Plant hormones: auxins, tropisms and their uses (HT)

Plants don't have brains or muscles, but they do respond to the environment using plant hormones. The most important class at GCSE is auxins, which control growth — and the directional growth responses called tropisms.

What is a tropism?

A tropism is a directional growth response to a stimulus.

• Phototropism — growth in response to light.
• Gravitropism (geotropism) — growth in response to gravity.

A "positive" tropism means growth towards the stimulus; "negative" means away.

• Shoots are positively phototropic (grow towards light) and negatively gravitropic (grow upwards).
• Roots are negatively phototropic (grow away from light, into the soil) and positively gravitropic (grow downwards).

This makes biological sense: shoots find light for photosynthesis, roots find water and anchorage.

How auxins control phototropism in shoots

Auxins are made in the tips of growing shoots and roots. In shoots:

1. Light shines from one side onto a shoot tip.
2. Auxin moves to the shaded side.
3. On the shaded side there is more auxin, which causes the cells to elongate (grow longer) faster.
4. The shaded side grows faster than the lit side, so the shoot bends towards the light.

Key fact: in shoots, auxin promotes elongation; the shaded side has more auxin → grows more → shoot bends towards light.

How auxins control gravitropism in roots

In roots, auxin inhibits elongation (it works in the opposite direction!).

• A root lying horizontally will accumulate auxin on the lower side.
• More auxin on the lower side inhibits elongation there.
• The upper side grows more, so the root bends downwards (towards gravity).

A useful summary:

	Shoot	Root
Effect of auxin	Promotes elongation	Inhibits elongation
Side with more auxin	Shaded / lower (gravity)	Lower side (gravity)
Result	Bends towards light / upwards	Bends downwards

Investigating tropisms (required practical)

Classic setup: place cress seedlings in dishes lined with damp cotton wool. Cover one side; light the other. After a few days, the shoots curve towards the light. Controls include seedlings in all-round light (no curving) and in total darkness (etiolated, but straight). Use a clinostat (slow-rotating turntable) to "remove" gravity in gravitropism experiments.

Uses of plant hormones in horticulture and agriculture

Auxins and other hormones are used commercially:

• Weed killers (selective herbicides). Synthetic auxins (e.g. 2,4-D) make broad-leaved weeds grow uncontrollably and die, but leave narrow-leaved cereal crops unharmed.
• Rooting powders. Cuttings dipped in auxin form roots faster, allowing easy cloning of desirable plants.
• Tissue culture promoters. Auxin and cytokinin in agar cause stem cells in plant tissue to divide and develop into new plants.

Other plant hormones (you only need awareness):

• Gibberellins — used to end seed dormancy, promote flowering and produce larger fruits (e.g. seedless grapes).
• Ethene — used to control ripening of fruit during storage and transport (so fruit ripens just as it reaches the shop).

Links

Connects back to B4.1 (photosynthesis — why phototropism evolved). The use of auxin to clone plants links to B6.7 / B6.8 (genetic engineering and cloning).