ALTERNATE BEARING AFFECTS NITROGEN, PHOSPHORUS, POTASSIUM AND STARCH STORAGE POOLS IN MATURE PISTACHIO TREES

The relationship between crop load and the functional storage of selec ted macronutrients and starch was assessed to develop nutrient budgets and best management fertilization practices in orchards. Functional s torage represents the amount of nutrients and starch redistributed fro m perennial tree parts in support of the spring growth flush. Function al storage was influenced by: (a) nutrient and starch accumulation pri or to dormancy; and (b) nutrient and starch demand by vegetative and r eproductive organs in spring. Lightly cropping (off-year) trees stored 7, 14 and 2 times as much N, P and K, respectively, as heavily croppi ng (on-year) trees. Similar to many biennial plant species, nutrients that accumulated during the vegetative phase in off-year trees were us ed to support reproductive growth during the subsequent on-year. Soil nutrient uptake contributed more to storage pools than leaf nutrient r esorption in off-year-trees, while the reverse was true in on year tre es. Net nutrient resorption from senescing leaves accounted for all of the N and P and a third of the K stored in on-year trees. Only betwee n 20-33% of the N, P and K stored in perennial tissues of off-year tre es could be attributed to leaf nutrient resorption. This is the first study to determine the amounts of nutrients stored in the perennial pa rts of mature, field-grown trees and the relative contributions of lea f nutrient resorption and soil nutrient uptake to functional storage i n trees. (C) 1998 Annals of Botany Company