2018 ASHS Annual Conference

In spring, sweet cherry bloom, fruit set, and spur leaf development are dependent on storage carbohydrates. Subsequent fruit and new shoot development relies on photoassimilates supplied by new spur, and eventually shoot, leaves. Thus, horticultural techniques that increase spur leaf size would be expected to increase the carbohydrate supply capacity for the remainder of fruit development. Leaf photosynthetic capacity is determined by physical features, such as specific leaf area (the ratio of leaf area to leaf dry mass) and stomata density, and biochemical features (particularly nitrogen content per unit area). This study examined post-bud break foliar applications of cytokinin (6-benzylaminopurine, 6-BA), gibberellin (GA₃), the combination 6-BA plus GA₄₊₇, and several nitrogen fertilizers to improve spur leaf size. Foliar applications were made to individual spurs of ‘Sam’ sweet cherry trees before sunrise by hand-held trigger-pump sprayer as follows: T1: 0.5% urea; T2: 150 ppm 6-BA; T3: 30 ppm GA₃; T4: distilled water (control); T5: 2.0% Ca[NO₃]₂; T6: 1.7% KNO₃; and T7: 150 ppm 6-BA plus 30 ppm GA₄₊₇. Applications were made when the first spur leaf was large enough to measure, usually between full-bloom and the petal fall, with five replications. For each spur leaf, length and width was measured every ~60 growing degree days (GDD, base 7.2°C) from emergence to complete expansion. Small sections were cut from leaf margins to determine cell number and size. Statistical comparisons (Wilcox, 0.05) showed that all of the phytohormone treatments significantly increased final leaf area, while none of the nitrogen treatments differed from the control. Ca[NO₃]₂ (T5) caused young leaf marginal burning and cupping and was omitted from further data collection. To simplify the leaf growth curve analyses (173 curves total), mean growth curves were ranked for the five largest leaves for each spur and grouped by the largest to the smallest final leaf area. These mean growth curves revealed 6-BA plus GA₄₊₇ (T7) increased overall spur leaf area by ~50%, and 6-BA (T2) and GA₃ (T3) increased overall spur leaf area by ~20%. Analysis of leaf cell number and size was highly variable, but treatments that increased spur leaf area appeared to largely increase cell elongation. The results are promising for developing horticultural treatments to increase spur leaf area in spring, though further research on timings and rates is needed.