15 Plant Responses

Key Idea: Plants generally respond to their environment by growing to or away from a stimulus or by responding in a way that affects some physiological process. Even though most plants are firmly rooted in the ground, they can still respond to changes in their external environment, mainly through changes in patterns of growth. These responses may involve relatively sudden physiological changes, as in flowering, or a steady growth response, such as a tropism. Many of these responses involve annual, seasonal, or circadian (daily) rhythms.

Life cycle responses

Plants use seasonal changes (such as falling temperatures or decreasing daylength) as cues for starting or ending particular life cycle stages. Such changes are mediated by plant growth factors, such as phytochrome and gibberellin, and enable the plant to avoid conditions unfavourable to growth or survival. Examples include flowering, dormancy and germination, and leaf fall.

Rapid responses to external stimuli

Plants are capable of quite rapid responses. Examples include the closing of stomata in response to water loss (below), opening and closing of flowers in response to temperature, and nastic responses. These responses may follow a circadian rhythm and are protective in that they reduce the plant’s exposure to abiotic stress or grazing pressure.

Tropisms

Tropisms are growth responses made by plants to directional external stimuli, where the direction of the stimulus determines the direction of the growth response. A tropism may be positive (towards the stimulus), or negative (away from the stimulus). Common stimuli for plants include light, gravity, touch, and chemicals.

Plant competition and allelopathy

Although plants are rooted in the ground, they can still compete with other plants to gain access to resources. Some plants produce chemicals that inhibit the growth of neighbouring plants. Such chemical inhibition is called allelopathy. Plants also compete for light and may grow aggressively to shade out slower growing competitors.

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Plant responses to herbivory

Many plant species have responded to grazing or browsing pressure with evolutionary adaptations enabling them to survive cropping or deter browsers. Examples include rapid growth to counteract the constant loss of biomass (grasses), sharp spines or thorns to deter browsers (acacias, cacti), or toxins in the leaf tissues (eucalyptus).

1. Identify the stimuli plants typically respond to:

2. Describe the adaptive advantage of a plant responding appropriately to the environment:

3. Describe one adaptive response of plants to each of the following stressors in the environment:

(a) Low soil water:

(b) Falling autumn air temperatures:

(c) Browsing animals:

(d) Low air temperatures at night: