Morphology, Anatomy and Physiology of Flowering Plants

Morphology, Anatomy and Physiology of Flowering Plants

Plant Anatomy

Morphology of a flowering plant:

Monocot vs. Dicot Traditionally, the flowering plants have been divided into two major groups, or classes: Monocotyledonae and Dicotyledonae (Cotyledon: seed leaf of the embryo of a plant that contains food for germination)

A comparison of monocots and dicots

Leaf Venation

Floral Number

Root Structure (a) The fibrous branching roots of the monocotyledon (b) The tap root structure with lateral roots of the dicotyledon.

The Root System Plant cells and tissues depend on shoots for sugar anchorage, absorption of water and minerals root hairs are epidermal extensions Vascular tissue system - central cylinder (stele) which consists of xylem and phloem dicots - xylem radiates from the stele's center in two or more spokes with phloem between the spokes monocots - a central pith is ringed by vascular tissue in an alternating pattern of xylem and phloem lateral roots; arise from pericycle (layer of cells just inside the endodermis that may become meristematic)

Dicot Root

Plant cells and tissues Tissue types of the stem: Epidermis: surface of the stem made of a number of layers often with a waxy cuticle to reduce water loss. Cortex Tissue: Forming a cylinder of tissue around the outer edge of the stem. Often contains cells with secondary thickening in the cell walls which provides additional support.

Plant cells and tissues Tissue types of the stem: Vascular bundle: contains xylem, phloem and cambium tissue. Xylem: a longitudinal set of tubes that conduct water from the roots upward through the stem to the leaves. Phloem transports sap through the plant tissue in a number of possible directions. Vascular cambium is a type of lateral meristem that produces the secondary xylem and phloem through cell division. In the center of the stem can be found the pith tissue composed of thin walled cells. In some plants this section can degenerate to leave a hollow stem.

Dicot Stem

Dicot Stem

Monocot Stem

Leaf Anatomy

Leaf Anatomy

Ecological Adaptations for Leaves The plant structures previous are examples of mesophytic ('normal') plants that are generally associated with temperate climates or where there is no water stress. Xerophytes are plants that have adaptations to reduce water loss or indeed to conserve water. They occupy habitats in which there is some kind of water stress. Examples of such water stress habitats include: -Desert (high temp, low precipitation) -High Altitude & High Latitude ( low precipitation or water locked up as snow or ice) -Rapid drainage (sand dunes)

Xerophytic Adaptations for Leaves Have reduced leaves (narrow or needle-shaped) to reduce SA and reduce water loss Some have leaves that can roll up to create a humid pocket of air inside rolled leaf Hairy" leaves to reduce transpiration Stomata are in pits surrounded by hairs, reduces water loss Reduced number of stomata Thickened waxy cuticle Deep roots and low growth Fleshy water storage tissue (succulent)

Xerophytic Adaptations for Leaves This is a cross section of a grass leaf that occupies a sand dune habitat. They are able to tolerate the poor water retention of the soil (sand) and the drying effects of wind by limiting water loss. The Thick waxy upper epidermis extends all the way around as the leaf rolls up. This places the stomata in an enclosed space not exposed to the wind. The stomata are in pits which allows boundary layer of humidity to build up which also reduces water loss by evaporation The hairs on the inner surface also allow water vapor to be retained which reduces water loss through the pores. The groove formed by the rolled leaf also acts as a channel for rain water to drain directly to the specific root of the grass stem

Xerophytic Adaptations for Leaves This plant was photographed in SE Asia where it was growing very rigorously. The temperatures in this environment has an average over 30 C with a high potential for water loss. The leaves of this plant have waxy cuticle on both the upper and lower epidermis The waxy repels water loss through the upper and lower epidermal cells. If an epidermal cell has no cuticle water will rapidly be lost as the cellulose cell wall is not a barrier to water loss. In the background there are a variety of other plants all of which have this xerophytic adaptation of a waxy cuticle

Xerophytic Adaptations for Leaves Conifer distribution is often associated with the northern taiga forests of North America, Scandinavia and Russia. Like a rolled leaf, study of the internal structure shows it has effectively no lower epidermis. This adaptation is required as northern climates have long periods in which water is actually frozen and not available for transpiration. Plants in effect experience water availability more typical of desert environments. This type of adaptation means that confers have their distribution extended beyond the northern forests to a variety of water stress climates.

