Plant Biology

Plant Hormones: Auxin

1. Auxin, promotes plant growth by facilitating the elongation of developing cells. Auxin does this by increasing the concentration of H+ in primary cell walls, which, in turn, activates enzymes that loosen cellulose fibers. The result is an increase in cell wall plasticity. In response, turgor pressure causes the cell wall to expand, thus generating growth. Auxin is produced at the tips of shoots and roots, where, in concert with other hormones, it influences plant responses to light (phototropism) and gravity (geotropism). In addition, auxin is active in leaves, fruits, and germinating seeds. Structurally, auxin is a modified tryptophan amino acid. After synthesis from tryptophan, it is actively transported (using ATP) from cell to cell in a specific direction (polar transport), by means of a chemiosmotic process.

Plant Tissues: Ground Tissue

1. Ground tissues include three basic kinds of cells that differ mostly by the nature of their cell walls. • Parenchyma cells, the most common component of ground tissue, have thin walls and serve various functions including storage, photosynthesis, and secretion. • Collenchyma cells, which have thick but flexible cell walls, serve mechanical support functions. •Sclerenchyma cells, with thicker walls than collenchyma, also provide mechanical support functions.

Many flowering plants initiate flowering in response to changes in the photoperiod. Flowering plants can be divided into three groups, as follows:

1. Long-day plants flower in the spring and early summer when daylight is increasing. 2. Short-day plants flower in late summer and early fall when daylight is decreasing. These plants flower when daylight is less than a critical length (or when night exceeds a critical length). 3. Day-neutral plants do not flower in response to daylight changes. Some other cue, such as temperature or water, triggers flowering.

water transportation mechanisms

1. Osmosis. Water moves from the soil through the root and into xylem cells by osmosis. A concentration gradient between the soil and the root is maintained in two ways—by the continuous movement of water out of the root by xylem and by the higher mineral concentration inside the stele maintained by the selective passage of ions through the endodermis. To a certain extent, the movement of water into the root by this concentration gradient forces water up the xylem. This osmotic force, called root pressure, can be seen as guttation, the formation of small droplets of sap (water and minerals) on the ends of leaves of grasses and small herbs in the early morning. Under most environmental conditions, however, the forces generated by root pressure are too small to have a major effect on the movement of water in plants, especially large plants such as trees. 2. Capillary action. Capillary action or capillarity, is the rise of liquids in narrow tubes. It also contributes to the movement of water up xylem. Capillary action results from the forces of adhesion (molecular attraction between unlike substances) between the water and the capillary tube (a tube with a narrow bore). These forces combine to pull water up the sides of the tube. As a result, a meniscus, or crescent-shaped surface, forms at the top of the water column. In active xylem cells, however, water forms a continuous column without menisci. Thus, the effect of capillary action is minimal, confined to minute cavities in the cellulose microfibrils of the cell wall. 3. Cohesion-tension theory. Although root pressure and capillary action may make minor contributions to water movement under special conditions, most water movement through xylem is explained by cohesion-tension theory. The major concepts of this theory are as follows: • Transpiration, the evaporation of water from plants, removes water from leaves, causing a negative pressure, or tension, to develop within the leaves and xylem tissue. • Cohesion between water molecules produces a single, polymerlike column of water from roots to leaves. Cohesion is the molecular attraction between like substances. In water, cohesion results from the polarity of water molecules, which causes hydrogen bonding to occur between adjacent water molecules. As a result, the water molecules within a series of xylem cells (vessels or tracheids) behave as a single, polymerlike molecule. • Bulk flow of water through xylem cells occurs as water molecules evaporate from the leaf surface. When a water molecule is lost from a leaf by transpiration, it pulls up behind it an entire column of water molecules. In this way, water moves by bulk flow through the xylem by a pulling action generated by transpiration. Since transpiration is caused by the heating action of the sun, the sun, then, is the driving force for the ascent of sap through plants.

