Plant hormones

Overview

Plants may appear stationary, but they are highly responsive organisms capable of detecting and reacting to environmental stimuli such as light, gravity, and moisture. This topic, Plant Hormones (4.4), explores the chemical messengers that coordinate these crucial growth responses. Understanding plant hormones is fundamental to Biology because it demonstrates how multicellular organisms communicate internally without a nervous system.

This topic connects strongly with your prior knowledge of cell structure, osmosis, and human endocrinology. Examiners frequently use plant hormones to test your ability to apply scientific principles to novel contexts, particularly in agriculture and horticulture. You can expect a mix of short recall questions on specific hormones, diagram-based questions requiring you to predict plant growth, and extended 6-mark questions evaluating the commercial uses of these chemicals.

Key Concepts

Concept 1: Auxins and Tropisms

Tropisms are directional growth responses to environmental stimuli. Phototropism is the response to light, while gravitropism (or geotropism) is the response to gravity. These responses are controlled by auxins, a family of plant hormones produced in the tips of shoots and roots (the apical meristems).

Auxins work by controlling cell elongation. However, the crucial point that examiners test is that auxins have opposite effects in different parts of the plant:

• In shoots, a high concentration of auxin promotes cell elongation.
• In roots, a high concentration of auxin inhibits cell elongation.

Phototropism in Shoots

When light shines on a shoot from one side, auxin produced at the tip diffuses down the stem and accumulates on the shaded side. Because auxin promotes elongation in shoots, the cells on the shaded side grow faster and longer than those on the illuminated side. This unequal growth causes the shoot to bend towards the light (positive phototropism). This adaptation ensures the plant's leaves receive maximum light for photosynthesis.

Gravitropism in Roots

When a root is growing horizontally, gravity causes auxin to accumulate on the lower side. Because auxin inhibits elongation in roots, the cells on the lower side grow more slowly than those on the upper side. This unequal growth causes the root to bend downwards, towards the pull of gravity (positive gravitropism). This ensures roots grow deeply into the soil to anchor the plant and access water and mineral ions.

Concept 2: Gibberellins

Gibberellins are another important group of plant hormones. They are primarily responsible for initiating seed germination, promoting stem elongation, and triggering flowering.

When a seed is exposed to the correct conditions (water, oxygen, and suitable temperature), it releases gibberellins. These hormones stimulate the breakdown of starch reserves in the seed into glucose, providing the energy required for the embryo to grow. In mature plants, gibberellins work alongside auxins to control the lengthening of stems between leaves (internodes).

Concept 3: Ethene

Unlike other plant hormones, ethene is a hydrocarbon gas (C₂H₄). It is produced by aging leaves and ripening fruit. Ethene controls two main processes:

1. Fruit Ripening: It stimulates the conversion of starch to sugars and the breakdown of cell walls, making fruit softer and sweeter.
2. Abscission (Leaf Fall): It stimulates enzymes that digest the cell walls at the base of the leaf stalk, causing the leaf to detach from the plant in autumn.

Mathematical/Scientific Relationships

While there are no specific equations to memorize for plant hormones, you must understand the proportional relationship in tropisms:

• Rate of bending \propto Difference in auxin concentration between the two sidesThe greater the concentration gradient of auxin across a stem or root, the more pronounced the unequal growth, and the sharper the angle of bending.

Practical Applications

The commercial application of plant hormones is a major focus for examiners. You must be able to describe these uses and evaluate their advantages and disadvantages in agriculture.

Uses of Auxins:

• Rooting Powders: Plant cuttings are dipped in rooting powder containing synthetic auxins. This stimulates rapid root development, allowing growers to produce large numbers of identical plants (clones) quickly.
• Selective Weedkillers: Broad-leaved weeds absorb large quantities of synthetic auxins, causing them to grow uncontrollably and die. Narrow-leaved crop plants (like wheat and grass) are unaffected, increasing crop yields.
• Tissue Culture: Auxins are added to the growth medium in tissue culture to stimulate the division of cells and the formation of roots in plantlets.

Uses of Gibberellins:

• Ending Seed Dormancy: Seeds can be treated with gibberellins to make them germinate earlier than they normally would, or to ensure all seeds in a batch germinate simultaneously.
• Promoting Flowering: Gibberellins can induce flowering in plants regardless of the environmental conditions (e.g., day length), allowing growers to supply flowers year-round.
• Increasing Fruit Size: Spraying gibberellins on crops like seedless grapes causes the stems to elongate, giving the grapes more space to grow larger.

Uses of Ethene:

• Controlling Ripening: Fruit is often picked while unripe and firm, making it less easily damaged during transport. Once it reaches its destination, ethene gas is pumped into the storage chambers to trigger rapid, synchronized ripening just before sale.