Preformed and neoformed shoot growth as related to canopy development, carbohydrate partitioning and yield characteristics of pistachio (Pistacia vera L.) trees

The California pistachio industry currently uses three seedling rootstocks, Pistacia atlantica (Atl), P. integerrima (PGI) or P. atlantica x P. integerrima (UCB). Trees grown on PGI and UCB rootstocks are much more vigorous than those on Atl. Inherent with this greater vigor is the production of long-shoots in the top of the canopies that makes the trees more difficult and costly to manage than those grown on Atl, but generally with an increase in yield. The objectives of this project were to (1) characterize and determine the origins of long-shoots, (2) determine better ways to manage trees on PGI and UCB rootstocks, and (3) determine how long-shoots affect yield.; All terminal vegetative buds, regardless of rootstock or canopy position, had from 8--9 preformed nodes. This indicated that the growth differences observed were due to the stimulation of greater neoformation in trees on the more vigorous rootstocks. Early season regulated deficit irrigation (RDI) during was effective in suppressing long-shoot growth at the whole tree level without affecting yield. This suppression was the result of a reduction in the number of shoots producing neoformed growth, not shoot length of those that did develop neoformed growth. This mechanism of control was not ideal since it did not uniformly suppress long-shoot growth. Pruning into two-year-old or older wood significantly stimulated the production of neoformed growth on all rootstocks. When pruning was restricted to one-year-old wood neoformed growth was similar to unpruned control trees. Pruning studies on young trees indicated that when trees are young and the primary objective is canopy development approximately one-half to two-thirds of the parent shoot length should be removed to produce the most vigorous re-growth. However, as trees mature pruning should remove no more than one-fourth the length of the parent shoot so as to maximize intra-shoot competition, reduce individual shoot vigor and develop fruit wood. Long-shoots were found to produce significantly more clusters and consequently more total yield than short-shoots. However, it appears that the affect of long-shoots on yield may have been related to their full sun position high in the canopy and not their length per se.