Pistachio inflorescence bud abscission dynamics as a function of embryo weight, crop load and vegetative growth

Alternate bearing in pistachio (Pistacia vera L.) is particularly severe. The developing nuts are a major sink, draining branch carbohydrates, producing inflorescence bud abscission and decreasing the following year's production. We investigated inflorescence bud drop as a function of branch crop load, embryo weight, and vegetative growth. The main objective of this study was to investigate the mechanism of alternate bearing. We also tested the efficacy of eight different foliar nutrient combinations for reducing inflorescence bud abscission: Acadian A LSC®, Acadian B, low-biuret urea, Acadian A LSC® + low-biuret urea, Acadian B + low-biuret urea, MaxCell®, MaxCell® + low-biuret urea, and untreated control (sprayed with water). The trial was conducted during the 2016 growing season on eight-year-old Kerman pistachio trees planted at 5.2 m in-row and 6.1 m between rows, on PG1 rootstock, 8 replications for each treatment. Average tree yields as dry in-shell marketable split nuts were not significantly different among treatments ranging from 27 to 31 kg tree(-1)). The treatment did not affect inflorescence bud abscission. The bud abscission percentage was significantly higher in bearing branches (~70%) versus non-bearing branches (~15%). In fruiting shoots, the percentage of inflorescence bud abscission was significantly and positively correlated with embryo growth. Length, width and weight of new wood and one-year-old wood correlated significantly and positively with nut production, and negatively with inflorescence bud abscission. The data collected support the hypothesis suggested in the literature that alternate bearing is function of crop load and carbohydrate depletion. Results reported here are the first step toward characterization of optimal shoot parameters, length, width and leaf area, for optimal and regular crop production.