Plant Physiology

Water Movement and Transpiration Transpiration is the loss of water form the leaves and stem of plants.

Abiotic Factors Affecting Transpiration Humidity This is a measure of the water vapour in air and is normally expressed as a percentage. Light absorbed by the leaf warms the water within the mesophyll tissue and it enters the vapor phase in the space above the pore called the sub stomatal air space (SSAS). With the pore open water vapor diffuses out down a step water vapor gradient. The water vapor forms diffusion shells of changing % humidity. The steepest gradient is found at the edge of the pore where effectively most water loss occurs. In high humidity the diffusion gradient is not as steep and the rate of diffusion is less. In high humidity the diffusion shells of water vapor from one pore to the next and the steep gradient associated with the edge of the pore is lost. The formation of this boundary layer of high humidity reduces the rate of transpiration

Abiotic Factors Affecting Transpiration Wind Moving air reduces the external water vapor concentration such that the gradient between the sub stomatal air space and the surrounding air increases. Still air allows the build up of boundary layers as shown above and so reduces the rate of transpiration.

Abiotic Factors Affecting Transpiration Temperature The leaf absorbs light and some of the light energy is lost as heat. Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30 C, a leaf may transpire three times as fast as it does at 20 C. The effective evaporation is in the sub stomatal air space and increases the gradient of water vapor between the sub stomatal air space and the surrounding air.

Abiotic Factors Affecting Transpiration Light The rate of plants transpire is faster in the light than in the dark. This is because light stimulates the opening of the stomata and warming of the leaf. The mechanism of pore opening relates to the Guard cell becoming turgid. The mechanism: (a) The guard cell absorbs light and produces ATP in the light dependent reaction. (b) The ATP is used to drive proton pumps that pump out H+. The inside of the cell becomes more negative. (c) Potassium ions enter the cell which increases the solute concentration. (d) Water moves from the surrounding tissue by osmosis. The turgid cell increases the pressure potential and the cell expands. The cells have asymmetric thickening with lignin on the stoma-side wall. The cells expand more on the outside wall. The pore opens in the middle as each cell bends

Root Hairs The extension of the cell wall increases the surface area for the absorption of water and minerals at the cellular level providing both an increase in the cell wall (apoplastic pathway) and the cytoplasmic route (symplastic pathway) for the movement of water.

Process of Mineral Absorption From the Soil By Active Transport Plants secure the water and minerals they need from their roots. The path taken is soil roots stems leaves

Uptake of Water in the Roots (a) Water enters epidermal cell cytoplasm by osmosis. The solute concentration is lower than that of soil water due to the active transport of minerals from the soil water to the cytoplasm. Symplastic Pathway (b) to (d): water moves along a solute concentration gradient. There are small cytoplasmic connections between plant cells called plasmodesmata. Apoplastic Pathway(e) to (f):water moves by capillarity through the cellulose cell walls. Hydrogen bonding maintains a cohesion between water molecules which also adhere to the cellulose fibers. (g) The endodermis is the outer tissue of the vascular root tissue (h).

Water Conducting Xylem The cells are produced from the division of the cambium and then differentiate into xylem Form a continuous pipe from the root up through the stem to the leaves The cytoplasm breaks down to form the pipeline with perforated end walls Cells are dead at maturity uni-directional Their walls are thickened with secondary deposits of cellulose and are usually further strengthened by impregnation with lignin.

Loading of Water in the Xylem Minerals are actively loaded into the xylem which in turn causes water to enter the xylem vessel. Chloride for example is actively pumped by endodermal cells and pericycle cells. This creates a water potential gradient that moves water passively into the xylem. Pressure within the xylem increases which begins forcing water upward (Root Pressure). A pressure potential gradient (hydrostatic pressure) based on evaporation (tension) form the leaf is responsible for the upward movement of water in the xylem.

Water Transport in the Xylem Water molecules are weakly attracted to each other by hydrogen bonds. This action extends down the xylem creating a 'suction' effect. There is also adhesion between water molecules and the xylem vessels The cohesion and adhesion act together to maintain the water column all the way up from the root to the stomata. The rapid loss of water from the leaf pulls the water column creating 'tension' within the xylem vessel sufficient to cause the walls of the xylem to be bent inwards. In large trees this tension can be so great that the diameter of the tree can decrease during the summer months with peak rate of transpiration The movement of water is an example of mass flow due to a negative pressure potential.

