5.2 - Phytohormone

Definition and Properties

Phytohormones, or plant hormones, are chemical substances that stimulate and coordinate responses in plants at low concentrations.
Phytohormones are generally synthesised in a certain plant organ and transported to a target organ through the phloem.

Auxin
- Functions in phototropism and geotropism.
- Stimulates development of apical dominance in shoots and roots.
- Stimulates growth and elongation in root and shoot cells.
- Stimulates development of adventitious roots at the ends of stem cuttings.
- Stimulates cell division in the cambium during secondary growth.
- Inhibits abscission of fruits and young leaves.
- Inhibits growth of lateral buds.
Gibberellin
- Stimulates growth and elongation in stem cells.
- Stimulates development of leaves, flowers and fruits.
- Stimulates development and germination of seeds.
- Causes growth of flowering stems in dwarf plants.
- Inhibits development of roots.
Cytokinin
- Stimulates division and elongation of root and stem cells when auxin is present.
- Stimulates seed germination.
- Inhibits development of apical dominance.
- Delays leaf senescence.
- Stimulates growth of lateral buds.
Abscisic acid
- Inhibits plant growth.
- Stimulates abscission of mature fruits, leaves and flowers.
- Induces seed dormancy.
- Induces stomatal closing during drought.
- Inhibits bud growth and seed germination.
Ethylene
- Stimulates fruit ripening.
- Stimulates plant senescence.
- Stimulates abscission of leaves and fruits.

Auxin is synthesised at the coleoptile tip and stimulates cell elongation.
Coleoptile experiment:
- Removing the tip produces no growth.
- Replacing the tip directly or placing a centred auxin-containing agar block on the cut coleoptile produces upright growth.
- Placing the agar block off-centre produces curved growth.
Inference:
- Uniform auxin distribution causes the coleoptile to grow upwards.
- The side receiving more auxin grows faster; the coleoptile curves towards the side receiving less or no auxin.
External stimuli such as light and gravity influence auxin distribution.
High auxin concentrations stimulate cell elongation in shoots but inhibit cell elongation in roots.

Light from all directions produces uniform auxin distribution; the shoot grows upwards.
Light from one direction causes auxin to move away from the light and accumulate on the shaded side.
Cells on the shaded side elongate more than cells on the illuminated side.
The shoot bends towards the light and shows positive phototropism.

In a horizontally germinating seed, gravity causes auxin to accumulate on the lower sides of the shoot tip and root tip.
Shoot: High auxin concentration on the lower side stimulates cell elongation; the shoot bends upwards and shows negative geotropism.
Root: High auxin concentration on the lower side inhibits cell elongation; the root bends downwards and shows positive geotropism.