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Robin A. Cohen and Duane W. Greene

Two experiments were conducted to examine the effects of growth regulator application on cytokinin levels in developing apple leaves. In experiment 1, gibberellin GA4/7 (100 mg·liter-1) and benzyladenine (BA, 100 mg·liter-1) were applied alone or in combination as a foliar spray to `Golden Delicious' 7 days after full bloom. In experiment 2, BA was applied at 50 or 100 mg·liter-1 as a foliar spray to `Morespur McIntosh' 18 days after full bloom. Trans-zeatin ribosidelike (t-ZR) levels were determined by immunoassay. In experiment 1, GA1/7 had no effect and BA increased t-ZR-like levels. In experiment 2, BA increased t-ZR levels quadratically with application rate. The increase in endogenous cytokinin levels with BA treatment occured within 2 hours. Initially, t-ZR-like levels were 47 times higher in the BA (100 mg·liter-1) treated leaves as compared to check leaves. Over the next 8 days, t-ZR-like content in treated leaves decreased in a cubic manner.