Cell Turgidity and Plant Support Turgor Pressure: a) Water enters the cell by osmosis from the higher osmotic potential (solute potential) to the lower osmotic potential (solute potential). b) The volume of the cell cytoplasm increases forcing the plasma membrane outwards against the cell wall. c) The outward pressure is matched by an inward pressure, equal in magnitude but opposite in direction d) These pressures are called turgor pressures and provide mechanical support to the plant tissue. e) If a plant experiences a lack of water the cell becomes plasmolysed, wall pressure is lost and the plant wilts

Water Movement Thru Leaf Some of the light energy absorbed by the large surface area of the leaf is changed to heat. The heat raises the temperature of the leaf and water in the spongy mesophyll tissue is changed into water vapor. There is 100% saturation of the sub stomatal air space SSAS (b) which contrast with the very low % saturation of air with water vapour. With the stomata open, water evaporates into the air The water loss from the leaf draws new water vapor from the spongy mesophyll into the SSAS. In turn, the water molecules of the mesophyll space draw water molecules from the end of the xylem (a). Water is therefore drawn up the stem by cohesion between water molecules and adhesion to the xylem vessel walls. This transpiration 'pull or tension' extends all the way down the xylem to the root

Cohesion Tension Theory There is a gradient of negative pressure potential from the stomatal pore, through the leaf and down the xylem.

Guard Cells and Abscisic Acid Plants have a mechanism which closes the stoma at night (stimulated by the wavelength of light). However when a plant is suffering water stress (lack of water) there is another mechanism to close the stoma. (a) dehydrated (low water potential) of the mesophyll cell causes them to release abscisic acid. (b) Abscisic acid stimulates the stoma to close.conserving water.

Phloem Translocation 1. Translocation moves the organic molecules (sugars, amino acids) from their source through the tube system of the phloem to the sink. Phloem vessels still have cross walls called sieve plates that contain pores. 2. Companion cells actively load sucrose (soluble, not metabolically active) into the phloem. 3. Water follows the high solute in the phloem by osmosis. A positive pressure potential develops moving the mass of phloem sap forward. 4. The sap must cross the sieve plate. 5. The phloem still contains a small amount of cytoplasm along the walls but the organelle content is greatly reduced. 6. Companion cells actively unload (ATP used) the organic molecules 7. Organic molecules are stored (sucrose as starch, insoluble) at the sink. Water is released and recycled in xylem.

Translocation & Transpiration 1. Source produces organic molecules 2. Glucose from photosynthesis produced 3. Glucose converted to sucrose for transport 4. Companion cell actively loads the sucrose 5. Water follows from xylem by osmosis 6. Sap volume and pressure increased to give Mass flow 7. Unload the organic molecules by the companion cell 8. Sucrose stored as the insoluble and unreactive starch 9. Water that is released is picked up by the xylem 10. Water recycles as part of transpiration to re supply the sucrose loading

Food conducting phloem A living tissue with food-conducting cells arranged into tubes that distribute sugar, amino acids, and other organic nutrients throughout the plant. Phloem lack nucleus, ribosomes and a distinct vacuole Phloem cells have a perforated end wall Also have companion cells - nucleated cell with ribosomes that serves sieve-tube cells and connected by plasmodesmata Transports food made in the leaves to the roots and to nonphotosynthetic parts of the shoot system, from source to sink.

Food conducting phloem Proton pumps do the work that enables the cells to accumulate sucrose and minerals. The ATP-driven pumps move H+ concentration across the plasma membrane. Another membrane protein uses this energy source to co-transport sucrose in the cell along with returning hydrogen ions. Bi-directional - transports sugars down in summer for storage and up in spring to provide energy for new leaf growth

Plant Reproduction

Plant Reproduction Structure of an animal-pollinated dicotyledonous plant. a) Sepals cover the flower structure while the flower is developing. In some species these are modified to ' petals'. b) Petals surround the male and female flower parts. Function is to attract animal pollinators. c) Stigma is the surface on which pollen lands and the pollen tube grows down to the ovary. d) Style connects the stigma to the ovary. e) Ovary contains the ovules (contain single egg nuclei). f) Filaments support the anthers that contain the pollen. Together they are called the stamen.

