Seasonal changes in starch content in pistachio organs as related to crop load

The alternate bearing in pistachio (Pistacia vera L.) results from inflorescence buds abscission differentiated on current season growth during the year of heavy crop. Inflorescence bud drop has been directly correlated with the presence of fruits on 1-year shoot, and in particular with the kernel grow that acts as the major sink of nutrients. Experimental evidences suggest the involvement of competition between reproductive organs for the available resources. The annual carbohydrates storage was analysed in mature pistachio trees characterized by low, medium and high crop load (about 1, 7 and 11 kg tree-1 of dry in-shell nuts). The experiment was carried out in 2013 in a commercial pistachio orchard, using ‘Bianca’ grafted onto P. terebinthus L. in Sicily (Italy, 37°30’ Lat N). Starch concentration was measured in sample tissues of trunk wood, current-year shoot, one-year-old and two-year-old shoots throughout the season from the dormancy phase (March) until the post-harvest period (September). During dormancy, the trunk represented the main storage organ having starch concentration ranging from 60 to 120 mg g-1 of dry weight, whereas the tissues of the other organs examined showed values below 20 mg g-1 of dry weight. The deepest decrement of starch concentration after bud-break until bloom was observed in the trunk, whereas, starch concentration started increasing after the initial growth flush. The patterns of starch in the tissues varied according with the crop load; trees bearing the highest crop load (11 kg tree-1) showed a constant starch concentration during the growing season, in contrast with the trees characterized by lower crop load values. Before the onset of dormancy, starch rapidly increased in all the shoots type and in the trunk wood and no effects of tree crop load were observed.