Fruitlet carbohydrate status and relative growth rate explain apple abscission rates irrespective of flower position

The setting of apple (Malus x domestica Borkh.) fruitlets is primarily determined by their relative sink strength, which depends on the balance between carbohydrate supply and demand and the number of competing sinks. It is generally accepted that limits in carbohydrate supply predispose small fruitlets, i.e., weak sinks, to abscission. This study examined the relationship among carbohydrate status, fruitlet growth, and abscission in 'Gala' and 'Honeycrisp' over two seasons. Fruitlets were sampled every other day after chemical thinning applications according to their flower position in the cyme (king vs. lateral) and absolute fruitlet weights and compared to untreated controls. At each sample date, fruitlets were selected from three distinct size classes: 90^th, 50^th, and 10^thpercentile of the sample population, each with an associated fruit set prediction according to the fruitlet size distribution model (FSDM). Soluble sugars (glucose, fructose, sucrose, sorbitol) and starch were quantified at each sample date. Small fruitlets (10^th percentile) consistently exhibited reduced soluble sugar concentrations and relative growth rates prior to abscission, while larger fruitlets (50^thand 90^th percentiles) maintained higher sugar levels; thus, having greater sink strength and a higher retention probability. Sorbitol and sucrose concentrations were often dynamic and generally reflected modeled tree-level carbon balance, based on the MaluSim model; these sugars also declined markedly after chemical thinner applications were made indicating stress. Starch accumulation further distinguished stronger from weaker sinks, with exponential accumulation in 90^thpercentile fruitlets under thinning stress. Positional effects were evident, as king fruitlets generally maintained higher sugar concentrations than laterals; however, this effect was confounded by king fruit generally being larger. When position was ignored, fruit weight could explain a fruitlet's abscission fate. When the FSDM predictions were within 90% of the actual fruit set, fruitlets destined to abscise had markedly reduced concentrations of soluble sugars and lower growth rates than those predicted to be retained, before any visible signs of fruit abscission were apparent. These data underscore the physiological basis of abscission by highlighting the central role of carbohydrate availability and sink strength that determine fruitlet survival, supported by the FSD model.