To be useful for indicating plant water needs, any measure of plant stress should be closely related to some of the known short- and medium-term plant stress responses, such as stomatal closure and reduced rates of expansive growth. Midday stem water potential has proven to be a useful index of stress in a number of fruit tree species. Day-to-day fluctuations in stem water potential under well-irrigated conditions are well correlated with midday vapor-pressure deficit, and, hence, a nonstressed baseline can be predicted. Measuring stem water potential helped explain the results of a 3-year deficit irrigation study in mature prunes, which showed that deficit irrigation could have either positive or negative impacts on tree productivity, depending on soil conditions. Mild to moderate water stress was economically beneficial. In almond, stem water potential was closely related to overall tree growth as measured by increases in trunk cross-sectional area. In cherry, stem water potential was correlated with leaf stomatal conductance and rates of shoot growth, with shoot growth essentially stopping once stem water potential dropped to between −1.5 to −1.7 MPa. In pear, fruit size and other fruit quality attributes (soluble solids, color) were all closely associated with stem water potential. In many of these field studies, systematic tree-to-tree differences in water status were large enough to obscure irrigation treatment effects. Hence, in the absence of a plant-based measure of water stress, it may be difficult to determine whether the lack of an irrigation treatment effect indicates the lack of a physiological response to plant water status, or rather is due to treatment ineffectiveness in influencing plant water status. These data indicate that stem water potential can be used to quantify stress reliably and guide irrigation decisions on a site-specific basis.

Over-fertilization (i.e., the application of fertilizer nitrogen (N) in excess of the tree or vine capacity to use it for optimum productivity) is associated with high levels of residual nitrate in the soil, which potentially contribute to groundwater and atmospheric pollution as a result of leaching, denitrification, etc. Overfert-ilization also may adversely affect productivity and fruit quality because of both direct (i.e., N) and indirect (i.e., shading) effects on flowering, fruit set, and fruit growth resulting from vegetative vigor. Pathological and physiological disorders as well as susceptibility to disease and insect pests also are influenced by the rate of applied N. Over-fertilization appears to be more serious in orchard crops than in many other crop species. The perennial growth habit of deciduous trees and vines is associated with an increased likelihood of fertilizer N application (and losses) during the dormant period. The large woody biomass increases the difficulty in assessing the kinetics and magnitude of annual N requirement. In mature trees, the N content of the harvested fruit appears to represent a large percentage of annual N uptake. Overfertilization is supported by a) the lack of integration of fertilizer and irrigation management, b) failure to consider nonfertilizer sources of plant-available N in the accounting of fertilizer needs, c) failure to conduct annual diagnosis of the N status, and d) the insensitivity of leaf analysis to over-fertilization. The diversity of orchard sites (with climatic, soil type, and management variables) precludes the general applicability of specific fertilization recommendations. The lack of regulatory and economic penalties encourage excessive application of fertilizer N, and it appears unlikely that the majority of growers will embrace recommended fertilizer management strategies voluntarily.

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Uptake and internal cycling of nitrogen (N) in mature walnut trees was studied over a period of 6 years using (15)N-depleted fertilizer and full-canopied walnut (Juglans regia L. cv Hartley) trees. The magnitude of internal N cycling, i.e., the availability of N for new growth from internal N pools, was quantified using both the percent annual depletion (PAD) and the N balance budget approaches. There was good agreement between the two measures, and about 60% of annual N demand was derived from N redistribution from internal pools. The remaining 40% of annual tree N demand was met by an influx of N from the soil/fertilizer pool. Trees were excavated, processed and analyzed after 6 years to determine total tree N content and labeled N recovery. Trees recovered 29.4% of the labeled N applied and, based on previous evidence, we assumed that tree accumulation of labeled N occurred entirely in the first year. Labeled N in the fruits and leaves harvested in the first year represented 26% of the total labeled N accumulated, and the remaining 74% of the labeled N accumulated that year was stored and used to support development of annual organs in subsequent years. In the first year, the early maturing catkins did not accumulate labeled N, indicating their exclusive reliance on internal N. Using the atom% (14)N excess values of catkins and an exponential decline equation to determine turnover rate, the Mean Residence Time (MRT) of storage N in the tree was estimated to be 2.0 years. The size of the cycling pool of storage N in the tree was estimated to be about 50% of the total N content of perennial tree parts. Our data support the hypotheses that: (1) in any given year, mature walnut trees store the majority of soil and fertilizer N absorbed and within 2 years following uptake the N is remobilized and used for new growth, and (2) about half of the total N content of the perennial parts of mature walnut trees is present as nonstructural N and is available for recycling.