| Exogenous growth regulators and endogenous cytokinins produced by rootstocks were investigated with regard to their effects on fruit set of sweet cherry.;All treatments caused some leaf epinasty, with the GA(,3) containing treatments most severe. GA(,3) also enhanced shoot elongation of the current season's growth.;Two cytokinin-like substances believed to be zeatin and zeatin riboside were detected in the xylem sap of sweet cherry scion branches on trees grafted to different rootstocks. The greatest cytokinin activity occurred during bloom and decreased rapidly throughout the fruit development period. The higher yielding and fruit setting rootstocks produced higher levels of cytokinin in the xylem sap of scion branches at each date sampled.;The strongest correlations between the effects of rootstock on xylem sap cytokinin level and yield data were with fruit retention between first and final set and with yield efficiency. The predominant activity of the cyokinin translocated from the rootstocks appeared to be through prevention of senescence and abscission of fruit between early fruit set and harvest.;Exogenous applications of the cytokinin N(,6) benzyladenine (N(,6)BA), the auxins 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and gibberellic acid (GA(,3)) increased fruit set with sweet cherry, even in the absence of normal embryo development. The phenoxy auxins, alone or combined with GA(,3) were most effective. Naphthaleneacetic acid (NAA) reduced final fruit set. 2,4-D enhanced maturity of the fruit, while treatments containing GA(,3) retarded maturity, induced fruit cracking and inhibited flowering and fruit set the following year. N(,6)BA appeared to counteract, in part, the inhibitory effects of GA(,3) on flower initiation. |
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