Plant Hormones – a.k.a Plant Growth Regulators

Plants do not move actively from place to place.They do not posses muscle or nervous systems.

But they do respond to:

•LIGHT•GRAVITY•CHEMICALS•CHANGES in TEMPERATURE•CONTACT

•These tend to initiate very small changes in growth/direction of growth – called tropisms.

Plant Growth Regulators – Positive Phototropism

Shoots of a plant will grow towards the light.Charles Darwin and his son carried out early experiments (see fig. 34.1).Later it was discovered that there is a bundle of tightly rolled leaves at the tip of a growing shoot called a coleoptile, this area is very sensitive to light.

An overview of some early experiments:•Removal of coleoptile stops the tropic response.•Inserting mica stops the tropic response.•Cutting off the tip, inserting agar and then replacing tiphad no effect.

Plant Growth Regulators – Positive Phototropism

The chemical responsible for this was called AUXIN – (Greek for to grow). It was later more specifically called IAA Indoleacetic Acid.

When one half of a shoot receives more light than the another, IAA is transported across to the shaded side, where it promotes cell elongation (not division).

IAA is synthesised from the amino acid TRYPTOPHAN. IAA increases the release of H+ in the cell, which reduce the pH, providing optimum conditions for enzymes that weaken the bonds between cellulose chains.

This in turn allows the cell to expand a little as it absorbs water

Plant Growth Regulators – Negative Phototropism

The chemical responsible for this was called AUXIN – (Greek for to grow). It was later more specifically called IAA Indoleacetic Acid.

When one half of a root receives more light than the another, IAA is transported across to the shaded side, where it inhibits cell elongation.

These is the complete reverse of the situation in the shoots.

Whilst the shaded part of the root is prevented from growing, the part exposed to light continues to grow, creating downward growth.

Plant Growth Regulators – Geotropisms

This is growth in response to gravity.

Negative geotropisms happen in the shoots – due to IAAPositive geotropisms happen in the roots, this is due to ABA – abscisic acid.

ABA collects on the lower side of roots, inhibiting cell elongation, creating a downwards growth.These is the complete reverse of the situation in the shoots.

Amyloplasts (starch grains) rest on lower cell walls of roots, triggering the release of ABA

Plant Growth Regulators – Stomatal Closure

ABA is also involved in stomatal closure.ABA has been nick named the stress hormone.If difficult conditions prevail – ABA is released.

In drought conditions the concentration of ABA can increase by 40 times, triggering stomatal closure.

It is a very quick response to stress that takes minutes, it is possible that ABA binds directly to guard cells, stopping proton pumps and so preventing G.cells from becoming turgid.

Plant Growth Regulators – Apical dominance

Apical dominance is the term given when the tip of the stem produces a chemical to inhibit lateral branches.

Some plants have strong apical dominance – poplarOther plants of weak apical dominance – oak

It is still unclear which PGR has dominant role here, but IAA, ABA & Cytokinins are all thought to be involved.

Plant Growth Regulators – Controlling germination

In some seeds, GIBBERELLINS are involved in contolling germination.

When a seed is released from a plant, it is dormant – it is dry and is not metabolically active.

(1)On application of water, the embryo of the seed produces Giberellin.

(2)One of the outer layers (aleurone layer) of the seed synthesises amylase in response to this.

(3)Amylase then breaks down all the stored starch into maltose and then to glucose.

(4)Glucose is used as tissues begin to respire.(5)Gib. regulates the genes that synthesise

amylase therefore it is a slow process

Plant Growth Regulators – Controlling ripening

Only on PGR is a gas – ETHENE or ETHYLENE.

It is produced by fruits in the early stages of ripening,

it then stimulates further ripening.

Ethene has also been observed to;

(1)Cause leave fall aka abscission.(2)Release buds and seeds from dormancy.

Plant Growth Regulators – Economic Importance

1 Controlling dormancy:ABA is applied to keep vegetables/seeds in a

dormant state.

2 Ripening & harvesting fruit:IAA can keep fruit/leaves on trees.Ethene can cause fall of fruit/leaves.Fruit can be picked unripe, transported and then ripened.

3 Selective weed killers:Synthetic auxins exist that promote respiration in dicots, this is then used to promote monocot growth (lawns).

4 Rooting hormones:IAA on a cut end of a stem leads to root growth, therefore ideal when taking cuttings.