The objective of this study was to investigate the effect of nitrogen fertilizers and plant hormones applied to sweet cherry (Prunus avium L.) spurs in early spring on spur leaf growth and morphological features, including leaf area, leaf thickness and adaxial epidermis cell size. In preliminary studies (2017) with isolated spurs, three plant hormone treatments, gibberellic acid (GA3, 30 ppm), 6-benzylaminopurine (BA, 150 ppm) and 6-benzylaminopurine + gibberellic acid (150 ppm BA + 30 ppm GA4+7) increased total spur leaf area 30%, 37% and 47%, respectively, compared to an untreated control. One nitrogen fertilizer treatment (2.0% calcium nitrate, Ca[NO3]2) caused phytotoxicity to emerge spur leaves, and the others (0.5% urea and 1.7% potassium nitrate, KNO3) showed no significant differences with the control treatment. At the microscopic level, there were no statistically significant differences in leaf thickness among treatments, but the leaf adaxial epidermis cell size was increased from 20% to 40% in the plant hormone treatments, which supports that hypothesis that plant hormones increased leaf size by promoting larger cell size. In follow-up studies at the whole tree level (2018), three experiments examined plant hormone application rates, timing, and gibberellin type. For the Rate experiment, the 150 ppm BA + 30 ppm GA3 and the 75 ppm BA + 15 ppm GA3 applications increased total leaf area per spur by 59% and 55%, respectively. For the Timing experiment, 30 ppm GA3 applied twice (first when three emerging leaves were present and second after accrual of an additional 100 Growing Degree Days, GDD using a base temperature of 7°C) increased total leaf area by 36% relative to the control. For the Gibberellins experiment, 30 ppm GA3 increased total leaf area by 33% relative to the control. However, unlike in 2017 with the isolated spur study, none of the plant hormone treatments to whole trees in 2018 increased the area of individual leaves relative to the control; for the Timing and Rate experiments, the larger total leaf area per spur was due primarily to the emergence of more leaves per spur.
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