Plant Physiology: Chemical Coordination in Plants

Plants, unlike animals, do not possess a nervous system for coordination. Instead, they rely on a sophisticated system of chemical coordination through specialized organic compounds called plant hormones or phytohormones. These substances are produced in very small quantities in one part of the plant and transported to other parts, where they exert specific physiological effects, regulating growth, development, and responses to environmental stimuli.

1. Plant Growth Regulators (Phytohormones)

Plant growth regulators (PGRs) are chemical messengers that influence various aspects of plant life, from germination to senescence. They can be broadly classified into growth promoters and growth inhibitors.

1.1. Auxins

Auxins are a group of plant hormones primarily involved in cell elongation and differentiation. The most common natural auxin is Indole-3-acetic acid (IAA).

• Physiological Effects:
  • Cell Elongation: Promote the elongation of cells, particularly in shoots, leading to stem growth. This is the basis of phototropism and geotropism.
  • Apical Dominance: The apical bud (tip of the shoot) produces auxins that inhibit the growth of lateral (axillary) buds, leading to a single main stem. Removal of the apical bud (decapitation) promotes lateral bud growth.
  • Root Initiation: Promote the formation of adventitious roots on stem cuttings, making them useful in horticulture for vegetative propagation.
  • Fruit Development: Promote fruit growth and prevent premature fruit drop. Synthetic auxins are used to induce parthenocarpy (fruit development without fertilization).
  • Differentiation of Xylem and Phloem: Influence the differentiation of vascular tissues.
  • Herbicide Action: High concentrations of synthetic auxins (e.g., 2,4-D) act as selective herbicides, killing broadleaf weeds but not grasses.

1.2. Gibberellins (GAs)

Gibberellins are a large group of plant hormones, with Gibberellic acid (GA₃) being the most studied. They are primarily known for promoting stem elongation.

• Physiological Effects:
  • Stem Elongation: Cause significant increase in stem length, especially in dwarf varieties, by promoting both cell elongation and cell division.
  • Seed Germination: Break seed dormancy and promote germination, often by stimulating the synthesis of enzymes (like α-amylase) that mobilize stored food reserves.
  • Flowering: Can induce flowering in some long-day plants even under short-day conditions.
  • Fruit Development: Promote fruit growth, particularly in grapes, leading to increased size.
  • Bolting: Promote bolting (stem elongation just before flowering) in rosette plants.

1.3. Cytokinins

Cytokinins are plant hormones primarily involved in cell division (cytokinesis) and differentiation. Zeatin is a natural cytokinin found in corn kernels and coconut milk.

• Physiological Effects:
  • Cell Division: Promote cell division in the presence of auxins, crucial for tissue culture and organogenesis.
  • Delay Senescence (Aging): Delay the aging and yellowing of leaves by promoting nutrient mobilization.
  • Break Apical Dominance: Promote the growth of lateral buds, often acting antagonistically to auxins.
  • Chloroplast Development: Promote the development of chloroplasts in leaves.
  • Nutrient Mobilization: Help in the movement of nutrients to areas where they are needed.

1.4. Abscisic Acid (ABA)

Abscisic Acid (ABA) is a plant hormone often referred to as a stress hormone because it plays a crucial role in plant responses to environmental stresses.

• Physiological Effects:
  • Stomatal Closure: Promotes the closure of stomata during water stress, reducing water loss through transpiration.
  • Seed Dormancy: Induces and maintains seed dormancy, preventing premature germination under unfavorable conditions.
  • Bud Dormancy: Promotes dormancy in buds, especially in perennial plants during winter.
  • Inhibits Growth: Generally acts as a growth inhibitor, counteracting the effects of growth-promoting hormones.
  • Abscission: Promotes abscission (shedding) of leaves, fruits, and flowers, especially under stress conditions.

1.5. Ethylene

Ethylene is a gaseous plant hormone, unique among PGRs. It is primarily involved in fruit ripening and senescence.

• Physiological Effects:
  • Fruit Ripening: Promotes the ripening of climacteric fruits (fruits that continue to ripen after harvesting, e.g., bananas, apples, tomatoes) by stimulating the production of enzymes that soften the fruit and change its color and flavor.
  • Senescence and Abscission: Accelerates the aging of leaves and flowers and promotes their shedding.
  • Triple Response in Seedlings: In dicot seedlings, it causes inhibition of stem elongation, increased radial swelling of the stem, and horizontal growth of the hypocotyl, helping the seedling navigate obstacles in the soil.
  • Flowering: Can induce flowering in some plants (e.g., pineapple).

2. Tropic Movements in Plants

Tropic movements (or tropisms) are directional growth responses of a plant organ towards or away from a specific external stimulus. The direction of growth is determined by the direction of the stimulus. These movements are regulated by plant hormones, particularly auxins, which cause differential growth.

2.1. Phototropism

Phototropism is the growth movement of a plant organ in response to a light stimulus.

• Mechanism: When light falls unevenly on a plant shoot, auxins migrate to the shaded side. Higher concentrations of auxins on the shaded side promote greater cell elongation there, causing the shoot to bend towards the light.
• Types:
  • Positive Phototropism: Growth towards light (e.g., shoots, leaves).
  • Negative Phototropism: Growth away from light (e.g., roots, though less common and often weak).
• Example: A sunflower turning its head to follow the sun; a houseplant bending towards a window.

2.2. Geotropism (Gravitropism)

Geotropism (or gravitropism) is the growth movement of a plant organ in response to gravity.

• Mechanism: Gravity causes auxins to accumulate on the lower side of horizontally placed roots and shoots. However, roots are more sensitive to auxin than shoots. In shoots, higher auxin concentration on the lower side promotes growth, causing the shoot to grow upwards. In roots, the higher auxin concentration on the lower side inhibits growth, causing the root to grow downwards.
• Types:
  • Positive Geotropism: Growth towards gravity (e.g., roots).
  • Negative Geotropism: Growth away from gravity (e.g., shoots).
• Example: A germinating seed's radicle (root) growing downwards into the soil, and its plumule (shoot) growing upwards.

2.3. Hydrotropism

Hydrotropism is the growth movement of a plant organ in response to water.

• Mechanism: Roots grow towards areas of higher water concentration. While the exact mechanism is complex, it involves the perception of water gradients and differential growth, likely influenced by auxins and other hormones.
• Type: Roots typically exhibit positive hydrotropism.
• Example: Plant roots growing towards a leaky water pipe or a moist patch of soil, even against the force of gravity.

2.4. Thigmotropism

Thigmotropism is the growth movement of a plant organ in response to touch or physical contact.

• Mechanism: When a plant part (like a tendril) comes into contact with a solid support, the cells on the side opposite to the contact grow faster than the cells on the side of contact. This differential growth causes the plant part to coil around the support.
• Type: Often positive thigmotropism.
• Example: The coiling of tendrils of climbing plants (e.g., pea plants, grapevines) around a support structure like a fence or another plant.

2.5. Chemotropism

Chemotropism is the growth movement of a plant organ in response to a chemical stimulus.

• Mechanism: The plant organ grows towards or away from a specific chemical substance due to a concentration gradient of that chemical.
• Types: Can be positive (towards the chemical) or negative (away from the chemical).
• Example:
  • Positive Chemotropism: The growth of a pollen tube towards the ovule in response to chemical signals released by the ovule during fertilization. This ensures that the pollen tube reaches the egg cell for successful reproduction.
  • Negative Chemotropism: Roots growing away from harmful chemicals in the soil.

These tropic movements allow plants to adapt and survive in diverse environments by optimizing their position for light, water, and support, and by avoiding harmful conditions.