Plant Physiology Notes

Plant Physiology

  • Transport of Substances in Plants
  • Symbiosis of Plants with Fungi and Bacteria
  • Nutrition of Plants Without Chlorophyll
  • Carnivorous Plants
  • Plant Hormones
  • Secondary Compounds
  • Light and Plant Growth and Development

Transport of Substances in Plants

  • Transpiration Stream:
    • Transport of water and ions from the soil through the roots to the tips and leaves. This relies on the evaporation of water from the leaves, known as transpiration.
    • Occurs in the xylem (tracheids and vessels).
  • Assimilate Stream:
    • Organic substances produced during photosynthesis are transported in the opposite direction, from the leaves to other parts of the plant.
    • Occurs in the phloem (sieve tubes).
  • In developing fruits, buds, and growing tips, both streams flow in the same direction.

Xylem and Phloem Transport

  • Xylem Flow: Towards the apex of the plant. Primarily transports water (H2OH_2O).
  • Phloem Flow: Transports assimilates (S). Includes assimilate flow, originating from the source (leaves) and moving towards the sink (areas of consumption).
  • The flow involves water transport and assimilate movement. Water is taken up by the roots from the soil, and assimilates are consumed.

Assimilate Flow

  • Assimilate Sap:
    • Consists of sugars (mainly sucrose, the transport sugar), amino acids, and hormones.
    • Moves through the phloem, specifically sieve tubes with companion cells. Companion cells actively pump assimilates from leaf cells into the sieve tubes.
    • High concentration of substances in the phloem of leaf veins creates pressure, causing water to enter from adjacent tissues.
    • At the site of assimilate consumption, the concentration is lower, driving the flow in that direction.
    • The assimilate stream also flows upwards towards growing tips and fruits alongside the transpiration stream.

Transpiration Stream

  • Transpiration:
    • The release of water vapor through the leaf surface, primarily through leaf stomata (regulated by guard cells and subsidiary cells).
    • Driven by the difference in water potential between the roots and the surrounding air.
    • This generates negative pressure in the leaf veins. The water column rises due to capillarity and cohesion, maintaining an unbroken stream.

Transpirational Pull

  • Continuous evaporation of water through stomata reduces the water potential in the leaves compared to cells further away from the evaporation site.
  • The resulting gradient of water potential draws water through the plant.
  • Cohesion and Adhesion: Water molecules adhere to each other (cohesion) and to the inner surface of the xylem (adhesion), creating a continuous water column.

Transport in Leaves and Stems

  • Xylem Sap: Contains inorganic substances and organic compounds (sugars stored in roots and tubers), hormones, and other substances.
  • Transport occurs through xylem vessels.
  • The text gives the example of maple sap being suitable for making sweet drinks.

Transport in Roots

  • Facilitated by the transpiration stream and root pressure.
  • The plant absorbs water and minerals through root hairs via osmosis, moving from areas of higher to lower water potential.
  • Active transport into the root xylem generates root pressure, pushing water upwards.
  • Guttation: Occurs when water is exuded through leaf tips at 100% air humidity due to lack of transpiration (e.g., "weeping" in pruned trees during springtime).

Root Structure

  • Root Cap: Protects the root tip.
  • Root Hairs: Increase surface area for water and nutrient absorption.
  • Epidermis: Outermost layer.
  • Cortex: Ground tissue inside the epidermis.
  • Endodermis: Innermost layer of the cortex, containing the Casparian strip.
  • Casparian Strip: Impermeable to water, ensuring that water and nutrients pass through the cell membranes for controlled uptake.
  • Xylem: Vascular tissue that transports water and minerals.
  • Phloem: Vascular tissue that transports sugars and other organic compounds.

Leaf Structure

  • Upper Epidermis: The top layer of the leaf.
  • Cuticle: Waxy covering on the epidermis that prevents water loss.
  • Palisade Parenchyma Cells: Elongated cells containing chloroplasts, located below the upper epidermis and are the primary site of photosynthesis.
  • Spongy Parenchyma Cells: Irregularly shaped cells with air spaces, located below the palisade parenchyma, which facilitate gas exchange.
  • Lower Epidermis: The bottom layer of the leaf, containing stomata.
  • Stomata: Pores on the leaf surface that allow for gas exchange and transpiration. They have guard cells to regulate opening and closing
  • Vein: Contains the xylem and phloem, which transport water, nutrients, and sugars.

