Preformation in vegetative buds of pistachio (Pistacia vera): relationship to shoot morphology, crown structure and rootstock vigor.

Effects of rootstock, shoot carbohydrate status, crop load and crown position on the number of preformed leaf primordia in the dormant terminal and lateral buds of mature and immature 'Kerman' pistachio (Pistacia vera L.) trees were investigated to determine if rootstock vigor is associated with greater shoot preformation. There was no significant variation in preformation related to the factors studied, suggesting strong genetic control of preformation in 'Kerman' pistachio. The growth differences observed among trees on different rootstocks were associated with greater stimulation of neoformed growth in trees on the more vigorous rootstocks. However, most annual extension growth in mature tree crowns was preformed, contrasting with the relatively high rate of neoformation found in young tree crowns. Large amounts of neoformed growth in young trees may allow the trees to become established quickly and secure resources, whereas predominantly preformed growth in mature trees may allow for continued crown expansion without outgrowing available resources. We hypothesized that the stimulation of neoformed growth by the more vigorous rootstocks is associated with greater resource uptake or transport, or both. Understanding the source of variation in shoot extension growth on different rootstocks has important implications for orchard management practices.