11.1.3 Fruit set

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An ovary must be stimulated in some way for fruit growth to occur; this is normally by pollination and fertilisation. This important principle was established as early as the 1960s when Nitsch and others (Sections 9.2.2, 9.3.1) showed that gibberellins and auxins are involved in the pollination stimulus. Subsequent hormone production by the fertilised ovary is critical to stimulating fruit development (Nitsch 1970).

By implication, a suitable balance of growth regulators applied to unpollinated fruitlets can result in fruit set, and in practice gibberellins GA4 and GA7 are very effective in setting parthenocarpic (seedless) apple fruit (Dennis 1986). By contrast, parthenocarpy is rare in kiwifruit, although repeated appli-cations of napthaleneacetic acid (NAA) with benzyladenine (BA) and gibberellin have been successful (Hopping 1986). Such results confirm that growth regulators — alone or in combination — can trigger cell division in ovaries or related tissues that ultimately become fruits.

Seedless fruits have arisen via human selection of genotypes in which ovaries produce an adequate supply of growth regulators without any stimulation from the germinating pollen and developing seed (triploid banana), or where fertilisation is closely followed by seed abortion (stenospermocarpic, as in sultana grapevines; see Chapter 11 frontispiece). In the absence of pollination, levels of endogenous hormones such as auxins and gibberellins normally fall markedly (Nitsch 1970) and flowers abscise or fruitlets stop growing. The molecular signals responsible for this reduction are still not known.