Regulation of sweet cherry fruit abscission: the role of photo-assimilation, sugars and abscisic acid

SummaryPremature late-season fruit abscission is a major concern for UK sweet cherry growers, but its causes are not understood. Competition for resources such as photo-assimilates and plant hormones may trigger fruit abscission. We therefore examined the roles of photo-assimilation, sugars and abscisic acid (ABA) in the regulation of late-season fruit abscission. The experimental material consisted of phloem-girdled short shoots (‘spurs’) with two distinct ranges of leaf area to fruit number (LA:FN) ratios. These ratios were achieved by selecting spurs with similar leaf areas, but different fruit numbers. Our previous research showed that fruit rapidly abscise from spurs with a low LA:FN ratio (within 14 – 21 d after girdling; DAG), while fruit on spurs with a high ratio are typically retained. Photo-assimilation of leaves was measured over 15 DAG. Fruits were collected for the determination of sorbitol, ABA and 1-aminocyclopropane-1-carboxylic acid concentrations 0, 1, 3, 5, 7 and 9 DAG. After girdling, photo-assimilation of leaves from spurs with a low LA:FN ratio was greater than that of leaves from spurs with a high LA:FN ratio. However, fruit on spurs with a low LA:FN ratio potentially received three-fold less photo-assimilate per fruit compared to fruit on spurs with a high LA:FN ratio. Before the symptoms of abscission appeared, sorbitol concentrations in fruit were 30% lower on spurs with low LA:FN compared to fruit on spurs with a high ratio. As the symptoms of abscission developed for fruit on spurs with a low LA:FN ratio, fruit ABA concentrations increased as fruit sorbitol concentrations declined. Experiments to determine if the exogenous application of ABA induced fruit abscission showed a rapid induction of fruit loss. These results support the hypothesis that source limitation, through the supply of photo-assimilates, can cause late-season fruit abscission in sweet cherry. However, the delayed rise detected in fruit ABA concentration appears more consequential than abscission-related.