Plant Sensory Systems and Responses

Core Concepts

Plant hormones: the chemical signaling toolkit

Hormones act at low concentrations, often locally, and their effects depend on the target tissue. The five classical groups:

• Auxin (IAA): promotes cell elongation, apical dominance (suppression of lateral buds), root initiation, and fruit development; it drives tropisms because it can be redistributed.
• Cytokinins: promote cell division and shoot growth and delay senescence; the auxin/cytokinin ratio directs root vs. shoot formation.
• Gibberellins (GAs): promote stem elongation, seed germination, and flowering (bolting); they break dormancy.
• Abscisic acid (ABA): the stress hormone — stomatal closure during drought, seed and bud dormancy. Despite the name, ABA is not the main trigger of abscission; that role belongs largely to ethylene.
• Ethylene: the only gaseous hormone — fruit ripening, senescence, abscission, and the seedling "triple response" (shortened, thickened stem, exaggerated hook) as shoots push through soil.

Tropisms: directional growth responses

A tropism is growth toward or away from a stimulus — directional and irreversible.

• Phototropism: shoots bend toward light. Classic experiments (Darwin's coleoptile, Boysen-Jensen's separations, Went's agar blocks) showed that a mobile chemical redistributes so the shaded side gets more auxin and elongates faster. Modern view: light triggers auxin transport away from the lit side.
• Gravitropism: shoots grow up (negative) and roots grow down (positive). Dense starch-filled statoliths settle to the lower side of cells, signaling gravity; auxin accumulates there. In stems, higher auxin promotes elongation, so the lower side grows faster and the shoot bends up; in roots, higher auxin inhibits elongation, so the root bends down — the same hormone, opposite responses in different tissues.
• Thigmotropism: growth in response to touch — tendrils wrap around supports.

Nastic movements: reversible, non-directional responses

Nastic movements do not depend on stimulus direction and are often reversible turgor changes. The touch-sensitive Mimosa pudica folds its leaflets within seconds: potassium and water rush out of motor cells, collapsing turgor, and an electrical signal can spread the response. The Venus flytrap snaps shut with a similar rapid mechanism. Nyctinasty ("sleep movements") is the daily folding rhythm of leaves such as bean leaflets.

Photoperiodism: measuring night length with phytochrome

Many plants time flowering to day length — more precisely, to night length:

• Short-day plants flower when nights are longer than a critical length (poinsettia, chrysanthemum, soybean).
• Long-day plants flower when nights are shorter than the critical length (spinach, lettuce, wheat).
• Day-neutral plants flower regardless (tomato, cucumber).

The detector is phytochrome, a pigment with two interconvertible forms: Pr (absorbs red light, inactive) and Pfr (absorbs far-red light, active). Red light converts Pr → Pfr; far-red converts Pfr → Pr; Pfr slowly reverts to Pr in darkness. Daylight (rich in red) drives the pigment toward Pfr; at night Pfr decays, and if the night is long enough, Pfr falls below the flowering threshold. The proof that night length matters: a brief red-light flash interrupting the dark period prevents short-day plants from flowering (it restores Pfr), and far-red immediately after reverses the effect. Some plants also need cold (vernalization) before flowering.

Circadian rhythms and dormancy

Plants have internal ~24-hour circadian rhythms that persist in constant conditions: leaf movements, stomatal opening, and hormone levels rise and fall on a daily cycle, entrained by light. Dormancy is a survival strategy: seeds and buds arrest growth until conditions are safe. ABA promotes dormancy; gibberellins and cues such as cold, light, or water break it — which is why many seeds need a winter chill or light before germinating.

Defense responses

Physical barriers (thorns, spines, trichomes, thick cuticles, bark) are backed by constitutive chemicals — secondary metabolites such as alkaloids (caffeine, nicotine), tannins, and phenolics — and by induced defenses: wounding triggers jasmonate signaling, ramping up chemical defenses; pathogen attack triggers salicylic acid signaling and the hypersensitive response, in which cells around the infection die rapidly to contain it, followed by systemic acquired resistance throughout the plant. Some plants release volatiles that attract natural enemies of herbivores — indirect defense.

Common Confusions