9 - Plant Biology

Leaf Structure

cuticle, xylem, phloem, upper and lower epidermis, palisade mesophyll cells, spongy mesophyll, vascular bundle, guard cells and stomata, chloroplast.

Plant Structure

stem, leaf, bud, reproductive structure (flower or spore) and root system.

Stomata

the pores of a leaf. For gas exchange the stomata need to be open, thus transpiration is affected by the level of photosynthesis and is a consequence of gas exchange.

Transpiration

the loss of water vapor from leaves and stems. Light converts water within leaves into vapor which evaporates via stomata. New water is absorbed from the roots and soil, creating a pressure difference between the leaves - low and roots - high. Water flows through the xylem along the pressure gradient to replace lost water, transpiration stream process.

Capillary action

The up movement of liquids through slim tube, cylinder or permeable substance due to adhesive and cohesive properties of water.

water properties

Cohesion, water is attracted to water because water molecules are polar meaning they form hydrogen bonds. Cohesion lets water to move up through xylem towards leaves in a continuous stream.

Adhesion, water is attracted to other substances. Xylem wall is polar and forms intermolecular bonds with water, as they move up the xylem via capillary action, they pull inward on the xylem walls to create further tension.

Xylem Structure

Xylem is a tube consisting of dead hollow cells with no protoplasm allowing for free movement of water, as the cells are dead water movement is a passive process in one direction only. Cell walls contain many pores/pits that allow water transfer between cells. Cell walls reinforced by lignin to provide strength as water in transported under tension. Xylems consist of tracheids which are present in all vascular plants and vessel elements which are only found in some. Tracheids are tapered cells that exchange water through pits thus slow water transfer. Vessel elements have fused end walls that form a continuous tube and have faster water transfer.

Xylem

the vascular plant tissue that transports water and minerals from the roots to the entire plant. Water can move up due to adhesion, able to attach to polar molecules, and cohesion, water sticks to itself. Xylem tissue is dead. XYLEM TRANSPORT WATER UP

Evaporation

Water loss from leaves and conversion into vapor, which then diffuses through stomata.

Some light absorbed by the leaves becomes heat energy and evaporates water within the spongy mesophyll. Then, vapor diffuses through stomata and this creates a negative pressure gradient within the leaf. A transpiration pull or tension force is created in the leaf cell walls which draws water from xylem under tension due to adhesive attraction between water and cell walls.

Transpiration rate

The rate at which water vapor is lost from leaves regulated by opening and closing of stomata.

Turgor

Pressure exerted by fluid in a cell that presses the cell membrane against the cell wall.

Regulating transpiration rates

The transpiration rate is regulated by opening and closing of stomata

Guard cells flank the stomata and can close the opening by becoming more flaccid in response to cellular signals.

When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid, ABA.

ABA triggers the output flow of potassium from guard cells and decreases water pressure within the cells, loses turgor.

A loss of turgor makes stomatal pores close, as guard cells become flaccid and block the opening.

Transpiration rates are higher when stomatal pores are open. Pores are responsible for gas exchange in leaves hence photosynthesis affects transpiration, as well as, humidity, temperature, light intensity and wind.

Transpiration stream

The movement of water up through the xylem from the roots to the leaves against gravity. Water rises through xylem vessels due to cohesion and adhesion.

Phloem

the vascular plant tissue that transports water and soluble organic nutrients, namely sucrose from photosynthesis or source to the sink. Phloem consists of; sieve tube elements and companion cells, and its tissue contains living cells.

PHLOEM TRANSPORTS UP AND DOWN.

Phloem structure

Sieve element cells are long narrow cells, connected by porous sieve plates at transverse end to form the sieve tube where organics can flow. Sieve elements have thick rigid cell walls to withstand hydrostatic pressures to facilitate flow.

Companion cells metabolically support the sieve element cells by facilitating the loading and unloading of materials at sources and sinks. Possess infolding plasma membranes to increase SA:Vol ratios for more material exchanges. Many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink. Contain certain transport proteins within the plasma membrane to move materials in or out the sieve tube.

