MACRONUTRIENT ALLOCATION TO LEAVES AND FRUIT OF MATURE, ALTERNATE-BEARING PISTACHIO TREES - MAGNITUDE AND SEASONAL PATTERNS AT THE WHOLE-CANOPY LEVEL

Estimates of leaflet and fruit macronutrient (N, P, K, Ca, and Mg) acc umulation and resorption were developed in six (three heavily cropping , on-year and three noncropping, off-year) mature pistachio (Pistacia vera L. 'Kerman') trees over three growing seasons during three stages of phenology [the spring growth flush (April to June); seed fill Gate June to September); and leaf senescence (September to November)]. Cro p load influenced total nutrient content per tree in annual organs (le aves and fruit), the relative allocation of nutrients between leaves a nd fruit, temporal patterns of nutrient accumulation in annual organs, and the magnitude of net leaf nutrient resorption per tree prior to l eaf fall, In off-year trees, macronutrient accumulation in annual orga ns (leaves) was concentrated during the spring flush of growth. In con trast, significant macronutrient accumulation in annual organs of on-y ear trees (leaves plus fruit) occurred not only during the spring flus h of growth but also during seed fill, Duration and magnitude of macro nutrient accumulation were greater in on-year vs, off-year trees, Frui t N and P demand during seed fill was partially met by a net decrease in the N and P contents of the pericarp, These decreases in pericarp n utrient content during seed fill were equivalent to 32% and 26% of emb ryo accumulation of N and P, respectively, Fruit demand for N, P, and K during the spring flush of ''on'' years was accompanied by reduced l eaf N, P, and K contents per tree, Net leaf N, Ca, and Mg resorption p er tree during leaf senescence differed with crop load, Net leaf N res orption was significantly greater in off-year trees than on-year trees , Leaf N resorption presumably represents an important component of th e N pool stored in perennial tree parts during dormancy, The greater l eaf N resorption following ''off' years was a function of greater leaf N concentration and greater leaf biomass per tree, In contrast, net l eaf resorption of Ca and Mg was greater in on-year vs, off-year trees, Experimental validation of the magnitude and periodicity of nutrient uptake by mature pistachio trees is needed during the alternate-bearin g cycle, especially in light of the potential contribution of current fertilization practices to groundwater contamination.