Pollination - the placement of pollen onto the stigma of a carpel (use wind, animals etc.) Double Fertilization one sperm unites with egg to form zygote other sperm combines with the two polar nuclei to form 3N nucleus in the large central cell of the embryo sac which gives rise to the endosperm (food storing tissue) Plant Reproduction

Seed Germination

Seed Structure a) Testa protects the plant embryo and the cotyledon food stores b) Radicle is the embryonic root c) Plumule is the embryonic stem d) Cotyledons contain food store for the seed e) Micropyle is a hole in the testa ( from pollen tube fertilization) through which water can enter the seed prior to germination f) Scar is where the ovule was attached to the carpel wall

Seed Germination Seeds require a combination of oxygen for aerobic respiration water to metabolically activate the cells temperature for optimal function of enzymes for their successful germination. Each seed has its own particular combination of the above three factors. In some species these processes need to be proceeded by other more specialized conditions such as: fire freezing passing through digestive system of a seed dispersing animal washing to remove inhibitors (beans) erosion of the seed coat (Poppy) The particular conditions required by a seed allows it to match germination to favorable conditions

Seed Germination The metabolic events of seed germination: a) Water absorbed and the activation of cotyledon cells b) Synthesis of gibberellin which is a plant growth substance. c) The gibberellin brings about the synthesis of the carbohydrase enzyme amylase d) Starch is hydrolyzed to maltose before being absorbed by the embryonic plant e) The maltose can be further hydrolyzed to glucose for respiration on polymerized to cellulose for cell wall formation.

Plant Hormone Auxin

The role of auxin in phototropism (example of the control of plant growth) Auxins are a class of plant growth hormones (growth regulating factor) The most common auxin is IAA (Indolacetic acid). Auxin is associated with the phenomenon of phototropism - bending-growth towards a source of light.

The role of auxin in phototropism (example of the control of plant growth) auxin produced at the apical meristem transported to a growing area or the zone of cell growth Auxin is laterally transported to cells on the shade side results in cell expansion allowing the shoot to "grow" toward light source

The role of auxin in phototropism (example of the control of plant growth) A tropism is a bending-growth movement either toward or away from a directional stimulus. Phototropism is the bending-growth towards the unilateral source of light. Auxins are a class of plant growth hormones (growth regulating factor) Auxins are one of atleast three major classes of plant growth regulators. Unlike animal hormones plant hormones can provide a range of responses from the tissues. The most common auxin is IAA ( Indolacetic acid). IAA was discovered in 1932 and is believed to be the principle auxin in higher plants. Auxin is associated with the phenomenon of phototropism.

The role of auxin in phototropism (example of the control of plant growth) Charles Darwin studies of auxin effects are published a book called, 'The Power of movement'. Darwin studied phototropism using the germinating stem of the canary grass (Phalaris canariensis). The cylindrical shoot is enclosed in a sheath of cells called the coleoptile.

The role of auxin in phototropism (example of the control of plant growth) Darwin set out to determine which region of the coleoptile is sensitive to light. (a) When there is a unilateral light shinning on one side of a coleoptile there is a bending growth movement towards the light. (b) Decapitation of the tip results in no bending growth suggesting that this region is possibly sensitive to the light stimulus. (c) The opaque cap prevents light from reaching the tip without damaging the tip as in (b). There is no bending-growth response. (d) The buried coleoptile (except tip) show that it is not the lower stem section that is responding to light but rather the tip. Darwin's experiments suggest that the tip is the region sensitive to light. Darwin concluded, " when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend".

The role of auxin in phototropism (example of the control of plant growth) Boysen- Jensen experiments of 1913 showed that the substance traveling down the coleoptile stem was of a chemical nature. (e) The mica plate is an un-reactive substance that is inserted on one side of the stem. With the mica in place and the unilateral distribution of light there is no bending-growth. This suggests that the growth promoting substance is prevented from moving down the shaded side of the stem. (f) The mica placed on the exposed side does not prevent bending-growth. In combination with the previous observation this suggests the growth promoting substance is chemical and moving down the shaded side.

The role of auxin in phototropism (example of the control of plant growth) (g) The coleoptile is decapitated (h) A gelatine block permeable to chemical diffusion is placed between the stem and the root tip. (i) The reconstructed coleoptile still shows the bending-growth response with the unilateral distribution of light. Again these Boysen-Jensen experiments add confirmation that the growth promoting substance is chemical in nature.