Sieve elements cannot sustain metabolic activity without companion cells, as they have no nuclei and less organelles to maximize flow rate. Many plasmodesmata exist between sieve elements and companion cells, connecting to the cytoplasm of the two cells to mediate the symplastic exchange of metabolites. Phloem also contains schlerenchymal and parenchymal cells.

Water Conservation in Xerophytes

Xerophytes are plants that tolerate dry conditions like deserts and have adapted to conserve water. They have high rates of transpiration due to high temperatures and low humidity in deserts. They have fewer leaves which are also smaller reducing the SA available for water loss; rolled leaves mean less exposure of stomata to the air reducing evaporative water loss; thick and waxy cuticles prevent water loss from the leaf surface; the pits have stomata surrounded by hairs which trap water vapor reducing transpiration, the plants grow low to the ground and thus are exposed to less wind and are more shaded preventing water loss, and CAM physiology allows for plants to open stomate at night.

Water conservation in Halophytes

Halophytes are plants that tolerate salty conditions like swamps and have adapted to conserve water. They lose water due to a high intake of salt from soils and water is drawn from plant tissue via osmosis. They isolate toxic ions and salts within cell walls or vacuoles, salts are concentrated in particular leaves which drop off, roots are structured to exclude 95% of salt, some plant parts possess salt glands and actively eliminate salt, and halophytes can flower only at specific times to minimize salt exposure.

Plant Experiments and Models of Water Transport

The movement of water up the xylem is modeled via capillary tubing, filter or blotting paper, and porous pots, and the transpiration rate is estimated via potometer. To estimate measure the distance moved by an air bubble every minute.

Translocation

the movement of organics like sugars from sources, where the organics are synthesized so leaves, to sinks, where the organics are stored so roots, fruits, and seeds, via phloem, a vascular tube system. Sugars are mainly transported as sucrose as it is soluble but metabolically inert, in the form of plant sap a viscous nutrient-rich fluid. The rate of translocation or phloem transport is affected by dissolved sugar concentrations, thus affected by the rate of photosynthesis, rate of cellular respiration, rate of transpiration, and diameter of the sieve tubes.

Identification of Xylem and Phloem in microscopic images of stem and root

In monocot roots: Xylem vessels are larger and closer to the center pith, phloem is smaller and externally located. In dicot roots: xylem is located near the center and phloem surrounds it, xylem may form x shape where the phloem is located in the gaps.

In monocot stems: Phloem is positioned externally. In dicot stems: vascular bundles arranged in a circle around the pith, and phloem and xylem is separated by cambium.

Phloem Loading

Organics from source are actively loaded into phloem sieve tubes via companion cells. Materials pass into sieve tube via interconnecting plasmodesmata or symplastic loading, or materials are pumped across the intervening cell wall by membrane proteins or apoplastic loading.

Apoplastic loading of sucrose into phloem sieve tubes uses active transport and ATP. H+ is transported out of phloem cells by proton pumps and ATP hydrolysis. H+ concentration builds up outside of the cell hence proton gradient. H+ passively diffuses back into phloem cell via co-transport protein which needs sucrose movement, resulting in sucrose buildup within phloem sieve tube for transport from the source.

Mass flow at the source

High concentrations of solutes in the phloem at the source lead to water uptake via osmosis, and incompressibility of water allows for transport along hydrostatic pressure gradients.

Solutes are actively transported into the phloem by companion cells making the sap solution hypertonic, have higher osmotic pressure than surroundings. Hence water is drawn from xylem via osmosis and water moves towards higher solute concentrations. As water is incompressible it builds up within phloem and hydrostatic pressure increases forcing the phloem sap to move towards areas of lower pressure - Mass flow. Overall, phloem transports solutes away from source towards sink.

Mass flow at the sink

Raised hydrostatic pressure causes phloem contents to flow towards sinks.

