allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.
Just like the stem, the leaf contains vascular bundles composed of xylem and phloem. The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.
Leaf design in coniferous plants that live in cold environments have leaves that are reduced in size and needle-like in appearance, like spruce, fur, and pine. These needle-like leaves have sunken stomata and a smaller surface area, which help in reducing water loss. In hot climates, cactus plants have leaves that are spines, which help conserve water along with their thick stem trunks. Many aquatic plants have leaves with wide lamina that can float on the water surface, and a thick, waxy cuticle on the leaf surface that repels water.
Transport of Water and Solutes in Plants
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants.
Plants are designed with hydraulic engineering and can transport water throughout their physiology to the top of a tree over 100 meters tall. Hydraulic forces also enable plants enough force to break rocks and ceramic materials through water potential.
Water potential is a measure of potential energy in water. Water potential is recognized by the Greek letter psi (Ψ) and is expressed in units of pressure called megapascals. Water potential of pure water is given a value of zero and water potential values in different parts of the plant are given in relation to pure water.
Water potential (Ψ) is expressed in mathematical form as:
Ψ system = Ψ total = Ψs + Ψp + Ψg + Ψm
Ψs = water potential of the solute
Ψp = water potential of pressure
Ψg = water potential of gravity
Ψm = matric potential of water
The water potential system can refer to the water potential of soil water, root water, stem water, leaf water, or water in the atmosphere. Water moves from higher water potential to lower water potential. In order for water to move from the soil through the plant to the atmosphere, or transpiration, water potential of the soil must be greater than the root, which must be greater than the stem, which must be greater than the leaf, which must be greater than the atmosphere.
A plant can control water movement by controlling individual components of the water potential.
Solute potential (osmotic potential) is related to the solute concentration in molarity. This relationship is expressed by the van't Hoff equation, which is:
Ψs = -MiRT
M= molar concentration
i = van't Hoff factor, the ratio of the amount of particles in the solution to amount of formula units dissolved.
R = Ideal gas constant
T = temperature in Kelvin degrees
Therefore, Ψs decreases with increased solute concentration, and a decrease in Ψs leads to a decrease in Ψ total. The internal water potential of a plant cell is more negative than pure water because of increased solute concentration and because of the water potential difference, water can move from the soil through the roots, stem, and leaves of a plant and into the atmosphere by osmosis, or osmotic potential. Intelligent Design at work. Plant cells can regulate the amount of solute by adding and removing solute molecules and therefore have control over total water potential.
Pressure potential, Ψp (or turgor potential) may be positive or negative and positive Ψp will increase Ψtotal while negative Ψp will decrease Ψtotal. Positive turgor pressure is produced inside plant cells by the cell wall. A plant can manipulate Ψp by manipulating Ψs and the process of osmosis. Increasing the cytoplasmic solute concentration will reduce Ψs and Ψtotal, which will cause osmosis to occur and increase Ψp. The opening and closing of the stomata also affect the Ψp. Stomata opening causes water to evaporate from the leaf, reducing Ψp and Ψtotal, and allowing water to flow from the petiole or leaf stem into the leaf.
Gravity potential Ψg is always negative to zero in a plant with no height and always removes or consumes potential energy from the system. The force of gravity pulls water downwards toward the soil, reducing the total amount of potential energy in the water in the plant Ψtotal. The taller the plant, the more influential Ψg becomes and in short plants, Ψg is negligible. In tall trees, gravity potential is very strong and must be overcome to enable water to rise to the length of the tree by osmosis as water is pulled from the soil into the roots, trunk, leaves, and out through the stomata. Plants are unable to manipulate Ψg.
Matrix potential, Ψm, is always negative to zero and is lowest or most negative in dry systems like seeds and dry soil and zero in water-saturated systems. The binding of water to a matrix or solid surface always removes or consumes potential energy from the system and is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophillic solute molecules, whereas in Ψm, the other components are insoluble, hydrophillic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophillic, producing a matrix for adhesion of water. The matrix potential is very large or negative for dry tissues like seeds or dry soils. However, the matrix potential becomes zero when saturated with water. Matrix potential cannot be manipulated by plants.
Solutes, pressure, gravity and matric potential affect the transport of water in plants as water moves from a higher total water potential location to a lower total water potential location. The water potential is also directly related to Gibbs Free Energy, so higher water potential will have higher Gibbs Free Energy. Gibbs Free Energy is the energy associated with a chemical reaction that can be used to do work and is expressed as ΔΨ.
Transpiration is the loss of water from the plant through evaporation at the leaf surface and is the main cause of water movement in the xylem tissue. Transpiration causes a negative pressure at the leaf surface which is highest in dry conditions and water from the roots is pulled up because of this pressure or tension. When stomata holes close and transpiration stops, water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem tissue, and the cohesion of water molecules to each other.
At the cellular level, water saturates the cell walls and oxygen-carbon dioxide exchange occurs through intercellular air spaces as a requirement for photosynthesis. The water on the wet cell wall and cell surface evaporates through the air spaces in the leaf cells. The evaporation causes greater tension on the water in the cells and increases the pull on the water through the xylem tissue vessels. Small perforations between vessel tissue elements reduce the number and size of gas bubbles by a cavitation process. The gas bubbles in the xylem tissue interrupt the stream of water through the plant, or embolism. Taller trees have more cavitation events.
