Phytohormones links named stimuli, coordinating structures, signalling routes and effectors so body systems or plant tissues produce an integrated biological response.
Coverage 2012–2025 · Updated 16 Jul 2026
Phytohormones coordinate plant decisions.
Plant hormones are chemical signals made in small amounts that alter growth or development in target tissues. Their effects depend on concentration, tissue sensitivity, transport and interactions with other hormones.
hormone source/transport; target tissue; cellular response; environmental context.
The same auxin signal can promote shoot elongation but, with a different concentration or tissue, influence rooting.
A hormone is not a single instruction with one universal effect across every plant tissue.
This objective is assessed through structured response, commonly using Identify / State / Calculate.
Identify / State / Calculate / Compare / Contrast
Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.
Treating low oxygen as the only trigger and omitting carbon-dioxide-driven pH change.
Representative question
State the name and site of production of two phytohormones that regulate the growth of seedlings.
| Name of phytohormone | Site of production |
|---|---|
Plant signalling integrates directional growth, phytohormones, and responses to stimuli. Tropisms are directional growth responses in seedlings; experiments compare shoot or root growth under lateral light or gravity stimuli. Shoots show positive phototropism toward lateral light; auxin redistribution causes greater elongation on the shaded side. Phytohormones are plant signalling chemicals controlling growth and development; auxin, cytokinin, and ethylene coordinate responses to stimuli and internal state. PIN auxin efflux carriers actively move auxin between plant cells; asymmetric carrier placement maintains auxin gradients during tropic responses. Auxin activates proton pumping into the apoplast; acidified cell walls loosen, allowing water uptake, elongation, and bending. Auxin generally promotes root formation; cytokinin promotes shoot formation and division; their ratio coordinates root-shoot growth and plant tissue culture outcomes. Ethylene is a gaseous phytohormone that promotes fruit ripening; ripening fruit releases more ethylene, creating positive feedback in nearby fruit.