The role of auxin in phototropism (example of the control of plant growth) The experiments of Paal (1919) confirm the work of Boysen-Jensen. (j) Decapitation of the coleoptile tip. (k) Replacement of the coleoptile tip but asymmetrically over one side of the coleoptile stem. (l) With NO LIGHT, there is a bendinggrowth, with the overlapped side experiencing the growth. Paal also suggested that in the dark or light from all sides the growth promoting substance was sent uniformly down the stem.

The role of auxin in phototropism (example of the control of plant growth) Auxin, the growth promoting substance, was first isolated by F. W. Went in 1926. The actual structure shown above for auxin was not determined until 1932. (m) Went's experiments extended the work of Paal, Boysen and Jensen by isolating the auxin onto agar gel. (n) The gel was cut up into block as a way of quantifying the dose of auxin used. (o) The agar block (containing auxin) are placed asymmetrically on the stem. (p) The angle of bending-growth was measured.

The role of auxin in phototropism (example of the control of plant growth) Since Auxin (IAA3) was synthetically produced more rigorous quantitative bio-assay can be performed. This graph measures the bending-growth against the concentration of IAA3. Note that the graph suggests: Increasing IAA3 increases the bendinggrowth angle. Optimal angle of bending-growth is achieved between 0.2-0.25 mg Higher levels seem to have reducedbending growth.

The role of auxin in phototropism (example of the control of plant growth) Transport of Auxin: Auxin is transported through the cell membrane of the adjacent plant cells by protein carriers in the plasma membrane. These carriers transport the anion of auxin in polar direction, from the top of the cell to the bottom of the cell. However the stimulus of light would seem to result in the introduction of these carriers into the side of the cell membranes so that the IAA3 can now be laterally transported.

The role of auxin in phototropism (example of the control of plant growth) The role of auxin: Since IAA3 is a 'hormone' there must be some link between the signal molecule and the sub cellular responses and the cellular responses. It appears that it is the receiving cell that determines the exact cellular response rather than IAA3 having very specific responses across all cells. As we have noted one of the major functions of auxins is the promotion of growth. Research has shown that in some tissues IAA3 promotes mitosis whilst in other tissue, it promotes cell enlargement.

Cell elongation in response to auxin: the acid growth hypothesis

Flowering

Phytochrome and the Control of Flowering Flowering Cues: Plant have to coordinate the production of flowers to coincide with the best reproductive opportunities. Flowering depends on day length Length of night is significant (not day length) and there must be an unbroken period of night There is a critical night length or minimum length of night controls the flowering Short and Long day Plants: Short day plants (SDP) typically flower in the spring or autumn when the length of day is short. Long day plants (LDP) typically flower during the summer months of longer photoperiod. Some plants are day neutral (ex. Tomato)

Phytochrome and the Control of Flowering Phytochrome System: The receptor of photoperiod is located within the leaf. The chemical nature of the receptor is a the molecule PHYTOCHROME. Phytochrome exists in two interconvertible forms Interconversion of phytochrome pigment can measure daylength Red variety (Pr) of phytochrome is inactive form Far red (Pfr) is active form Under daylight condition red is converted into far red variation Far red slowly converts back to red in the dark (at night)

Phytochrome and the Control of Flowering Flowering in SDP: Short day plants flower when the night period is long. In day light or red light, Phytochrome Red (Pr) is converted to Phytochrome Far Red (Pfr). The conversion actually only requires a brief exposure to white or red light. In the dark, Pfr is slowly converted back to Pr. A long night means that there is a long time for the conversion. Under short day conditions (long night) at the end of the night period the concentration of Pfr is low. In SDP, low Pfr concentration is the trigger for flowering.

Phytochrome and the Control of Flowering Flowering in LDP: Long day plants flower when the night period is short. In day light (white or red) the Pr is converted to Pfr. During periods when the day light period is long but critically the dark period is short, Pfr does not have long to breakdown in the dark. Consequently there remains a higher concentration of Pfr. In LDP, high Pfr concentration is the trigger to flowering.

Photoperiodic control of flowering

Reversible effects of red and far-red light on photoperiodic response

Phytochrome and the Control of Flowering