Bud abscission dynamics in pistachio branches with different crop load

The alternate bearing phenomenon in Pistachio (Pistacia vera L.) has a peculiar mechanism of regulation. Unlike the other species, low crop load years (OFF) are associated with premature fall of inflorescence buds rather than the lack of flower differentiation. Behind the mechanism, several authors have suggested an involvement of nutritional factors (carbohydrates, nitrogenous substances and mineral elements) mediated, probably, by hormonal signals; it is an extremely complex phenomenon that is not fully understood. This work aims to monitor the dynamics of flower bud abscission in branches with different fruit loads to identify the time window in which the signal that triggers the mechanism of falling turns on. On three bearing plants, 30 years old, five branches per tree with different fruit loads were selected: non-bearing (T0), with one infructescence (T1), two (T2), three (T3) and four infructescences (T4). The total number of fruits, the number of fruits per infructescence, the number of leaves, and the number of axillary buds were monitored weekly during the entire growing season (May-September). The number of fruits per infructescence was inversely related to the total number per branch. The leaf area per branch was significantly different among the test groups, with 2127±28 cm2 in T0 and 1291±320 cm2 in T1, T2, T3 and T4 on average. As expected, the percentage of total bud drop was directly related to the branches’ fruit load; the flower bud drop was 32±6% vs. 73±24% for nonbearing and bearing branches respectively. The trial’s findings revealed that branches with a higher number of fruits lose inflorescence buds earlier than the fruitless branches. This suggests that the presence of fruits on a branch can trigger the shedding of buds, leading to an earlier bud abscission. Additionally, nonbearing branches showed a slight drop in inflorescence buds, when fruits were present in the adjacent branches, which further confirmed the hypothesis of the semi-autonomy of branches reported also by other studies.