Plant Hormones Explained: Essential Revision Guide for OCR A-Level Biology

Plant Movement and Responses

Plants might seem stationary, but they're constantly moving and responding to their surroundings. Tropism is how plants grow directionally based on external stimuli - think of it as their way of navigating the world.

Phototropism makes shoots grow towards light (positive), whilst geotropism pulls roots down with gravity (positive) and pushes shoots up against it (negative). You'll also see chemotropism when pollen tubes follow chemical trails to reach eggs, and thigmotropism when climbing plants like ivy wrap around supports.

Plants need these responses to avoid being eaten, reduce stress from harsh conditions, and survive long enough to reproduce successfully. It's basically their survival toolkit.

Key Point: All tropisms can be positive (towards stimulus) or negative (away from stimulus) - this determines the plant's growth direction.

Plant Growth Centres and Hormones

Plant growth happens in specific areas called meristems - clusters of young, dividing cells that haven't yet specialised. Apical meristems at root and shoot tips control length, whilst lateral meristems handle width growth.

There are five major plant hormones working like chemical messengers: auxins, cytokinins, gibberellins, abscisic acid, and ethene. They travel through the plant's transport system and bind to specific receptors on target cells.

Each hormone has different jobs throughout the plant's life cycle. Some promote germination, others control flowering, and several manage fruit development and leaf drop.

Remember: Plant hormones work at incredibly low concentrations, making them tricky for scientists to study - that's why we're still discovering how they work!

Auxins and Gibberellins in Action

Auxins are the superstars of plant growth, controlling shoot elongation by making cell walls stretchy. They create acidic conditions that activate enzymes to loosen cellulose bonds, allowing cells to expand as they absorb water.

These hormones maintain apical dominance - keeping the main shoot growing whilst suppressing side shoots. This helps plants compete for light effectively. Auxins also prevent leaf fall in young leaves and promote root growth at low concentrations.

Gibberellins work alongside auxins to cause stem elongation and trigger flowering. They're essential for breaking down food stores during seed germination and stimulating pollen tube growth.

Study Tip: Remember that auxins and gibberellins often work together (synergistically) to help plants grow tall and reach sunlight.

Ethene and Abscisic Acid Functions

Ethene is the plant's aging hormone, triggering fruit ripening and leaf fall in autumn. It creates an abscission layer at the base of leaf stalks that eventually breaks, causing leaves to drop off deciduous trees.

Abscisic acid (ABA) is the plant's stress manager and growth inhibitor. It prevents seeds from germinating at the wrong time and closes stomata during drought by making guard cells lose water and become flaccid.

Plant hormones often work against each other - auxins prevent leaf loss whilst ethene promotes it, and auxins inhibit side shoots whilst gibberellins stimulate them. This creates a balanced control system.

Exam Focus: Understanding how ABA closes stomata through ion movement and water loss is a common exam question - learn the step-by-step process.

Gibberellins and Growth Control

Gibberellins are particularly important for stem elongation, working through both cell division and cell elongation mechanisms. Research on dwarf plants has shown that gibberellins are essential for normal height development.

These experiments have helped scientists understand how plant hormones coordinate growth responses, though the complete picture is still being pieced together through ongoing research.

Quick Revision: Gibberellins = growth and germination, making them essential for a plant's development from seed to mature plant.