Responses in Plants

Overview

Responses in plants occur in reaction to changes in the environment. Unlike animals, plants cannot move but they can change how they grow and develop due to their cell structure and organisation.
These responses are essential for plant survival, enabling them to adapt to various environmental conditions by moving towards or away from specific stimuli.

A tropism is the growth of a plant in response to a directional stimulus. It can be positive, towards the stimulus, or negative, away from the stimulus.
Phototropism is the growth of a plant in response to light, with positive phototropism being growth towards the light and negative phototropism meaning growth away from the light.
Geotropism (or gravitropism) is the growth of a plant in response to gravity, with positive geotropism being growth towards the earth (normally roots) and negative geotropism being growth away from the earth (normally shoots).
Hydrotropism is the growth of a plant in response to water, with positive hydrotropism being growth towards water.

Plant hormones, also known as phytohormones, are the chemicals that regulate plant growth and development.
Auxins are a type of plant hormone that is involved in the elongation of cells, regulating phototropism and geotropism. Auxins are produced in the growing apices of shoots and roots and move to other areas to stimulate growth.
Gibberellins are plant hormones that promote stem and leaf elongation, trigger the germination of seeds and stimulate flowering and fruit development.
Abscisic acid (ABA) is a plant hormone that regulates the closure of stomata and inhibits stem growth, ensuring the plant responds appropriately to stress conditions like drought.

Photoperiodism is the physiological reaction of organisms to the length of day or night. It affects flowering, germination, and the growth of leaves, and is also crucial for seasonal activities like the development of winter dormancy.
Short-day plants flower when the photoperiod is less than a certain critical length, while long-day plants flower when the photoperiod is greater than a certain critical length. Day-neutral plants flower irrespective of photoperiod length.
The hormone florigen is believed to regulate photoperiodism in flowering. It is produced in leaves in response to specific photoperiods and then transported to the shoot apical meristem, where it initiates flowering.

Plants have various defence responses against pests and pathogens. Some of these involve the production of chemical compounds to deter herbivores or inhibit the growth of pathogens.
Plants can also respond to damage by closing wounds, producing additional structural materials like lignin to reinforce cell walls, or shedding infected leaves to limit the spread of disease.
Some plants form mutualistic associations with other organisms to enhance their defensive capabilities. For example, many legumes form mutualistic relationships with nitrogen-fixing bacteria, which provide them with a source of nitrogen in exchange for sugars.