Photoperiodism (maintenance of circadian rhythm)

1. Pfr appears to reset the circadian-rhythm clock. Pfr is the active form of phytochrome and appears to maintain photoperiod accuracy by resetting the circadian-rhythm clock. 2. Pr is the form of phytochrome synthesized in plant cells. Pr is synthesized in the leaves of plants. 3. Pr and Pfr are in equilibrium during daylight. During daylight, Pr is converted to Pfr, since red light is present in sunlight. Some far-red light is also present in sunlight, so some of the Pfr is converted back into Pr. In this manner, an equilibrium between the two forms of phytochrome is maintained during daylight. 4. Pr accumulates at night. At night, the levels of Pfr drop. This is because there is no sunlight to make the conversion from Pr to Pfr. Also, Pfr breaks down faster than Pr, and in some plants, Pfr is metabolically converted back into Pr. Furthermore, the cell continues to make Pr at night. Thus, Pr accumulates at night. 5. At daybreak, light rapidly converts the accumulated Pr to Pfr. An equilibrium between Pr and Pfr is again attained. 6. Night length is responsible for resetting the circadian-rhythm clock. If daylight is interrupted with a brief dark period, there is no effect on the circadian-rhythm clock. In contrast, flashes of red or far-red light during the night period can reset the clock. If a plant is exposed to a flash of red light during the night, Pr is converted back to Pfr, a shorter night period is measured, and the circadian rhythm is reset. If a flash of far-red light follows the red light, then the effect of the red light is reversed, and the night length is restored to the night length in effect before the far-red flash. In a series of alternating flashes of red and far-red light, only the last flash affects the perception of night length. Thus, red light shortens the night length and far-red restores the night length.

Phototropism

1. Phototropism, the response to light, is achieved by the action of the hormone auxin. The process is described as follows: • Auxin is produced in the apical meristem, moves downward by active transport into the zone of elongation, and generates growth by stimulating elongation. • When all sides of the apical meristem are equally illuminated, growth of the stem is uniform and the stem grows straight. • When the stem is unequally illuminated, auxin moves downward into the zone of elongation but concentrates on the shady side of the stem. Auxin that would have normally accumulated on the sunny side ends up on the shady side. • The higher concentration of auxin in the shady side of the stem causes differential growth; that is, since auxin generates growth by stimulating elongation, the shady side grows more than the sunny side. When the shady side grows more than the sunny side, the stem bends toward the light.

Structure of the Leaf

1. The epidermis is a protective covering of one or more layers of cells. As in other aerial portions of the plant, the epidermis is covered by the cuticle, a protective layer consisting of the waxy material cutin. The cuticle reduces transpiration, or the loss of water through evaporation. Specialized epidermal cells may bear trichomes (hairs, scales, glands, and other cell outgrowths). 2. The palisade mesophyll consists of parenchyma cells equipped with numerous chloroplasts and large surface areas, specializations for photosynthesis. Photosynthesis in leaves occurs primarily in this tissue. The parenchyma cells are usually tightly packed in one or more layers at the upper surface but can occur at both surfaces of vertically oriented leaves of plants adapted to dry habitats. 3. The spongy mesophyll consists of parenchyma cells loosely arranged below the palisade mesophyll. The numerous intercellular spaces provide air chambers that provide CO2 to photosynthesizing cells (and O2 to respiring cells). 4. Guard cells are specialized epidermal cells that control the opening and closing of stomata. Stomata are openings in the epidermis that allow gas exchange between the inside of the leaf and the external environment. 5. Vascular bundles consist of xylem and phloem tissues. Xylem delivers water for photosynthesis, while phloem transports sugars and other carbohydrate by-products of photosynthesis to other areas of the plant. There are usually specialized mesophyll cells called bundle sheath cells that surround the vascular bundles in such a way that no vascular tissue is exposed to intercellular spaces. In this way, air bubbles cannot enter vessels where they could impede the movement of water. In addition, bundle sheath cells provide the anaerobic environment for CO2 fixation in C4 plants.

Plant Tissues: Dermal Tissue

2. Dermal tissue consists of epidermis cells that cover the outside of plant parts, guard cells that surround stomata, and various specialized surface cells such as hair cells, stinging cells, and glandular cells. In aerial portions of the plant, the epidermal cells secret a waxy protective substance, the cuticle.

