(a) Describe an experiment to demonstrate the influence of auxins on the growth of plant shoot. (b) List three uses of auxins in agriculture. (c) Describe b...

Answer Details

(a) Experiment to demonstrate the influence of auxins on the growth of a plant shoot (phototropism)

• Materials: several germinating oat or maize seedlings (coleoptiles), sharp blade, a box with a hole cut in one side, and a dark cupboard.
• Method: Take three sets of seedlings.
  • Set A: leave the tip of the coleoptile intact (control).
  • Set B: cut off the extreme tip of the coleoptile.
  • Set C: cut off the tip, then replace it on the stump.
• Place all sets in the box so that light enters only from one side (unilateral light) and leave for a day or two.
• Observations: Set A (tip intact) bends and grows towards the light. Set B (tip removed) does not grow or bend. Set C (tip replaced) grows and bends towards the light like Set A.
• Explanation/Conclusion: the growing tip produces a hormone (auxin) which, in unilateral light, moves to the shaded side of the shoot and causes greater cell elongation there, so the shoot bends towards the light. Removing the tip removes the source of auxin, so no growth or bending occurs. This shows that auxin produced by the tip controls the growth (and phototropic curvature) of the shoot.

(b) Three uses of auxins in agriculture

• Used as a rooting hormone to promote the growth of roots on stem cuttings.
• Used as selective weed killers (herbicides) to destroy broad-leaved weeds.
• Used to prevent premature fruit drop and to promote the development of fruits (including seedless/parthenocarpic fruits).

(c) Mechanism of transmission of impulses through a nerve fibre

A resting nerve fibre is polarised: the outside of the membrane is positively charged and the inside negatively charged (resting potential), maintained by the sodium-potassium pump. When the fibre is stimulated strongly enough (above threshold), the membrane at that point becomes depolarised: sodium ions rush in, reversing the charge so the inside becomes positive. This change is the action potential (nerve impulse). The depolarised region stimulates the next region, so the impulse travels as a wave along the fibre. Behind the impulse the membrane is quickly repolarised (returns to resting state). At the end of the fibre the impulse reaches a synapse, where a chemical (neurotransmitter) is released to carry the impulse across the gap to the next neurone.