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.

Six cross-incompatibility groups, which contain most of commercially important California almond cultivars [Prunus dulcis (Mill.) D.A. Webb, syn. Prunus amygdalus Batch], and their self-incompatibility (S) allele genotypes are identified. Incompatibility groups include `Mission' (SaSb), `Nonpareil' (ScSd), and the four groups resulting from the `Mission' × `Nonpareil' cross: (SaSc), (SaSd), (SbSc), and (SbSd), as represented by `Thompson', `Carmel', `Merced' and `Monterey', respectively. All seedlings from the `Mission' × `Nonpareil' cross were compatible with both parents, a result indicating that these two cultivars have no alleles in common. Crossing studies support a full-sib relationship for these progeny groups and the origin of both parents from common germplasm. Cultivars in these six groups account for ≈ 93% of present California production, a result demonstrating a limited genetic base for this vegetatively propagated tree crop.

The spatial and temporal pattern of noninfectious bud failure (BF) expression (BFexp) was studied during seven growing seasons in a population of `Carmel' almond trees originating from twelve commercial propagation sources. All progeny trees were grown in a single experimental site with high prevailing summer temperatures. BFexp increased continuously but irregularly in each nursery population as measured as the proportion of trees showing BF and as an average BFexp rating. Populations from the 12 nurseries represented increasing clonal generations from the original seedling tree and showed increasing levels of BF, as well as a decreasing shape value and increasing scale value derived by a failure statistics model. Models for development, distribution and hazard functions were defined for each of the 12 sources studied. Only sources from the original tree and source A demonstrated potential for commercial use. A significant correlation was found between average yearly increase in BFexp and the average daytime temperature for the previous June. The June period coincides with a specific stage in the seasonal growth cycle when vegetative buds mature.

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Fruits of ‘Delicious’ apple (Malus domestica Borkh.) were grown under differing radiant flux densities (rfd nm) from 45 days post-bloom until harvest. The rfd nm affected red fruit color, soluble solids, starch content and size, but not firmness, pH or total acidity at harvest or after 105 days of storage at −0.5°C. Levels of rfd nm sufficient to enhance red color development in red sports of ‘Delicious’ were not necessarily sufficient to insure flesh quality.