9.7 Growth Responses in Plants

Plants cannot move from place to place, but they respond to environmental stimuli through growth responses — changes in the direction or rate of growth. These responses allow plants to optimize their position for light, anchor themselves in soil, and time their reproduction with the seasons.

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Tropic Movements (Tropisms)

A tropism is a directional growth response in which the direction of growth is determined by the direction of the stimulus. Tropisms can be positive (growth toward the stimulus) or negative (growth away from the stimulus).

Types of Tropisms

Mechanism of Phototropism

Phototropism is mediated by auxins (primarily Indole Acetic Acid, IAA):

1. Light strikes one side of the shoot tip.
2. Auxins migrate laterally away from the light source to the shaded side.
3. Higher auxin concentration on the shaded side promotes greater cell elongation there.
4. The shoot bends toward the light source — positive phototropism.

Mechanism of Geotropism

In a horizontally placed seedling:

• Auxins accumulate on the lower side due to gravity.
• In shoots: high auxin promotes elongation → lower side grows faster → shoot curves upward (negative geotropism).
• In roots: roots are more sensitive to auxin; high auxin inhibits elongation → lower side grows slower → root curves downward (positive geotropism).

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Plant Hormones and Growth Regulation

Plant growth responses are coordinated by phytohormones (plant hormones). The major classes are:

Apical Dominance

Apical dominance is the suppression of lateral (axillary) bud growth by the apical bud.

• The apical bud produces auxins that move downward (basipetal transport).
• High auxin concentration in lateral buds inhibits their growth.
• Cytokinins produced in roots move upward and promote lateral bud growth, counteracting auxin.
• Removing the apical bud (pruning/pinching) removes the auxin source → lateral buds are released from inhibition → bushy growth.

Practical application: Gardeners prune plants to encourage bushier growth by removing apical dominance.

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Photoperiodism

Photoperiodism is the physiological response of plants to the relative lengths of the light period (day) and dark period (night) in a 24-hour cycle. It primarily controls flowering.

Classification of Plants by Photoperiodic Response

Phytochrome — The Photoperiod Sensor

Phytochrome is the pigment responsible for detecting day/night length:

• Exists in two interconvertible forms:
  • Pr (absorbs red light, 660 nm) → converts to Pfr
  • Pfr (absorbs far-red light, 730 nm) → converts to Pr (or reverts slowly in darkness)
• During the day, Pfr accumulates (active form).
• During the night, Pfr slowly converts back to Pr.
• The ratio of Pr:Pfr signals the length of the dark period to the plant.

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Vernalization

Vernalization is the induction or acceleration of flowering by exposure to a prolonged period of cold temperatures (typically 0–10°C for several weeks), mimicking winter conditions.

Key Features

• Ensures plants do not flower prematurely in autumn; they flower in spring/summer after winter has passed.
• The cold stimulus is perceived by the shoot apex (meristematic cells).
• The effect is mitotically stable — once vernalized, the plant retains the memory of cold exposure through cell divisions.
• Devernalization can occur if the plant is exposed to high temperatures immediately after cold treatment.

Examples

• Winter wheat and rye: require vernalization to flower in spring.
• Biennial plants (e.g., carrot, cabbage): grow vegetatively in year 1, flower in year 2 after experiencing winter cold.

Vernalization vs. Photoperiodism

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Summary