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K. Shackel, S. Southwick and B. Lampinen

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. Methods for the measurement of plant water potential (Ψ) are available, but conflicting results have led to disagreement as to whether any of these give an appropriate biological index of plant water stress. Some pressure chamber results may be attributed to an artifact of water loss following excision. Leaf and stem Ψ however, in addition to being numerically different, may not be equivalent indices of plant stress, and midday stem Ψ has proven to be a useful index of stress in a number of fruit trees. Day to day fluctuations in midday stem Ψ under well irrigated conditions is well correlated to midday Vapor Pressure Deficit, and hence can be used to predict a non-stressed baseline. A 50% decline in water use at both the leaf and canopy level were associated with relatively small reductions (0.5 to 0.6 MPa) in midday stem Ψ from this baseline in prune. In cherry, midday stem Ψ was correlated to both leaf stomatal conductance and rates of shoot growth, with shoot growth essentially stopping once midday stem Ψ dropped to between -1.5 to -1.7 MPa. In pear, increased fruit size, decreased fruit soluble solids and increased green color were all associated with increases in midday stem Ψ.

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R.E. Moran, S.M. Southwick, K.G. Weis and B. Lampinen

Secondary or “rat-tail” bloom, a major site for fireblight infection of `Bartlett' pear, comprised 10% of the total bloom in 1997 and 20% in 1998. We are striving to find production practices that can be economically applied to reduce the number of “rat-tails.” Of the five known types of secondary clusters in pear, four occur on `Bartlett', the most numerous being types I and V. Type I rat-tails occur on the bourse at the base of normal clusters and bloom from 10 to 30 days after normal bloom. Type V rat-tails occur mostly at pruning sites and have one to three flowers per cluster, blooming 20 to 50 days after normal bloom. GA 3 or GA4+7 + BA were applied at 100 mg•L-1 in 1997 to reduce rat-tail bloom in 1998. In 1998, neither GA3 nor GA4+7 + BA had an effect on normal bloom or type I rattails. GA3 reduced type V rat-tails when applied at either 2 June, 2 July, or 15 Aug. but had no effect on type V clusters when applied at full bloom, petal fall, 16 June, or 15 July. GA4+7 + BA reduced the number of type V rat-tails when applied at either 2 June, 16 June, 2 July, and 15 July but had no effect when applied at full bloom, petal fall, or 15 Aug. Dormant pruning horizontal shoots resulted in as many rat-tails as vertical shoots, and heading cuts a similar number as stubbing cuts. Dormant pruning 1-year wood resulted in fewer rat-tails than 2-year wood. Summer pruning 21 or 49 days after bloom resulted in fewer rat-tails than pruning 10 days after harvest, but was similar to pruning 89 days after bloom. These and other results from ongoing work will be presented toward development of an integrated fire blight reduction strategy.

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

Prunes trees are believed to be relatively tolerant of water stress, and because prune fruit are dried, a low fresh to dry weight ratio of the fruit will reduce energy requirements for fruit drying and will represent an economic benefit to the grower. In previous research, we found that, under some orchard conditions, irrigation deprivation was associated with a number of economically beneficial effects, including a lower fresh to dry weight ratio of the fruit, increased return bloom, and final saleable crop yield. Analysis of these results was complicated by the effects of irrigation on alternate bearing, and the fact that tree water stress could be substantially different under different soil conditions for the same level of irrigation deprivation. Taking these factors into account, however, indicated that economic yield in prune could be maintained or increased by managing trees at a moderate level of water stress. An experiment was established to determine whether midday stem water potential could be used to guide irrigation and achieve a target level of water stress during the growing season, and whether a moderate level of water stress would be economically beneficial to prune production. By managing prune trees at a moderate level of water stress (midday stem water potential reaching about –1.5 Mpa by the end of the season) over 3 years, an average savings of 40% in applied irrigation water was obtained. Modest increases in return bloom, and an improved fruit dry to fresh weight ratio, occurred in moderately water stressed trees, although overall yield was not changed. The substantial savings in water, without reducing yield, should represent a net economic benefit to growers, depending on the price they pay for water.

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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.