Plant Hormones: Gibberellins

2. Gibberellins are a group of plant hormones that, like auxin, promote cell growth. The more than 60 various related gibberellins are abbreviated GA1, GA2, GA3, etc., for gibberellic acid. They are synthesized in young leaves, roots, and seeds but are often transported to other parts of the plant. For example, gibberellins produced in the roots and transported to shoot tips interact with auxins to stimulate shoot growth. Gibberellins are also involved in the promotion of fruit development and of seed germination, and the inhibition of aging in leaves. High concentrations of GA can cause the rapid elongation of stems (called bolting). For example, bolting occurs in rice plants when a fungus that produces GA attacks the plant.

Gravitotropism

2. Gravitropism (or geotropism), the response to gravity by stems and roots, is not well understood. In general, both auxin and gibberellins are involved, but their action depends on their relative concentrations and the target organ (root or stem). The role of auxin appears to agree with the following: • If a stem is horizontal, auxin produced at the apical meristem moves down the stem and concentrates on its lower side. Since auxin stimulates cell elongation, growth of the lower side is greater than that of the upper side, and the stem bends upward as it grows. • If a root is horizontal, auxin is produced at the apical meristem (root tip), moves up the roots, and, as in stems, concentrates on the lower side of the root. However, in roots, auxin inhibits growth. This is because concentrations of auxin are higher in roots than in stems. Dissolved ions, auxins, gibberellins, and other hormones do not respond to gravity. They remain evenly distributed in a solution, regardless of the presence or directional pull of gravity. Therefore, auxins do not concentrate in the lower parts of stems or roots in direct response to gravity. Starch, on the other hand, is insoluble in water and does respond to gravity. It is believed that specialized starch-storing plastids called statoliths, which settle at the lower ends of cells, somehow influence the direction of auxin movement.

Plant Hormones: Cytokinins

3. Cytokinins are a group of hormones that stimulate cytokinesis (cell division). Structurally, they are variations of the nitrogen base adenine. They include naturally occurring zeatin and artificially produced kinetin. Cytokinins are produced in roots (and perhaps elsewhere) and are transported throughout the plant. They have a variety of effects depending upon the target organ and, sometimes, the presence (and concentration) of auxin. In addition to stimulating cell division, cytokinins influence the direction of organ development (organogenesis). For example, the relative amounts of cytokinins and auxin determine whether roots or shoots will develop. Cytokinins stimulate the growth of lateral buds, thus weakening apical dominance (the dominant growth of the apical meristem). Cytokinins have been found to delay senescence (aging) of leaves and are often sprayed on cut flowers and fruit to prolong their usefulness.

Thigmotropism

3. Thigmotropism is a response to touch. When vines and other climbing plants contact some object, they respond by wrapping around it. The mechanism for this kind of differential growth is not well understood.

Plant Tissues: Vascular Tissue

3. Vascular tissue consists of two major kinds of tissues, xylem and phloem. The two usually occur together to form vascular bundles. • Xylem functions in the conduction of water and minerals and also provides mechanical support. Xylem transports up, in addition to the primary cell wall that all plants have, xylem cells have a secondary cell wall that gives them additional strength. • Phloem functions in the conduction of sugars. Phloem is made up of cells called sieve-tube members that form fluid-conducting columns called sieve tubes. phloem can transport up or down as needed.

Plant Hormones: Ethylene

4. Ethylene (H2C = CH2) is a gas that promotes the ripening of fruit. During the later stages of fruit development, ethylene gas fills the intercellular air spaces within the fruit and stimulates its ripening by enzymatic breakdown of cell walls. Ethylene is also involved in stimulating the production of flowers. In addition, ethylene (in combination with auxin) inhibits the elongation of roots, stems, and leaves and influences leaf abscission, the aging and dropping of leaves.

Plant Hormones: Abscisic Acid

5. Abscisic acid (ABA) is a growth inhibitor. In buds, it delays growth and causes the formation of scales in preparation for overwintering. In many species of plants, ABA maintains dormancy in seeds. Dormancy in these seeds is broken by an increase in gibberellins or by other mechanisms that respond to environmental cues such as temperature or light. In some desert species, seed dormancy is overcome by the leaching of ABA from seeds by rains. Although ABA is named for the process of abscission, its influence on the abscission of leaves, flowers, and fruits is controversial.

