Annals of Botany

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Little green apples

As I write this, the apple trees in my garden are in full bloom; I look forward to a good harvest in the late summer and on through the autumn. However, between now and then, many of the fruit that initially set will fall off, a phenomenon known as the “June drop”. This is, in effect, a self-thinning activity that ensures an economical allocation of resources to fruit and seed production. There is a distinct hierarchy in this process. The central flower in a cluster of five opens first (and is thus usually pollinated first) and it is the fruit forming from this flower that is most likely to be conserved. Commercial growers of apples often encourage fruitlet abscission by spraying the trees with hormone solutions, as described by Dal Cin et al. (Padova and Trento, Italy, pp. 1195–1202). In common with many abscission events in flowering plants, ethylene is involved. In both a “conventional” variety, “Golden Delicious” and in a “spur variety”, “Red Delicious”, there was increased transcription of the gene encoding ACC-oxidase, the last enzyme in the ethylene biosynthesis pathway. In “Red Delicious” this occurred in all fruitlets but in “Golden Delicious” it was confined to lateral fruitlets. However, in both varieties, increased ethylene production occurred only in lateral fruitlets (implying a further level of control in “Red Delicious”). Even though ethylene is clearly involved, one of the hormones used by growers to promote fruitlet abscission is the cytokinin benzylaminopurine (BA): application of BA causes an increase in transcription of the ACC-oxidase gene and ethylene output. However, only in “Golden Delicious” was fruitlet abscission stimulated. Despite this, data from measurement of shoot growth suggest that BA stimulates fruit drop in “Golden Delicious” by promoting vegetative growth at the expense of reproduction. A spur type such as “Red Delicious” does not respond because shoot growth is regulated in a different way.