Down-regulation of photosynthesis flowing girdling, but contrasting effects of fruit set and retention, in two sweet cherry cultivars

Sweet cherry (Prunus avium) trees were manipulated to analyse the contribution of soluble sugars to sink feedback down-regulation of leaf net CO₂ assimilation rate (A_net) and fruit set and quality attributes. Total soluble sugar concentration and A_net were measured in the morning on fully expanded leaves of girdled branches in two sweet cherry cultivars, ‘Kordia’ and ‘Sylvia’ characterised typically by low and high crop load, respectively. Leaves on girdled trees had higher soluble sugar concentrations and reduced A_net than leaves on non-girdled trees. Moreover, RuBP carboxylation capacity of Rubisco (V_cmax) and triose-phosphate utilisation (TPU) were repressed in the girdled treatments, despite J_max remaining unchanged; suggesting an impairment of photosynthetic capacity in response to the girdling treatment. Leaf A_net was negatively correlated to soluble sugars, suggesting a sink feedback regulatory control of photosynthesis. Although there were significantly less fruit set and retained in ‘Kordia’ than ‘Sylvia’; girdling had contrasting effects in each cultivar. Girdling significantly increased fruit set and fruitlet retention in ‘Sylvia’ cultivar, but had no effect in ‘Kordia’ cultivar. We propose that low inherent sink demand for photoassimilates of ‘Kordia’ fruit could have contributed to the low fruit retention rate, since both non-girdled and girdled trees exhibited similar retention rate and that increases in foliar carbohydrates was observed above the girdle. In ‘Sylvia’ cultivar, the carbohydrate status may be a limiting factor for ‘Sylvia’ fruit, since girdling improved both fruit set and retention, and leaf soluble solids accumulation.