Seed Development and Germination

After a seed reaches maturity, it remains dormant until specific environmental cues are encountered. The most important environmental cue is water. Others may include specific temperatures (cold or warm), light, or seed coat damage (for example, by fire or by the action of enzymes from the digestive tracts of animals). In some cases, there may be a required dormancy period, during which germination will not occur, regardless of the presence of external environmental cues. Germination begins with the of water. The water initiates the activity of various enzymes, which activate biochemical processes including respiration. In addition, the absorption of water causes the seed to swell and the seed coat to crack. The growing tips of the radicle produce roots that anchor the seedling. Elongation of the hypocotyl follows, producing a young shoot. In the young seedling, growth occurs at the tips of roots and shoots, called apical meristems. These are areas of actively dividing, or meristematic, cells. This kind of growth, typical in seedlings and young plants, is called primary growth. The growth of a root can be divided into areas based on the activity of its cells. The root tip, or root cap, protects the apical meristem behind it. The dividing cells of the apical meristem form the zone of cell division. Newly formed cells absorb water and elongate, forming the next region, the zone of elongation. Since elongation actually makes the root tip get longer, this zone is technically responsible for our perception of growth. Behind this zone is the zone of maturation. Here, cells mature into xylem, phloem, parenchyma, or epidermal cells. Root hairs may form as extensions of epidermal cells.

Pollination Gymnosperms

Angiosperms have animals and insects carry their pollen from plant to plant, while most gymnosperms rely on the wind. Angiosperms have more advanced tissues and diversity than current day gymnosperms, in large part due to more effective pollination. Bees are huge pollinators! They fly from flower to flower transferring pollen grains!

Plant Defense Mechanisms

B) Chemicals: Some plants fend off predators by using an array of toxins. Some of these compounds are called alkaloids. They include nicotine, caffeine, morphine, cocaine, strychnine, and quinine. These alkaloids have pharmacological effects on both humans and animals. Some alkaloids inactivate enzymes, others affect DNA repair mechanisms. Some plants release HCN to ward off insects and other predators. Clearly you see that plants have developed a stunning array of structural as well as chemical defenses to deter against predators.

Plant Development

Bottom Line: the stigma catches the pollen, it travels down the pollen tube and then finds a receptive ovule, inside the ovary. The male and female genetic material unite to form an embryo which will become a seed. Fruits, flowers, and what is called double fertilization are unique to angiosperms. What is double fertilization? The arrival of a pollen grain on the female reproductive part is called pollination. A pollen grain develops into a "pollen tube" which grows toward the ovary and contains two sperm. The egg fuses with one sperm and a zygote is made (2N). The other sperm fuses with two polar nuclei cells to make a 3N cell... this 3N cell will give rise to the nutritive tissue. (The 3N tissue represents the endosperm). After double fertilization, ovule becomes the seed, and the ovary becomes the fruit. Seeds are multicelled, and will safeguard the embryo. The embryo develops in our plant

Secondary Growth: Tree Bark

Each year, new layers of secondary xylem are produced by the vascular cambium. Recall that xylem tissue, which is the actual wood of a plant, is dead at maturity. However, only xylem produced during the more recent years remains active in the transport of water. This xylem is referred to as sapwood. Older xylem, located toward the center of the stem, is called heartwood and functions only as support. In many environments, conditions vary during the year, creating seasons during which plants alternate growth with dormancy. During periods of growth, the vascular cambium is actively dividing, and as the season draws to an end, divisions and growth slow and gradually come to an end. When the next season begins, the vascular cambium begins dividing again. The alternation of growth and dormancy produces annual rings in the secondary xylem tissue. These rings can be used to determine the age of a tree. Since the size of the rings is related to the amount of water available during the year, rings can also provide a rainfall history for the region.

primary plant grow

For many plants, including most monocots, actively dividing cells occur only at the apical meristems producing growth that increases the length of a shoot or root. This kind of growth is called primary growth, and the tissues that develop from this growth are primary tissues. Thus, primary xylem and primary phloem refer to vascular tissues originating from apical meristem growth.

