A carbon budget model to predict branch carbohydrate deficiencies as a function of water stress and crop load in pistachio (Pistacia vera L.)

The effect of irrigation and crop load on alternate bearing was studied in pistachio tree using a “branch carbon budget model” developed to calculate branch carbohydrate balance. Experiences were conducted in Sicily (37°26’N, 14°03’E, 360 m a.s.l.) on female trees of ‘Bianca’. Two treatments were applied: rainfed (T0) and 100 mm of irrigation (T100). At 29, 44, 65, 86, 103 and 121 DAFB, on one fully expanded leaf selected on three trees per treatment, were monitored leaf gas exchanges. At 15 days interval, three branches treatment-1 were excised and, in the lab, the following parameters were measured: total photosynthetically active leaf surface; number of leaves, fruits, shoot fresh weight (FW) and dry weight (DW). Photosynthetic response curves to PPFD were calculated on the base of data published in the literature. Irrigation positively affected branch carbon budget that resulted positive (6.87 g of C) at the end of the season, whereas in rainfed trees the carbon budget was negative (-13.98 g of C). In rainfed trees seasonal branch carbon assimilation was 44% lower as a consequence of reduced leaf area. 28 days after full bloom, when leaves reached full development, leaf area fruiting branch-1 in rainfed trees was 920 and 1300 cm2 in branches of irrigated tress. Branch of rainfed trees also showed intense leaf drop and yellowing that strongly reduced late season assimilation rate (Amax 4.5 µmol m-2 s-1 in branches of rainfed trees vs. 10 µmol m-2 s-1 in irrigated ones), affecting total seasonal carbon assimilation. Referring to the carbon budget of the branches, considering the interaction between crop load and irrigation, one cluster was the maximum sustainable crop load in rainfed trees while irrigation enabled the branches to sustain up to three clusters, maintaining a positive carbon budget until fruit ripening. The results reported confirm that the “branch carbon budget model”, after further validation, will be a valid tool to predict carbohydrate budget as function of crop load and irrigation, useful to help to manage orchard to reduce the severity of alternate bearing.