The role of leaves in citrus fruitlet abscission: Effects on endogenous gibberellin levels and carbohydrate content

In satsuma (Citrus unshiu, cv. Clausellina) mandarin trees, defoliation sho rtly after anthesis of old (previous season) or young (current season) leav es increased fruitlet abscission (up to 60%) during the following 12 weeks. Defoliation also advanced the beginning of abscission. In foliated plants, maximum rates of abscission were reached 41 d after anthesis (DAA), wherea s in plants without old or young leaves maximum abscission rates occurred 3 2 and 54 DAA, respectively. Gibberellin (GA) and carbohydrate concentration s were determined by GC-MS and HPLC, respectively. Defoliation did not subs tantially modify the endogenous levels of the active GA(1), although the pr ecursors and products of this GA changed mostly at 32 DAA. Thereafter, GA c oncentrations were similar in foliated and defoliated plants. Carbohydrate concentrations were law, and the highest differences were observed at 21 DA A. At this time, sucrose and hexoses were slightly lower and higher, respec tively, in fruitlets on defoliated plants. Higher abscission rates of fruit let of leafless inflorescences were also associated with reduced amounts of sucrose and higher hexose concentrations during this period. The data sugg est that removal of either old or young leaves advanced and increased absci ssion, and reduced the sucrose/hexose ratio of fruitlets during the initial period of fruit set.