Chemical control in plantsEdexcel A-Level Biology B: Revision notes

Plants coordinate growth with chemicals called plant growth substances (plant hormones). They are made in small amounts, often in specific tissues, and move by diffusion, active transport or in the phloem to target cells that carry the right receptor. Their effect depends on concentration and on the tissue.

Three you must know are:

• Auxins (e.g. IAA): cell elongation, apical dominance, root growth
• Cytokinins: promote cell division, growth of lateral buds, delay leaf ageing
• Gibberellins: germination and stem elongation

Auxin is made mainly in shoot tips and young leaves. It binds to receptors on the plasma membrane of target cells and activates proton pumps. H⁺ ions are pumped into the cell wall, so the wall becomes more acidic.

The low pH activates expansins, which loosen bonds between cellulose microfibrils. Water enters by osmosis and turgor pressure stretches the loosened wall, so the cell elongates.

Auxin also promotes root growth, but roots are far more sensitive: a concentration that stimulates stem elongation can inhibit root elongation. This is why rooting powders are dilute.

In apical dominance auxin made in the apical bud suppresses the growth of lateral buds, so the plant grows tall with little branching. Removing the apical bud lowers auxin near the lateral buds and they grow.

Cytokinins promote lateral bud growth, so the two substances are antagonistic: auxin suppresses lateral buds and cytokinin promotes them. The balance between them decides whether a lateral bud grows. Applying auxin to a cut stump restores suppression, which shows auxin is the inhibitor.

In a germinating cereal grain the embryo releases gibberellin. It diffuses to the aleurone layer, which makes and releases amylase. Amylase hydrolyses stored starch in the endosperm to maltose, which is hydrolysed to glucose and used in respiration by the embryo for growth.

Core Practical 14 investigates this with a starch agar assay:

• Place embryo-less grain halves on starch agar containing different concentrations of gibberellin (or cut wells filled with solutions)
• Incubate, then flood with iodine solution
• Starch is blue-black; a clear zone shows where amylase hydrolysed starch
• Measure the zone diameter or area; use a 0 concentration control and keep temperature and incubation time constant

Phytochrome is a light-sensitive pigment with two interconvertible forms. Pr absorbs red light (about 660 nm) and is converted to Pfr. Pfr absorbs far-red light (about 730 nm) and is converted back to Pr. In darkness Pfr slowly reverts to Pr. Pfr is the active form.

Flowering: plants respond to the length of the uninterrupted dark period. Short-day plants flower when nights are long and Pfr has fallen low. Long-day plants flower when nights are short and Pfr stays high. A red flash in a long night restores Pfr and can prevent short-day flowering; a far-red flash reverses it.

Photomorphogenesis is the effect of light on form and development. In light, Pfr inhibits stem elongation and promotes leaf expansion and chlorophyll formation. Seedlings in the dark are etiolated: long, pale and unexpanded.