Marginally nitrogen (N)-deficient, field-grown peach trees [Prunus persica (L.) Batsch (Peach Group) 'O' Henry'] were used to evaluate seasonal patterns of tree N uptake, vegetative growth, and yield following fall or spring fertilization. Sequential tree excavations and determinations of tree biomass and N contents in Feb. and Aug. allowed estimation of N uptake by fall-fertilized trees between September 1993 and mid-February 1994. Total N uptake (by difference) by spring- fertilized trees as well as additional N uptake by fall-fertilized trees over the spring.summer period was also determined. In fall-fertilized trees, only 24% of tree N accumulation between September 1993 and August 1994 occurred during the fall/dormancy period. Spring- and fall-fertilized trees exhibited comparable vegetative growth, fruit size, and yield despite lower dormant tree N contents and tissue N concentrations in the spring-fertilized trees. Fifty percent of tree leaf N content was available for resorption from leaves for storage in woody tree parts. This amount (N at ~30.kghhhhhhha-1) was calculated to represent more than 80% of the N storage capacity in perennial tree parts of fertilized peach trees. Our data suggest that leaf N resorption, even without fall soil N application, can provide sufficient N from storage to initiate normal growth until plant-available soil N is accessed in spring.

The effects of alternate bearing on recovery and loss of isotonically labeled fertilizer N and B and on the accumulation of carbohydrate and N reserves were assessed in mature 'Kerman' pistachio (Pistacia vera L.) trees. Total recovery of labeled fertilizer N applied once (in late January) was ≈ ≈ 60% greater if applied to trees entering an "off' than an "on" year, with respect to fruiting. Eleven percent more labeled B was recovered in off-than on-year trees. Five times more N (1 vs. 0.2 kg N) was lost from the tree in fruit and senescent leaflets from on-than off-year trees. In dormant trees, 144% and 22% more starch and N reserves, respectively, were present after off than on years. Thus, on-year trees were characterized by a greater reproductive demand for N and carbohydrates, reduced accumulation of C and N (i.e., storage) reserves in perennial tree parts, and reduced recovery of January-applied labeled fertilizer N than off-year trees. As B is absorbed passively, the higher transpiration that may accompany the 43% larger leaf area per tree and the probability of increased root growth probably contributes to its increased uptake during off years. The enhanced labeled N recovery in early spring by trees entering their off year preceded fruit and seed development in on-year trees. The differential tree capacity for nutrient uptake in spring may have been conditioned the previous rather than the current year. The increased uptake of labeled N by trees entering an off year (i.e., emerging from an on year) was associated with lower levels of carbohydrate and N reserves than for on-year trees that had just completed an off year. Future experimentation should assess the comparative capacity for nutrient uptake by on-and off-year trees at other stages of phenology, e.g., during seed development and postharvest.

[15N]‐depleted (NH4)2SO4 applied to the soil in 1985 resulted in residual labeling of about 16% of the storage nitrogen (N) pool of mature walnut (Juglans regia L. cv. Serr) trees in 1987. Application of [15N]‐depleted (NH4)2SO4 fertilizer to a different set of mature walnut trees in 1987 allowed monitoring of the kinetics and utilization of N from current year uptake in 1987 and resulted in >20% labeling of fruit N following completion of leaf expansion. Redistribution of storage N to the new growth predominated during the spring flush of growth although N derived from the soil during current‐year uptake contributed increasingly during leaf expansion. Labeled N from current year uptake accumulated preferentially in the leaves as compared with reproductive organs during leaf expansion but subsequent to leaf expansion, fruit were more highly labeled with N derived from current‐year uptake than leaves. Pistillate flower abortion was coincident with an apparent competition for N among developing vegetative and reproductive organs and preceded the period of significant N contribution from current‐year uptake.

Estimates of leaflet and fruit macronutrient (N, P, K, Ca, and Mg) accumulation 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 (late June to September); and leaf senescence (September to November)]. Crop load influenced total nutrient content per tree in annual organs (leaves and fruit), the relative allocation of nutrients between leaves and fruit, temporal patterns of nutrient accumulation in annual organs, and the magnitude of net leaf nutrient resorption per tree prior to leaf fall. In off-year trees, macronutrient accumulation in annual organs (leaves) was concentrated during the spring flush of growth. In contrast, significant macronutrient accumulation in annual organs of on-year trees (leaves plus fruit) occurred not only during the spring flush of growth but also during seed fill. Duration and magnitude of macronutrient accumulation were greater in on-year vs. off-year trees. Fruit 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 nutrient content during seed fill were equivalent to 32% and 26% of embryo accumulation of N and P, respectively. Fruit demand for N, P, and K during the spring flush of “on” years was accompanied by reduced leaf N, P, and K contents per tree. Net leaf N, Ca, and Mg resorption per tree during leaf senescence differed with crop load. Net leaf N resorption was significantly greater in off-year trees than on-year trees. Leaf N resorption presumably represents an important component of the N pool stored in perennial tree parts during dormancy. The greater leaf N resorption following “off” years was a function of greater leaf N concentration and greater leaf biomass per tree. In contrast, net leaf 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-bearing cycle, especially in light of the potential contribution of current fertilization practices to groundwater contamination.

Four adjacent heavily cropping 12-year-old `Petite d'Agen' prune (Prunus domestica L.) trees were selected, and two of the trees were defruited in late spring (28 May) after the spring growth flush and full leaf expansion. Trees received K daily through the drip-irrigation system, and 15N-depleted (NH4)2SO4 was applied twice between the dates of defruiting and fruit maturation. Trees were excavated at the time of fruit maturity (28 July) and fractionated into their component parts. The following determinations were made after tree excavation and sample processing: tree dry weight, dry weight distribution among the various tree fractions (fruit, leaves, roots, trunk, and branches), tree nutrient contents, within-tree nutrient distribution, total nonstructural carbohydrates (TNCs), and recovery of labeled N. Trees only recovered ≈3% of the isotopically labeled fertilizer N over the 6-week experimental period. Heavily cropping trees absorbed ≈9 g more K per tree (17% of total tree K content) during the 2-month period of stage III fruit growth than defruited trees. The enhanced K uptake in heavily cropping trees was apparently conditioned by the large fruit K demand and occurred despite greatly reduced levels of starch and TNCs relative to defruited trees. Fruit K accumulation in heavily cropping trees was accompanied by K depletion from leaves and perennial tree parts. Except for K, fruited and defruited trees did not differ in nutrient content.