Relations between the concentration of diffusable and extractable gibberellin-like substances and the alternate-bearing behaviour in apple as affected by chemical fruit thinning

The levels of diffusable and extractable gibberellin-like substances (GA_s) of apple fruits were investigated in the alternate-bearing ‘King of the Pippins’ and the regularly fruiting ‘Golden Delicious’, using the barley endosperm bioassay. During 1977 and 1978 there were marked differences between the two cultivars in both the chronological appearance and the absolute amounts of diffusable GA_s. The application of fruit-thinning chemicals (carbaryl, ethephon, NAAm) to ‘King of the Pippins’ suppressed the early increase of GA_s in diffusates, which normally occurs during the second week after full bloom. The following year, the thinned trees exhibited heavy flower- and fruit-set, while the untreated control trees showed a very poor bloom. The later appearance of the GA peak in diffusates from fruits of ‘Golden Delicious’ (5 weeks after full bloom), was not accompanied by a similar inhibitory effect on subsequent flowering.In contrast to their effect on the levels of diffusable GA_s, the thinning-treatments caused no real change in the amount of extractable GA_s, which was very similar in fruits from both cultivars.