Stomata

  • Guard cells are located in the epidermis and contain chloroplasts.
  • They have thicker cell walls towards the pore and thinner walls towards the subsidiary cells.
  • When water moves out of the guard cells into the subsidiary cells, the pore closes, and vice versa.

Factors Influencing Stomatal Opening and Closing

  • Environmental Conditions: Light and water availability.
  • Photosynthesis: Stomata open to allow CO2 to enter.
  • Potassium Ions (K+):
    • K+ ions enter guard cells, increasing the solute concentration.
    • Water follows by osmosis, making the guard cells turgid (swollen), which opens the stomatal pore.
  • Abscisic Acid:
    • Increases in leaves under water stress. It inhibits ion transport across membranes, preventing water from entering guard cells, causing the stomata to close.

Plant Nutrition

  • Photosynthesis: The primary mode of nutrition.
  • Symbiosis:
    • Mycorrhizae (Ectomycorrhizae and Endomycorrhizae)
    • Arbuscular Mycorrhizae
    • Root Nodule Symbiosis (with bacteria).
  • Parasitism
  • Carnivorous Plants

Symbiosis with Fungi and Bacteria

  • Mycorrhizae:
    • Increase the surface area for water and mineral uptake (90% of tree species benefit).
    • Endomycorrhizae: Fungal hyphae enter root cells.
    • Arbuscular Mycorrhizae: Hyphae form shrub-like structures (arbuscules) inside the cells, increasing phosphorus and nitrogen supply.
    • Herbicides can negatively affect mycorrhizal associations.
    • Orchids often rely on mycorrhizae.

Root Nodules

  • Legumes (e.g., Clover):
    • Roots contain nodules housing nitrogen-fixing bacteria.
    • Bacteria convert atmospheric nitrogen into amino acids, enriching the soil.
    • Nodules are formed due to plant growth hormones produced by bacteria.

Ectomycorrhizae

  • Hyphae develop around root tips.

Lichens

  • A symbiotic association between green algae/cyanobacteria and fungi.

Plants Without Chlorophyll

  • Parasitic Plants:
    • Obtain sugars and minerals by parasitizing fungi or tree roots.

Carnivorous Plants

  • Adaptations to nutrient-poor, acidic soils deficient in phosphorus and nitrogen.

Secondary Compounds

  • Produced during secondary metabolism.
  • Accumulate in parts that the plant can shed (e.g., falling leaves, cell walls, vacuoles, resin ducts).
  • Important for both physiological and ecological functions.
  • Types:
    • Terpenoids
    • Phenolic Compounds
    • Alkaloids

Terpenoids

  • Examples: Essential oils, sterols, rubber.
  • Functions:
    • Attracting insects for pollination.
    • Repelling herbivores through strong odors.
    • Preventing germination of other plants in their vicinity.
  • Sterols:
    • Glycosides (e.g., digoxin from Digitalis) are used to treat heart conditions.
  • Rubber:
    • Latex from rubber trees.

Phenolic Compounds

  • Examples: Plant pigments (anthocyanins (blue, red) and flavonols (yellow)), tannins, lignin.
  • Functions:
    • Attracting pollinators.
    • Preventing bacterial and fungal infections.
    • Providing structural support to cells.

Alkaloids

  • Examples: Nicotine, caffeine, morphine, cocaine, scopolamine.
  • Functions:
    • Toxic substances that deter animals from feeding.
  • Humans utilize many secondary compounds in the pharmaceutical, processing, and food industries.
  • Such as scopolamine is used to treat travel sickness.

Plant Growth and Development

  • Dependent on genetic information. Cell differentiation is enabled by gene expression.
  • Triggers for Gene Expression:
    • Light
    • Temperature
    • Plant Hormones

Plant Hormones (Phytohormones)

  • Organic compounds released by plants in small concentrations.
  • Transported from their site of production to other plant parts and bind to receptor (target) cells, inducing physiological changes.
  • Extremely low concentrations are effective (e.g., 10,00010,000 barley coleoptile tips yield 10−610^{-6} g of indoleacetic acid).
  • Types :
    • Auxins
    • Gibberellins
    • Cytokinins
    • Abscisic Acid
    • Ethylene

Auxins

  • Most Common: Indoleacetic acid (IAA).
  • Produced in the growing tip and transported downwards towards the roots. Also produced in embryos in seeds and pollen grains.
  • Functions:
    • Promotes cell elongation and root formation.
    • Inhibits growth of lateral buds and abscission of leaves and fruits.