Plant Hormones: Introduction

Hormones are substances that are produced by specialized cells in one part of an organism that influence the physiology of cells located elsewhere. They are small molecules, capable of passing through cell walls, that affect the division, growth (elongation), or differentiation of the cells. Very small quantities of hormones are required to alter cell physiology. However, the specific effect of a hormone depends upon the particular hormone, its concentration, the target cell, and the presence or absence of other hormones. A description of the five classes of plant hormones follows:

Pollination Angiosperms

Increases genetic variability Self-Pollination: Pollen grains are transferred from the anther to the stigma of the same flower or at the ovule in the case of a non-flowering plant like a gymnosperm. Cross-Pollination: Most flowers do this, we transfer pollen from the anther of a flower of one plant to the stigma of a flower on another plant of the same species.

What is Mycorrhizae

Mycorrhizae: this is the association of a nonpathogenic fungi with roots. Materials absorbed by the plant passes first through the fungi which allows them to gain nutrients. The fungus actually increases the surface area available for water and nutrient uptake. Some trees grow poorly without the fungus present!! Orchids are among the plants that also depend on this symbiotic relationship called mutualism.

The Seed

Note: A seed is produced after fertilization by pollen. A seed consists of an embryo, a seed coat, and some kind of storage material. The major storage material may be endosperm or cotyledons. Cotyledons are formed by using (digesting) the storage material in the endosperm. In dicots, such as peas, there are two fleshy cotyledons. Most of what you see when you look at the two halves of a pea seed are the two cotyledons (the remainder is a small embryo). In many monocots, such as corn, most of the storage tissue is endosperm, with a single cotyledon that functions to transfer nutrients from the endosperm to the embryo.

Root Nodules Contain What

On the roots of legumes such as peas and beans, we see swellings called root nodules that contain the nitrogen-fixing bacteria which convert atmospheric N2 into forms required by the plant, such as NO3 - and NH4 +. Once NO3 - and NH4 + are absorbed by the plant, they can use them to form amino acids, proteins, and other nitrogenous compounds. Soil microorganisms can also break down dead organisms to release NH4 + which can be oxidized into NO3 - in a process called nitrification. In denitrification we form N2 and a small amount of N2O as NO3 - and NO2 - are converted into these products. This is all part of what is called the nitrogen cycle.

secondary plant growth

Other plants, like conifers and the woody dicots, undergo secondary growth in addition to primary growth. Whereas primary growth extends the length of plant parts, secondary growth increases their girth, or lateral dimension (to the side) and is the origin of woody plant tissues. Secondary growth occurs at two lateral meristems, the vascular cambium and the cork cambium. These cells are meristematic, capable of dividing and producing new cells throughout the lifetime of the plant. The tissues that originate from the vascular cambium are the secondary xylem and the secondary phloem. The cork cambium gives rise to periderm, the protective material that lines the outside of woody plants.

anatomy of the plant (reproductive anatomy)

Pollination is the act of transferring pollen grains from the male anther of a flower to the female stigma. The top of the pistil is called the stigma, and is often sticky. Seeds are made at the base of the pistil, in the ovule. To be pollinated, pollen must be moved from a stamen to the stigma. When pollen from a plant's stamen (anther) is transferred to that same plant's stigma, it is called self-pollination.

Plant Responses to Stimuli: Tropisms

Since plants are anchored by their roots, they cannot move in response to environmental stimuli. Instead, they change their growth pattern. A growth pattern in response to an environmental stimulus is called a tropism. Three tropisms are described below:

Alteration of Generation

The bottom line is simply this... all land plants have this life cycle called alternation of generations which give rise to spores, egg, and sperm.

