Plant responses to signals — A/L Biology

What must I remember about Plant responses to signals?

• Plant hormones (phytohormones) are organic regulators synthesized in low concentrations that coordinate growth, development, and environmental responses.
• Auxins (IAA): Promote cell elongation in stems, apical dominance (suppressing lateral bud growth), phototropism, and root initiation in cuttings.
• Gibberellins (GA): Promote stem elongation (bolting), seed germination by triggering α-amylase production in aleurone layer, and fruit development.
• Cytokinins: Promote cell division (cytokinesis) with auxin, delay leaf senescence, and stimulate lateral bud growth (antagonistic to auxin).
• Abscisic Acid (ABA) and Ethylene: ABA acts as stress hormone promoting stomatal closure during drought and enforcing seed dormancy; Ethylene (gaseous) promotes fruit ripening, leaf/fruit abscission, and triple response in seedlings.

Test yourself (1 minute)

1. Photoperiodism & Phytochrome

   Plants detect day/night length using phytochrome pigment converting reversibly between Pr (inactive, absorbs red 660nm) and Pfr (active, absorbs far-red 730nm); critical dark period regulates flowering.

2. Phototropism mechanism

   Blue light activates phototropins on illuminated side, causing asymmetric lateral transport of auxin to shaded side where higher auxin induces faster cell elongation, bending shoot toward light.

3. Hormone antagonism

   Apical dominance reflects balance between auxin (from apex, inhibitory) and cytokinins (from roots, stimulatory) on axillary buds.

Practise Plant responses to signals questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 4

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Where does Plant responses to signals fit in the syllabus?

Year 1 · Term 3