Section 2: Plant Responses Preview Bellringer Key Ideas Plant Hormones Tropisms Seasonal Responses Plant Movements Summary

Bellringer What “senses” might plants have? Explain why it might be important for plants to sense environmental conditions.

Key Ideas Why are hormones important for plant growth and development? How do tropisms affect plants? What triggers seasonal change in plants? How do nastic movements affect plants?

Plant Hormones Plants can sense changes in the environment and respond to these changes in adaptive ways. A hormone is a chemical that is produced in one part of an organism and transported to another part, where it causes a response. Plant hormones are produced in small amounts but may have large effects on the growth and development of plants.

Plant Hormones, continued Hormones may stimulate or inhibit growth in a plant. Hormones are also important in plant responses to environmental stimuli. Plant responses often require the transfer of hormones from the part of a plant that detects a change to the part that responds to the change.

Plant Hormones, continued Auxins stimulate cell elongation. Gibberellins stimulate fruit development and seed germination. Cytokinins stimulate cell division. Ethylene promotes the ripening of fruit. Abscisic acid often slows growth in plants and helps maintain dormancy in seeds.

Visual Concept: Types of Plant Hormones

Tropisms Because most plants are anchored in one spot, they cannot move from an unfavorable environment to a more favorable one, as animals do. Tropisms, a plant’s responses to environmental stimuli, are triggered by the hormones that regulate plant growth. A tropism is a response in which a plant grows toward or away from a stimulus.

Tropisms, continued A positive tropism is when a plant grows toward a stimulus. A negative tropism is when a plant grows away from a stimulus. Directional movements in response to light are called phototropisms.

Tropisms, continued Growth responses to touch are called thigmotropisms. Responses to gravity are called gravitropisms.

Visual Concept: Tropism

Seasonal Responses Many responses to the seasons have evolved in plants. The principal way in which plants time seasonal responses is by sensing changes in night length. The response of a plant to the length of days and nights is called photoperiodism.

Seasonal Responses, continued In reference to photoperiodism, most plants can be categorized as one of three types: short-day plants, long-day plants, or day-neutral plants. Knowledge of photoperiodism is very important to the nursery and floral industries. Controlling the length of days and night in a greenhouse can force plants to flower at times of year when they ordinarily would not.

Flowering and Photoperiodism

Seasonal Responses, continued Temperature affects growth and development in many plants. For example, most tomato plants will not produce fruit if nighttime temperatures are too high. Dormancy is the condition in which a plant or seed remains inactive, even when conditions are suitable for growth.

Visual Concept: Dormancy

Plant Movements Some plant movements respond to an environmental stimulus but are not influenced by the direction of the stimulus. These movements are called nastic movements. Nastic movements are usually regulated by changes in the water content of special cells. Examples of nastic movements include the rapid closing of the leaves of a Venus’ flytrap and the closing of the leaves of a sensitive plant.

Summary Plant hormones are produced in small amounts but may have large effects on the growth and development of plants. Hormones may stimulate or inhibit growth in a plant. A tropism is a response in which a plant grows toward or away from a stimulus. Plant hormones are responsible for producing tropisms. The principal way in which plants time seasonal responses is by sensing changes in night length.

Summary, continued Some plant movements respond to an environmental stimulus but are not influenced by the direction of the stimulus. These movements are called nastic movements.