Solutes within phloem are unloaded by companion cells and transported into sinks causing the sap solution at the sink to become hypotonic, have lower solute concentration. Thus, water is drawn from phloem and back into xylem via osmosis ensuring that hydrostatic pressure at the sink is always lower than at the source, so sap always moves from source to sink, where organics are then metabolized or stored within vacuole tonoplasts.

Aphids and measuring phloem translocation rates

Aphids are insects which feed on phloem sap by using a protruding mouthpiece, stylet, They penetrate the sieve tube, and use to digestive enzymes to soften the intervening tissue layer and extract the sap. If stylet is broken, sap continues to flow due to hydrostatic pressure within sieve tube.

Aphids are used to measure phloem transport rates by collecting sap at various parts of a plant. Plant is grown in a chamber with radioactive CO2, the leaves convert it into radioactive sugars and they’re moved by phloem. Aphids are placed on different parts of the phloem and feed on the sap, then they’re stylet is sever and sap continues to flow. Sap is analyzed for radioactive sugars, and rate of phloem transport is calculated based on the time taken for the isotope to be detected at different parts of the plants.

DISTANCE FROM START / TIME FOR RADIOACTIVE TO TRAVEL

Meristems

Meristems are plant tissues consisting of undifferentiated cells which allow allow indeterminate growth. They are the totipotent stem cells counterpart of plants, but have specific parts where they grow and develop. Meristematic tissue allows plants to regrow or form new plants via vegetative propagation.

Apical meristems responsible for primary growth, lengthening of the plant, are found at the shoot and root tips, give rise to new leaves and flowers.

Lateral meristems responsible for secondary growth, thickening of the plant, are found at the cambium, produce bark.

Apical growth

Apical meristems cause primary growth, lengthening, at the shoot apex via cell enlargement and cell division, mitosis and cytokinesis. Differentiation of the dividing meristem causes a variety of stem tissues and structures. In the stem, growth occurs in nodes or sections, with remaining meristem tissue forming inactive axillary buds that have the potential to form new branching shoots.

Plant hormones and apical growth

Stem growth and node formation is controlled by plant hormones called auxins which are released from the shoot apex. Auxins promote growth in the shoot apex via cell elongation and division. Auxin production causes apical dominance, the prevention of growth in axillary buds ensuring that plants use energy to grow up towards light to outcompete other plants. As distance between terminal and axillary buds increases, the inhibition of axillary buds by auxin decreases.

Auxin

Auxins are a group of hormones produced by the shoot tip or root that regulate plant growth. Auxin efflux pumps create concentration gradients within tissues and change auxin distribution within plants, the pumps control direction of growth by finding which parts of tissue have high auxin levels. The pumps may change position within the membrane due to fluidity and be activated by various factors.

In the shoots auxin stimulates cell elongation and high auxin concentrations promote growth making the cells larger. In the roots auxin inhibits cell elongation and high auxin concentrations limit growth making cells smaller.

Auxin influences gene expression

Auxin influence cell growth rates by changing gene expression patterns. Auxin’s mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue.

In shoots: Auxin increases cell wall flexibility to promote plant growth via cell elongation. Auxin activates a proton pump in the plasma membrane causing H+ ion secretion into the cell wall. pH decreases and cellulose fibers in cell wall loosen, bonds between them break. Auxin increases the expression of expansins also increasing the elasticity, With flexible cell walls and influx of water the cells increase in size.

Tropisms

Tropisms describe the growth or turning movement of a plant in response to directional external stimulus. Phototropism is a growth movement in response to a unidirectional light source, light receptors trigger auxin redistribution from dark side of the plant. Geotropism is a growth movement in response to gravitational forces, auxin accumulates on the lower side in response to gravity.

In shoots high auxin concentrations promote cell elongation, so dark side of the shoot elongates and grows towards light, whereas the lower side of the shoot elongates and roots grow away from the ground.

In roots high auxin concentrations inhibit cell elongation, so dark side of the root becomes shorter and roots grow away from the light, the lower side of the root becomes shorter and roots turn downwards into the ground.