Darwin's Experiment

  • Demonstrates the influence of auxins on phototropism (growth towards or away from light).
  • Auxin redistribution causes a terminal bud to point toward the sun.

Went's Experiment

  • With agar blocks and cut coleoptile tips shows that auxin promotes cell elongation.
  • The side of the plant where the agar block is placed grows more.

Root Development Under Auxin Influence

  • The shoot tip inhibits the growth of lateral buds. Removing the shoot tip promotes the development of lateral buds.
  • Used in propagation to promote root formation in cuttings.

Cytokinins

  • Produced in root tips, embryos, seeds, and developing fruits.
  • Functions:
    • Promote cell division and growth.
    • Promote metabolism (accelerated assimilate flow).
    • Inhibit senescence.
  • Essential in biotechnology for tissue culture.

Plant Growth Based on Auxin and Cytokinin

  • Biotechnological applications: Artificial media with cytokinins and auxins for tissue cultures.

Micropropagation

  • Plant propagation using tissue culture techniques.
  • Individual cells, buds, roots, shoot tips, meristematic tissue, and embryos can be grown.
  • Foreign genes can be inserted into protoplasts (cells without cell walls) to create transgenic plants.
  • Nutrient media is sterile and contains vitamins, minerals, water, and hormones.

Gibberellins

  • Promote cell elongation in stems and leaves. Deficiency leads to dwarfism.
  • Functions:
    • Stimulates seed germination. Gibberellins produced by the germinating seed trigger enzyme production to break down starch.
    • Stem elongation
    • Flowering and fruit development
    • Root growth and differentiation

Abscisic Acid (ABA)

  • An inhibitor of growth and development.
  • Produced in leaves, shoots, fruits, seeds, and root tips.
  • Functions:
    • Inhibits germination, maintains seed dormancy.
    • Promotes senescence and abscission of leaves and fruits.
    • Plays a key role in stomatal closure under water stress.

Ethylene

  • A gaseous plant hormone primarily released by ripening fruits, senescing leaves, and flowers.
  • Functions:
    • Promotes abscission of leaves and fruits.
    • Inhibits longitudinal growth.
    • Promotes fruit ripening.
    • Inhibits lateral bud growth and promotes adventitious root growth (together with auxins).
  • CO2 inhibits ethylene production.

Influence of Light on Plant Growth

  • Light perception via phytochromes in leaf cells.
  • Phytochromes act as hormones, binding to specific receptor cells and influencing physiological processes.
  • Two Forms:
    • Sensitive to short-wave red light (F660F_{660}
    • Sensitive to long-wave far-red light (F730F_{730}) (physiologically active form).

Day and Night Light

  • Daylight (660 nm) promotes the formation of active phytochrome.
  • Night Light (730 nm) promotes the formation of the inactive phytochrome (F660F_{660}

Photoperiodism

  • Influence of day length on the formation of specific plant organs. Affects flowering, stem growth, and bud development.
  • Plant Types
    • Short-day plants (spring, autumn)
    • Long-day plants (summer)
    • Day-neutral plants

Influence of Temperature on Plant Growth and Development

  • Annuals remain as dormant seeds.
  • Deciduous trees - dormant buds.
  • Biennials flower in the second year.
  • Vernalization: The effect of cold on flowering. Activates enzymes that promote growth (gibberellins) and reduces abscisic acid levels.
  • Dormancy: A state of deep rest when metabolic processes are minimized.

Physiology of Movement

  • Tropisms: Slow movement of organs due to uneven growth.
    • Hydrotropism(response to water)
    • Gravitotropism(response to gravity)
    • Phototropism(response to light)
      • Plants detect gravity via cells in the root cap.

Nastic Movements

  • Rapid movements due to changes in osmotic potential in specialized cells (turgor changes).
  • Example: Mimosa pudica (sensitive plant).

Endogenous Movement

  • Movements of plant organs not triggered by external stimuli but by internal mechanisms (physiological clock).
  • Example: Leaf movement in beans (upwards during the day, downwards at night).

Taxism

  • Movement of an organism towards or away from a stimulus (e.g. Euglena and phototaxis).