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Harold McCutchan and K.A. Shackel

The relative sensitivity of plant- and soil-based measures of water availability were compared for prune trees subjected to a range of irrigation regimes under field conditions. Over the growing season, leaf- and stem-water potentials (ψ) measured at midday exhibited clear differences between frequently irrigated trees and unirrigated trees that were growing on stored soil moisture. Stem ψ was less variable than leaf ψ, and the daily variability in stem ψ was closely related to daily variability in evaporative demands, as measured by vapor pressure deficit (VPD). As a result of lower variability, stem ψ reflected the small stress effect of a moderate, 50% soil moisture depletion irrigation interval, whereas leaf ψ did not. The relation between soil water content and estimated orchard evapotranspiration (ET) was influenced by local differences in soil texture within the experimental plot. The relation between stem ψ and ET, however, was not influenced by soil texture and, in addition, was very similar to the relation between stem ψ and leaf stomatal conductance. Both relationships indicated that a 50% reduction in leaf and canopy level water loss characteristics was associated with relatively small reductions (0.5 to 0.6 MPa) in stem ψ. Stem ψ appears to be a sensitive and reliable plant-based measure of water stress in prune and maybe a useful tool for experimental work and irrigation scheduling.

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M.T. Stevenson and K.A. Shackel

Whole-tree effects of alternate bearing on the reproductive growth, vegetative growth, and carbohydrate storage in mature pistachio (scion Pistacia vera L., rootstock Pistacia atlantica Desf.) were examined. Although it is generally accepted that the “off” year in pistachio may be used to accumulate carbohydrate reserves, it was found that starch and sugars from the “off” year equaled only 8% of the construction cost of the “on” year's fruit load. The dry mass increase in perennial woody tissue of roots, trunk and branches in the “off” year (20.2 kg/tree), was 75% of the mass of fruit in “on” trees (26.4 kg/tree). When construction cost of wood and fruit is considered, “off” year woody tissue growth is 64% of “on” year fruit growth. Since previous studies have shown that leaf dry mass is generally 30% greater in “off” than “on” pistachio trees, we conclude that the overall production of biomass in “off” and “on” year pistachio is similar. Therefore, we suggest that the “off” year is not a period of substantial storage carbohydrate accumulation, but rather a switch in allocation from reproductive to vegetative growth. This is consistent with ecologically based theories of the adaptive advantage of masting behavior, and suggests that withholding water and fertilizer during the “off” year may not harm “on” year yield.

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K.A. Shackel, V. Novello, and E.G. Sutter

The relative contribution of stomatal and cuticular conductance to transpiration from whole tissue-cultured apple shoots of Malus pumila Mill. M.26 was determined with a modified steady state porometer. When shoots were exposed to 90% RH and high boundary layer conductance, large (73%) and, in some eases, rapid (2 to 3 hours) reductions in leaf conductance occurred, indicating functional stomata. Stomatal closure was also observed microscopically. A maximum estimate for the cuticular conductance of these apple leaves was 18 to 40 mmol·m-2·s-1, which is lower than previous estimates and close to the upper limit of naturally occurring leaf cuticular conductances. Hence, both stomatal and cuticular restrictions of water loss appear to be of importance in determining the water balance of tissue-cultured apple loots. The pathway of water transport in relation to water stress of tissue-cultured shoots is also discussed.

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M.T. Stevenson, K.A. Shackel, and L. Ferguson

Pistachio (Pistacia vera L.) is known to strongly exhibit alternate bearing. Over 19,500 individual shoots were measured on eight alternate bearing `Kerman' pistachio trees on P. atlantica Desf. rootstock. Average length of “on” year (5.4 cm) and “off” year new shoots (5.6 cm) were not significantly different. New shoot length distribution was skewed toward the shorter length categories, with a mode of 2 and 4 cm in “on” and “off” trees respectively. These results contrast with previous studies which have shown that “on” year new shoots of pistachio are much longer than “off” year new shoots. There were about twice as many “on” year new shoots in the 18 to 30 cm class as compared to “off” new shoots, and fruiting wood length was associated positively with fruit number. However, ≈80% of tree yield occurred on fruiting wood that was <10 cm long, with shoots longer than 15 cm contributing <5% to yield. We suggest that results from earlier studies regarding the bud abscission process in very long pistachio shoots should be confirmed on shorter shoots, which contribute significantly to yield. Shoots >30 cm in length may be important in establishing vegetative buds in a position above the main tree crown for canopy expansion during the following “off” year.

