Higher Gibberellins in Agriculture and Ethylene in Fruit Ripening
Part of Plant Hormones — GCSE Biology
This higher tier covers Higher Gibberellins in Agriculture and Ethylene in Fruit Ripening within Plant Hormones for GCSE Biology. Topic 11: Plant Hormones It is section 8 of 11 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Topic position
Section 8 of 11
Practice
15 questions
Recall
20 flashcards
Higher Gibberellins in Agriculture and Ethylene in Fruit Ripening
Gibberellins in agriculture: Gibberellins promote cell elongation and division, stimulate seed germination by activating enzymes that break down starch in the seed, and promote fruit development without fertilisation (parthenocarpy). Commercially, gibberellins are sprayed on seedless grapes to increase bunch size. In brewing, gibberellins promote germination of barley seeds (malting), releasing the sugars needed for fermentation. They can also extend the growing season of some plants by overriding dormancy.
Ethylene (ethene) in fruit ripening: Ethylene is unusual because it is a gas hormone. It triggers the breakdown of chlorophyll (fruit turns from green to yellow/red), softening of cell walls (fruit becomes softer), and conversion of starch to sugars (fruit becomes sweeter). Commercially, fruits such as bananas and tomatoes are picked unripe (green) when they are firm and easy to transport without bruising. Ethylene gas is then pumped into sealed ripening rooms at the destination to trigger ripening just before sale. This allows fruit to be transported worldwide without spoiling.
Understanding these agricultural applications is a Higher-tier focus because it requires applying hormone mechanisms to unfamiliar commercial contexts.
Quick Check: A plant shoot is illuminated from one side. Explain, in terms of auxin distribution and cell elongation, why the shoot bends towards the light.
Auxin is produced at the tip of the shoot. When light strikes from one side, auxin is redistributed laterally and accumulates on the shaded (dark) side of the shoot. The shaded side therefore has a higher concentration of auxin than the illuminated side. Auxin stimulates cell elongation, so cells on the shaded side elongate more than cells on the illuminated side. Because one side grows longer than the other, the shoot bends towards the light source — this is positive phototropism. The bending is not due to the plant seeking light, but is a direct mechanical consequence of unequal auxin distribution causing unequal growth rates.
Quick Check: Explain why applying high concentrations of synthetic auxin to broad-leaved weeds in a lawn causes those weeds to die, while the grass is unaffected.
Synthetic auxins act as selective weedkillers by exploiting the different sensitivities of broad-leaved plants (dicots) and grasses (monocots) to high auxin concentrations. Broad-leaved weeds have a larger surface area of leaves, absorb more synthetic auxin, and respond to very high concentrations with uncontrolled, abnormally rapid growth. This disordered growth disrupts normal cell function, uses up the plant's energy and resources rapidly, and ultimately causes the plant to die. Grasses (monocots), including lawn grass, absorb less of the auxin through their narrower leaves and also metabolise synthetic auxins differently, so they are much less affected. The weedkiller selectively kills the weeds without harming the lawn.
Quick Check: Describe and explain the effect of auxin on cell elongation in roots compared to shoots, and explain how this difference leads to positive gravitropism in roots.
In shoots, high auxin concentration promotes cell elongation (auxin is a growth stimulant). In roots, the same high concentration of auxin inhibits cell elongation (roots are far more sensitive to auxin). In a horizontal root, gravity causes auxin to accumulate on the lower side. The lower side of the root has a higher auxin concentration, which inhibits elongation there. The upper side has less auxin and elongates more. Because the upper side grows faster than the lower side, the root curves downward — positive gravitropism. In a shoot in the same situation (horizontal), auxin on the lower side would stimulate more elongation there, causing the shoot to curve upward — negative gravitropism. Same hormone, opposite responses in root versus shoot.