Function of the Stomata in Plants

The opening and closing of the stomata influence gas exchange, transpiration, the ascent of sap, and photosynthesis. When stomata are closed, CO2 is not available, and photosynthesis cannot occur. In contrast, when stomata are open, CO2 can enter the leaf, but the plant risks desiccation from excessive transpiration. Guard cells regulate the opening and closing of the stomata. Many factors seem to be involved in the mechanism that controls opening and closing of stomata. The following observations have been made: 1. Stomata close when temperatures are high. This reduces loss of water (but shuts down photosynthesis). 2. Stomata open when CO2 concentrations are low inside the leaf. This allows active photosynthesis, since CO2 is required. 3. Stomata close at night and open during the day. This may be in response to CO2 fluctuations caused by photosynthesis. During daylight hours, CO2 is low because it is used by photosynthesis, but at night, CO2 levels are high because of respiration. 4. Stomatal opening is accompanied by a diffusion of potassium ions (K+ ) into the guard cells (from surrounding subsidiary cells). An increase in K+ creates a gradient for the movement of water into the guard cell, which, in turn, results in guard cell expansion and the opening of the stomata. 5. When K+ enter a guard cell, they create an unbalanced charge state. In some plants, the charge is balanced by the movement of chloride ions (Cl- ) into the guard cells along with the K+. In other plants, H+ are pumped out of the cell. The H+ originate from the ionization of various organic substances within the cell.

Secondary Growth: Plant Structure

The vascular cambium originates between the xylem and phloem and becomes a cylinder of tissue that extends the length of the stem and root. Secondary growth in a stem is illustrated in. The cambium layer is meristematic, producing new cells on both the inside and outside of the cambium cylinder. Cells on the inside differentiate into secondary xylem cells; those on the outside differentiate into secondary phloem cells. Over the years, secondary xylem accumulates and increases the girth of the stem and root. Similarly, new secondary phloem is added yearly to the outside of the cambium layer. As a result, tissues beyond the secondary phloem are pushed outward as the xylem increases in girth. These outside tissues, which include the primary tissues (such as the epidermis and cortex), break apart as they expand and are eventually shed as they separate from the stem or root.

Transport of Sugar in Plants

Translocation is the movement of carbohydrates through phloem from a source, such as leaves, to a sink, a site of carbohydrate utilization. Translocation is described by the pressure-flow hypothesis, as follows: 1. Sugars enter sieve-tube members. Soluble carbohydrates, such as fructose and sucrose, move from a site of production, such as the palisade mesophyll, to phloem sieve-tube members by active transport. This develops a concentration of solutes (dissolved substances, sugars in this case) in the sieve-tube members at the source that is higher than that at the sink (a root, for example). 2. Water enters sieve-tube members. As a result of the movement of solutes into the sieve-tube members, the concentration of water inside the cell becomes less than in the area outside the cell. As a result, water diffuses into these cells, moving down the water concentration gradient. 3. Pressure in sieve-tube members at the source moves water and sugars to sieve-tube members at the sink through sieve tubes. When water enters the sieve-tube members in the leaves (or other source), pressure builds up because the rigid cell wall does not expand. As a result, water and sugars move by bulk flow through sieve tubes (through sieve plates between sieve-tube members). 4. Pressure is reduced in sieve-tube members at the sink as sugars are removed for utilization by nearby cells. As water and sugars move by bulk flow from source to sink, pressure begins to build at the sink. However, a sink is an area where carbohydrates are being utilized. Thus, sugars are removed from the sieve-tube members (by active transport), which increases the concentration of water within the sieve-tube members. Water then diffuses out of the cell (moving down the water concentration gradient), relieving the pressure

Transport of Water in Plants

Water and dissolved minerals enter the roots through root hairs by osmosis. There are two pathways by which the water moves toward the center of the root, as follows. 1. Water moves through cell walls and intercellular spaces from one cell to another without ever entering the cells. This pathway is called the apoplast and consists of the "nonliving" portion of cells. 2. Water moves from one cell to another through the symplast, or "living" portion of cells. In this pathway, it moves from the cytoplasm of one cell to the cytoplasm of the next through plasmodesmata, small tubes that connect the cytoplasm of adjacent cells. When water reaches the endodermis, it can continue into the vascular cylinder only through the symplast pathway. The apoplast pathway is blocked by the suberin that permeates the casparian strips. The endodermal cells allow water to enter the stele (vascular cylinder) but are selective as to which minerals are allowed to enter. For example, potassium (K+ ), an essential mineral, is allowed to pass, while sodium (Na+ , common in soils but unused in plants, is blocked. Once through the endodermis, water and minerals continue by the apoplast pathway to the xylem. The xylem tissue, consisting of tracheids and vessels, is the major conducting mechanism of the plant.