Transpiration is a passive process and does not require ATP or metabolic processes for water movement. Only the energy change from the pressure potential is needed to move water upward through the plant and this process is controlled. The atmosphere around the leaf drives transpiration and cause a large amount of water loss, up to 90 percent of water from the root can be taken by transpiration.
Leaves are covered by a waxy cuticle on their surface that prevents water loss and regulation of transpiration is accomplished by the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized guard cells that open and close in response to environmental conditions of light, water, and carbon dioxide gas. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. However, when stomata are open, water vapor is lost to the environment through transpiration. Plants must maintain balance between photosynthesis and water loss.
Plants that live in dry environments are designed to have a thicker waxy cuticle than those plants in wetter environments to control water loss. Other plants have a thick covering of trichome appendages or outgrowths that secrete oily substances that prevent air flow on the surface and reduce transpiration. Other plants have stomata sunken below the leaf's surface that help prevent water loss. Some plants also have multiple epidermal layers. These design features help reduce water loss by transpiration.
Products of photosynthesis are called photosynthates commonly in the form of sugars like sucrose. Structures that produce photosynthates for the growing plant are called sources. Sugars produced in sources such as leaves, need to be delivered to growing parts of the plant through the phloem in a process called translocation. The points of sugar delivery, such as roots, shoots, and seeds are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the stage of development and the season. The pattern of flow of photosynthates in the phloem can be in one direction or both directions through the plant, while in the xylem flow is only in one direction. The pattern of photosynthate flow will change as the plant grows and develops and photosynthates are sent where they are needed.
Translocation transport from source to sink occurs as photosynthates such as sucrose are produced in the mesophyll cells of photosynthesizing leaves and translocated through the phloem where they are used or stored. Photosynthates move through cytoplasmic channels called plasmodesmata across mesophyll cells. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles and the photosynthates are loaded onto these STEs. The sucrose is transported against the concentration gradient using ATP into the phloem cells using electrochemical potential of the proton gradient. In addition, a carrier protein called sucrose-H+ symporter carries the sucrose. Phloem STEs have reduced cytoplasmic contents and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells help assist the STEs with metabolic activities and produce energy for the STEs.
Once in the phloem, the photosynthates are translocated to the nearest sink. Phloem sap's high sugar content decreases solute potential, decreasing total water potential and causing water to move by osmosis from the xylem to phloem tubes and therefore increasing pressure. Increase in total water potential causes the bulk flow of phloem from source to sink. Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth or converted to starch or other polymers. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from high to low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.
Plant Sensory Systems and Responses
Responses to Light: Photomorphogenesis is the growth and development of plants in response to light and allows plants to optimize their use of light and space. Photoperiodism is the ability to use light to track time so that plants can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Phototrophism is a directional response that allows plants to grow towards or away from light. The very important response of plants to light is mediated by different photoreceptors, which are comprised of a protein covalently bonded to a light-absorbing pigment called a chromophore. The protein and pigment together are known as a chromoprotein. The different regions of the visible light spectrum including red and blue violet are responsible for structural development in plants. Sensory photoreceptors absorb light in these regions of the visible light spectrum because of the quality of light available in the daylight spectrum.
The phytochromes are a family of chromoprotein photoreceptors with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-inconvertible forms: Pr and Pfr. Pr absorbs red light and is immediately converted to Pfr. Pfr absorbs far-red light and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change in the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of the protein and therefore exposure to red light yields physiological activity. Pr is the inactive form of the protein. Exposure to far-red light inhibits phytochrome activity. Pr and Pfr represent the phytochrome system.
The phytochrome system acts as a biological light switch and it monitors the level, intensity, duration, and color of environmental light. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or in the nucleus it can directly activate or repress specific gene expression. In seeds, the phytochrome system is not used to determine direction and quality of light, but is instead used to determine if there is any light at all. In the dark, the phytochrome is in the Pr or inactive form and will not germinate unless exposed to light at the surface of the soil, where Pr is converted to Pfr and germination continues.
Plants also use the phytochrome system to sense the change of season. Photoperiodism is a biological response to the timing and duration of day and night and it controls flowering, setting of winter buds, and vegetative growth. Detection of seasonal changes is crucial to plant survival and day length is a better indicator of season than temperature and light intensity, which can vary. During the day, phytochrome molecules convert to active Prf form and back to Pf form in the evening. During long nights in the winter, all of the Prf form reverts, but during the short summer nights, some of the Prf remains at sunrise. The plant can sense the Pr/Prf ratio at dawn and can determine the length of the day/night cycle. This information can also be stored for several days and compared to determine the time of season so that the plant can modify its physiology accordingly.
Phototropism is the directional bending of a plant toward or away from a light source and is a response to blue wavelengths of light. Positive phototropism is bending toward light while negative phototropism is bending away from light. Phototropins are protein-based photoreceptors responsible for mediating the phototropic response. Phototropins contain a protein portion and a light-absorbing portion, called the chromophore. In phototropins, the chromophore is a covalently-bound molecule of flavin and phototropins are included in a class of proteins called flavoproteins.
