Alternate Bearing Affects Nitrogen, Phosphorus, Potassium and Starch Storage Pools in Mature Pistachio Trees

The relationship between crop load and the functional storage of selected macronutrients and starch was assessed to develop nutrient budgets and best management fertilization practices in orchards. Functional storage represents the amount of nutrients and starch redistributed from perennial tree parts in support of the spring growth flush. Functional storage was influenced by:(a)nutrient and starch accumulation prior to dormancy; and(b)nutrient and starch demand by vegetative and reproductive organs in spring. Lightly cropping (off-year) trees stored 7, 14 and 2 times as much N, P and K, respectively, as heavily cropping (on-year) trees. Similar to many biennial plant species, nutrients that accumulated during the vegetative phase in off-year trees were used to support reproductive growth during the subsequent on-year. Soil nutrient uptake contributed more to storage pools than leaf nutrient resorption in off-year-trees, while the reverse was true in on-year trees. 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 between 20–33% of the N, P and K stored in perennial tissues of off-year trees could be attributed to leaf nutrient resorption. This is the first study to determine the amounts of nutrients stored in the perennial parts of mature, field-grown trees and the relative contributions of leaf nutrient resorption and soil nutrient uptake to functional storage in trees.