Micropropagation

Micropropagation is a method used to produce a lot of identical plants from one parent plant. Plants reproduce asexually from meristems. A plant tissues are cultured in a lab to reproduce asexually.

Plant tissue, namely undifferentiated shoot apex are taken from a parent plant and sterilized. The sample is grown on sterile nutrient agar gel, treated with growth hormones like auxins to simulate shoot and root development, the growing shoots can be continuously divided and separated to form new samples, and once root and shoot develop the cloned plant is transferred.

Micropropagation uses

Rapid bulking - parent plants are cloned via micropropagation to produce large quantities of plants created via genetic modification.

Virus-free strains - plant viruses can destroy crops, cripple economies and cause famine. Viruses spread through infected plants via vascular tissue that meristems do not have, so propagating plants from non-infected meristems allow for reproduction of virus-free plant strains.

Rare species propagation - increase the amount of rare or endangered plant species, or species that are difficult to bread sexually.

Plant reproducition

Plants can reproduce though: vegetative propagation, asexual reproduction from a plant cutting. Spore formation such as mold. Pollen transfer, flowering plants or angiospermophytes.

In sexual reproduction of flowering plants male gametes in the form of pollen are transferred to female gametes - ova, through pollination, fertilization and seed dispersal.

Phases of plant reproduction

Pollination is first, pollen grains from an anther - the male plant structure are transferred to a stigma - the female plant structure, some plans however posses both structures and self-pollinate.

Then, fertilization where male gamete nuclei fuses with a female gamete nucleis to form a zygote.

Lastly, seed dispersal, where the seed leaves the parent plant either by wind, water, fruit, or animals. Hence reducing competition for resources between the germinating seed and parent plant.

Pollinators and Flowering Plants

Cross-pollination involves transferring pollen grains from one plant to the ovule of another. Pollinators, like bees, can transfer pollen and both species benefit as it is a mutualistic relationship. The plant can sexually reproduce and the animal can eat the sugar-rich nectar.

Flowering

Flowering refers to the development of flowers from the shoot apex, flowers are reproductive organs of angiospermophytes.

Abiotic factors such as suitable pollinators or photoperiodism cause changes the gene expression of the shoot apical meristem, it becomes bigger and the tissue differentiates to form flower structures - sepals, petals, stamen and pistil.

Flower Structure

Flowers are reproductive organs of angiospermophytes that contain male and female structures, monoecious flowers have both, dioecious only one.

Male part or Stamen consists of: anthers which produce pollen or the male gametes, filaments which are stalks supporting the anther and make it more accessible to pollinators.

The female part or Pistil consists of: stigma which are sticky receptive tips that catch pollen, style which connects the stigma and ovule, ovule which contains the female reproductive cells and develops into a seed after fertilization

Extra parts include petals, sepal which are leaves, and peduncle the stem.

Photoperiodism

Photoperiodism is a plant’s response to light and darkness where its detected by phytochromes.

Phytochromes exist as the active form Pfr that breaks into the inactive form Pr when it absorbs far red light ~725 nm, inactive breaks into active when absorbing red light ~600nm. More active form during day, more inactive during night.

In short day plants Pfr inhibits flowering and needs low Pfr gotten from long nights, long day plants need short nights.

Seed Structure

Testa is the outer seed coat protecting embryonic plant, micropyle is a small pore outside the seed for water, cotyledon stores food and forms embryonic leaves, plumule or epicotyl is the embryonic shoot, and radicle is embryonic root.

Germination

Germination is the sprouting of a seed after a period of dormancy, requires oxygen, water, and suitable pH.

Germination Additional Conditions

Fire – some seeds will only sprout after exposure to intense heat (e.g. after bushfires remove established flora)

Freezing – some seeds will only sprout after periods of intense cold (e.g. in spring, following the winter snows)

Digestion – some seeds require prior animal digestion to erode the seed coat before the seed will sprout

Washing – some seeds may be covered with inhibitors and will only sprout after being washed to remove the inhibitors

Scarification – seeds are more likely to germinate if the seed coat is weakened from physical damage