ASSESSMENT OF NITROGEN, PHOSPHORUS, AND POTASSIUM UPTAKE CAPACITY ANDROOT-GROWTH IN MATURE ALTERNATE-BEARING PISTACHIO (PISTACIA-VERA) TREES

We examined interrelationships between crop load, nitrogen (N), phosph orus (P), and potassium (K) uptake, and root growth in mature, alterna te-bearing pistachio (Pistacia vera L.) trees. Pistachio trees bear he avy (on-year) and light (off-year) fruit crops in alternate years. Upt ake and partitioning of N, P, and K among tree parts were determined d uring (a) spring flush (mid-March to late May), (b) nut fill (late May to early September), and (c) postharvest-leaf senescence (late Septem ber to early December). Nutrient uptake occurred primarily during nut fill in both on-year and off-year trees. In on-year trees, N and K upt ake increased by 35 and 112%, respectively, during nut fill compared w ith off-year trees. During this period, nutrients were allocated large ly to embryo development in on-year trees and to storage in perennial tissues in off-year trees. Nutrient uptake was negligible between harv est and leaf senescence. Although root growth was reduced during nut f ill in on-year trees compared with off-year trees, there was no relati onship between root growth and the uptake of N, P or K from the soil. Our data support the hypothesis that sink demand regulates the uptake and distribution of N, P, and K in pistachio trees.