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Bruce Lampinen, K. A. Shackel, S. Southwick, D. Goldhamer, and B. Olson

During this three year study, irrigation water was withheld from trees in a commercial drip irrigated french prune orchard (Butte County, CA), during different periods within the double sigmoid fruit growth pattern (stage I - III), and postharvest. Tree water stress associated with early season water deprivation was minimal, due to the presence of stored soil moisture and low evaporative demands. For mid and late season water deprivation there was no fruit growth stage that was particularly sensitive to water stress, although severe and prolonged stress caused smaller fruit with lower quality. For the three year average, irrigation treatments caused no statistically significant effects on fruit set or drop relative to the control, however most of the stress treatments increased return bloom relative to the control, resulting in higher fruit loads and higher yields. These results suggest that moderate water stress may enhance economic prune productivity.

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D.E. Kester, T.M. Gradziel, K.A. Shackel, and W.C. Micke

Noninfectious bud-failure (BF) is a genetic disorder in almond, associated with nursery source selection. Previously (Kester, PASHS, 1968), the latent potential for BF (BFpot) was shown to be heritable but its phenotypic expression (BFexp) varied among individual seedlings of a populations as a function of age. Vegetative propagation perpetuates BFpot of individual propagules (Kester and Asay, JASHS, 1978b) but the subsequent age of BFexp within individual plants is a function of accumulated exposure to high summer temperature and growth (Kester and Asay, JASHS 1978a). A recent 7-year “somatic heritability” study of 12 commercial nursery sources (Kester et al., HortScience 1998abst) portrays the total range of variability of BFpot and BFexp within the entire `Carmel' almond clonal population and includes a pattern of BF increase in consecutive vegetative propagation cycles that mimics patterns produced by phase change (i.e., juvenile > mature) phenomena (Hartmann et al., 1997). Although phase change potential is heritable in seedling populations, phase change expression is not (Kester, HortScience 1983). Furthermore phase changes can be reversed under particular conditions during consecutive vegetative propagations (Hartmann et al., 1997). In contrast, evidence shows that BF produces permanent changes in genotype that are heritable and irreversable. High correlations exist between BFpot of individual source blocks, individual trees and individual budsticks and the age and severity of BFexp in progeny trees. The apparent continuous change in BFpot and BFexp within clones appears to be the pattern of expression of different populations of increasingly defective (?) somatic cells that result from consecutive sequences of change during annual cycles of growth and generations of vegetative propagation.

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Kenneth A. Shackel, R. Scott Johnson, Charles K. Medawar, and Claude J. Phene

The heat balance method was used to estimate transpirational sap flow through 60- to 75-mm-diameter stems (trunks) of 3-year-old peach [Prunus persica (L.) Batsch. cv. O'Henry] trees under field conditions. On rare occasions, heat balance estimates agreed well with independent lysimetric measurements, but on most occasions, heat balance estimates of sap flow were unrealistic in both direction and magnitude. In some cases, the errors in sap flow approached two orders of magnitude and were always the result of a calculation involving division by a very small and sometimes negative temperature differential between the stem surface temperature above and below the gauge heater. The occurrence of negative temperature differentials under positive transpiration conditions may be inconsistent with a fundamental assumption in the heat balance model, namely that temperature differentials are solely a consequence of the dissipation of energy supplied to the gauge heater. In the absence of heating power applied to the gauge, temperature differentials exceeding - 1C were correlated with the rate of change in stem temperature, indicating that ambient conditions themselves can impose a bias in gauge signals and, hence, influence gauge accuracy. Our results suggest that the effect of ambient conditions on gauge signals should be critically evaluated before considering heat balance estimates of sap flow as reliable under any given conditions.

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S.M. Southwick, S.A. Weinbaum, T.T. Muraoka, W.R. Krueger, K.A. Shackel, and J.T. Yeager

Leaf dry weight per leaf area (LDW/LA); weight of leaf N per unit leaf area (LN/LA); leaf dry weight (LDW); and fruit quality, particularly sugar per fruit (SF); fruit fresh weight (FFW); and fruit dry weight (FDW) were measured over a range of daily average incident photosynthetic photon flux values (PPF) (50 to 1000 μmol·s-1·m-2) in 7-year-old prune (Prunus domestics L. syn. `Petite d'Agen') tree canopies. Linear or curvilinear relationships between these leaf attributes and fruit characteristics were significant over the PPF range. Analysis of LDW/LA or LN/LA may be used to indicate tree canopy locations in which fruit size and quality is limited by suboptimal PPF.

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K.A. Shackel, B. Lampinen, S. Southwick, W. Olson, S. Sibbett, W. Krueger, J. Yeager, and D. Goldhamer

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Kenneth A. Shackel, H. Ahmadi, W. Biasi, R. Buchner, D. Goldhamer, S. Gurusinghe, J. Hasey, D. Kester, B. Krueger, B. Lampinen, G. McGourty, W. Micke, E. Mitcham, B. Olson, K. Pelletrau, H. Philips, D. Ramos, L. Schwankl, S. Sibbett, R. Snyder, S. Southwick, M. Stevenson, M. Thorpe, S. Weinbaum, and J. Yeager

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.