Control and coordination in plants

Overview of Plant Responses

Plants respond to different environmental stimuli through a series of physiological changes. Unlike animals, plants do not have a nervous system, but their responses are nonetheless coordinated and enable them to adapt to their environment.
Some mechanisms of plant responses include tropisms, nastic movements, dormancy, germination, and flowering.
Growth hormones, also known as plant growth regulators, play a crucial role in modulating these responses.

Plant Tropisms

Tropisms are directional responses to environmental stimuli. They can be positive (towards the stimulus) or negative (away from the stimulus).
Phototropism is the response to light, where the plant grows towards (positive phototropism) or away from (negative phototropism) the light source.
Geotropism or gravitropism is the response to gravity, which causes roots to show positive geotropism (grow downwards) and shoots to show negative geotropism (grow upwards).
Thigmotropism is the response to touch or physical contact, as seen when a climbing plant wraps around a support.
Hydrotropism is the response to water, typically shown by roots growing towards moisture.

Plant Nastic Movements

Nastic movements are non-directional responses to stimuli; the direction of response is not dependent on the direction of the stimulus.
Examples include nyctinasty (daily movement in response to the onset of darkness or light), thermonasty (response to changes in temperature), and seismonasty (response to a physical shock).

Role of Plant Growth Regulators

Plant growth regulators, or phytohormones, mediate various aspects of plant growth and development. The primary types include auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
Auxins promote cell elongation, inhibit growth of lateral buds (apical dominance), and are involved in tropic responses.
Gibberellins stimulate stem elongation, seed germination, flowering, and fruit production.
Cytokinins promote cell division and delay leaf ageing.
Abscisic acid promotes leaf fall, dormancy in seeds and buds, and closing of stomata (guard cells) during drought.
Ethylene stimulates fruit ripening and promotes leaf abscission.

Control of Flowering

Plants time their flowering in response to environmental cues such as day length (photoperiod). This is regulated by a pigment called phytochrome, which exists in two interconvertible forms and senses red and far-red light.
Long-day plants flower when the night length is below a critical threshold, while short-day plants flower when the night length exceeds a threshold.
There are also day-neutral plants that flower regardless of the photoperiod.

Plant Dormancy and Germination

Dormancy is a period of reduced metabolic activity which allows plants to survive unfavourable conditions.
Seed dormancy can be induced by several mechanisms, including a hard seed coat, the presence of growth inhibitors, or immature embryos.
For germination to occur, seeds often need to encounter certain environmental conditions (such as water, oxygen, certain temperature range, and sometimes light). The activation of gibberellin synthesis is crucial to initiate germination.
The precise mechanisms of dormancy and germination are still areas of active research.