Other responses under the control of phototropins are leaf opening and closing, chloroplast movement, and the opening of stomata. Scientists today know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem.
Cryptochromes are another class of blue-light absorbing photoreceptors that also contain a flavin-based chromophore. Cryptochromes initiate the 24-hour activity cycle of the plant, which is also known as a circadian rhythm, using blue light clues.
Plant responses to gravity occur as shoots usually sprout upward and roots grow downward into the ground. Gravitropism enables roots to grow into the ground and shoots to grow upward toward sunlight. Growth of the shoot upward is called negative gravitropism and growth of roots downward is positive gravitropism.
Amyloplasts (statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in the shoots and specialized cells of the root cap. When a plant is tilted, the amyloplasts drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.
The mechanism of gravitropism occurs when amyloplasts settle to the bottom of gravity sensing cells in the root or shoot and contact endoplasmic reticulum (ER), which releases calcium ions that signals the cells to initiate polar transport of the hormone IAA to the bottom of the cell. In roots, high IAA causes cell elongation. This mechanism causes growth to slow at the root tip or lower root and grow normally at the upper root near the stem. In shoots, the mechanism is the opposite where higher concentration at the lower side causes growth upward. After vertical growth begins, the amyloplasts return to their normal position.
Plant sensory responses depend on hormones that act as chemical messengers. Plant hormones affect all aspects of plant life and morphogenesis and almost every cell in a plant can produce hormones and transport them throughout the plant.
Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. Auxins also control the differentiation of meristem into vascular tissue, and promote leaf development and arrangement. IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance or the inhibition of lateral bud formation, is triggered by auxins in the apical meristem. Flowering, fruit setting and ripening, and abscission (leaf falling) are other plant responses under the control of auxins. Auxins are also associated with the effects of blue light and red/fared light responses. Commercial use of artificial auxins is in agriculture as a growth hormone.
Cytokinins are hormones that produces cytokinesis (cell division) and up to 200 natural and artificial cytokinins are known. Cytokinins are most abundant in growing tissues where cell division is occurring.
Gibberellins (GAs) are a group of some 125 plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation.
Abscisic acid (ABA) is a plant hormone that accumulates as a response to stressful environmental conditions, such as dehydration, cold temperature, or short day lengths. ABAs counter the work of GAs and auxins by inhibiting growth and causing dormancy. The purpose of inhibiting growth is to protect the plant during extreme conditions. ABA also promotes the growth of winter buds or dormant buds. ABA in dry conditions can cause stomata to close to reduce water loss.
Ethylene is a plant hormone associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas, C2H4. Aging tissues produce ethylene. In fruit ripening, ethylene stimulates the conversion of starch and acids to sugars. Ethylene also causes leaf and fruit abscission, flower fading and dropping, and promotes germination in some plant grains. Ethylene is widely used in agriculture.
Recently discovered hormones that influence plant development include jasmonates, which work in defense responses to herbivory, when a plant is wounded by a predator. Volatile chemicals are released from the plant that attract predators of the herbivore.
Oligosaccharins are hormones that play a role in plant defense against bacterial and fungal infections. Strigolactones promote seed germination in some plants and inhibit lateral apical development in the absence of auxins in other plants. Strigolactones play a role in mycorrhizae, the symbiotic relationship of plant roots and fungi. Brassinosteroids promote development and physiological processes.
Thigmotrophism is the movement of a plant subjected to constant directional pressure such as touching or wind. Tendrils are threadlike plant parts that are touch sensitive and cause coiling in addition to helping the plant attach itself to an object for physical support.
Thigmonastic response is a touch response independent of direction of stimulus, such as in the Venus fly trap, where trigger hairs inside the trap cause the leaves to close quickly. Glands on the leaf surface secrete enzymes that slowly digest the prey and the resulting nutrients are absorbed by the plant and the leaves open again.
Thigmomorphogenesis is a slow development change in shape of a plant subjected to continuous mechanical stress, such as when trees are bent by strong winds. Strengthening tissue is produced to add stiffness and thickness to the tree trunk to resist force of the strong winds. Plant hormones such as ethylene and jasmonate are thought to be involved in this phenomenon.
The two main predators of plants are herbivores and pathogens. Herbivores are animals that chew on plants for their own food. Pathogens such as bacteria, fungi, and nematodes are microscopic or small organisms that cause diseases inside of plant cells and tissues.
Defense mechanisms in plants against predators include an impenetrable barrier, such as bark and waxy cuticles. Thorns and spines on plants are also used to protect from predators.
Secondary metabolites are chemicals produced by plants that are not associated with photosynthesis, respiration, or plant growth. Many metabolites are toxic and can be deadly. Some metabolites are alkaloids that have bad odors or tastes. Some are stimulants.
Mechanical wounds from predators initiate defense mechanisms inside the plant to stop the infection around the wound. Plants can also initiate abscission to remove a portion of its body that is damaged beyond repair.
Jasomonate hormones can create compounds that are toxic to predators or attract predators of pests of plants, including insects and parasites.