| To determine the role of ethylene in apple fruit set, sprays of aminoethoxyvinylglycine (AVG), an inhibitor of ethylene synthesis, silver thiosulfate (STS), an inhibitor of ethylene action, and (2-chloro-ethyl) phosphonic acid (ethephon), an ethylene generating compound, were applied in 1980 and 1981 to branches of trees of 3 apple cultivars. Additional branches were scored with a knife. When applied at full bloom 200 ppm AVG significantly increased set in all cultivars, but inhibition of ethylene evolution from excised flowers was slight and non-significant. Ethephon (40 to 100 ppm) did not affect fruit set significantly, yet markedly increased ethylene production. Fruit set was much better correlated with ethylene evolution in 'Delicious' than in 'McIntosh' or 'Golden Delicious', but r values were non-significant. Therefore the effects of AVG on fruit set appear to be independent of its effect on ethylene synthesis.;When flowers on bagged limbs were hand-pollinated, set decreased as pollination was delayed. Parallel response of AVG-treated and control flowers indicated that AVG had little or no effect on the effective pollination period. Supplemental hand pollination increased set in 'Delicious' but not in 'McIntosh', supporting the hypothesis that "basal gaps" between the stamens of 'Delicious' flowers limit fruit set by permitting bees to obtain nectar without transferring pollen to the stigmata. However, set of controls was similar in both cultivars.;Application of AVG (200 ppm) prior to "June" drop had variable effects upon fruit retention and did not significantly reduce ethylene evolution from excised fruitlets. Neither STS nor scoring consistently affected fruit drop or ethylene evolution, but ethephon (200 ppm) significantly increased both. Differences between large and small fruits in rate of ethylene evolution were apparently due to differences in size, rather than to differences in abscission potential. |
