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  • Author or Editor: S. Southwick x
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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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Abstract

Ethylene production by senescing flowers of calamondin (Citrus madurensis Lour.), at rates as high as 15 nl/g fresh weight-hour did not necessarily induce absicission. Moreover, combinations of gibberellic acid (GA), calcium dihydrophosphate and 6-benzylamino purine (BA), which are known to increase fruit set in citrus, did not significantly decrease ethylene production. Abscission of calamondin fruitlets and increases in fruit set appear to be independent of ethylene production.

Open Access

Abstract

Whole-tree sprays of gibberellic acid (GA) plus calcium dihydrophosphate Ca(H2PO4)2 increased fruit set of navel orange [Citrus sinensis (L.) Osb.] during 1979 and 1980. Gibberellic acid alone or a combination with Ca(H2PO4)2 or 6-benzylamino purine (BA) increased fruit set in 1980. Benzylamino purine did not increase fruit set or the fruit-setting effectiveness of GA. Calcium dihydrophosphate increased fruit set for approximately 5.5 weeks in 1980 even though leaves did not show signs of calcium deficiency. However, no increase in fruit set was observed 8.5 weeks after application. Fruit sprayed with GA were smaller than untreated fruit initially; however, no size differences were noted 8.5 weeks after full bloom.

Open Access

Soil applied potassium (K) may not alleviate K deficiency in fine textured California soils when high numbers of prunes per tree are produced leading to leaf necrosis and limb death. Because K demand is increased by fruit, K nitrate (KN) sprays appear to be a corrective option for growers in this situation. Our objectives were to determine best seasonal KN spray liming strategies to minimize K deficiency, quantify K uptake into leaves after spray and to evaluate spray effects on productivity. Results indicated that regardless of spray timing leaf K was increased by approximately 0.3% and three weeks later decreased 0.2%. Average leaf K in sprayed trees was 0.7% higher than untreated trees at harvest. Fruit fresh to dry weight ratios were lower (better) from summer sprayed trees than spring. Summer KN sprayed trees had yield efficiencies equal to those having soil applied K. Fruit size was similar regardless of K application method. Foliar KN sprays may be a viable K augmentation to soil application in heavy crop years on fine textured soils.

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Abstract

Treatments that inhibit production of citrus flowers, such as the presence of fruit, gibberellic acid (GA3) spray, and branch pruning, were examined on container-grown ‘Tahiti’ lime (Citrus latifolia Tan.) trees induced to flower by severe water stress lasting from 2 to 5 weeks. The presence of (or effects of recently removed) fruit located at the apex of branches inhibited production of new shoots as well as expression of the vegetative and floral nature of those shoots in basipetal lateral buds. Fruit ranging in size from 0.76 to 5.0 cm in equatorial diameter inhibited the shooting response. Shoot production and floral expression were normal on other branches. Fruit size or the amount of time fruit were present on branches were associated with the reduction in shoot and flower production. Aqueous GA3 sprays at 10−3 m concentration, applied at the onset or end of the floral inductive water stress treatment, inhibited expression of both vegetative and floral shoots. This inhibitory response dissipated as time elapsed after treatment. Pruning three branches from each tree replicate did not modify the flowering response in nonpruned water-stressed branches. The shooting response of branch units on these small trees responded independently to the presence or absence of fruit, GA3 treatment, and branch pruning.

Open Access

`Bing' sweet cherry is the most widely planted cultivar grown in the Western US because of widespread market acceptance. High prices are associated with early maturing `Bings' so growers are inclined to plant in early maturing growing regions. High numbers of less marketable, abnormally shaped (deep sutures, spurs, doubles) fruit tend to be produced in these regions. It is thought that abnormal fruit development is associated with high summer temperatures. Dataloggers equipped with thermocouples were located in 7 California cherry growing regions. Thermocouples were positioned throughout tree canopies, monitoring flower bud temperatures for 2 seasons from May to October. A Richard's function was used to describe the relation of average daily temperature (July, August, September to the percentage of fruit with deep suture. Correlation coefficients (R2) of 0.85 and higher were found, with increases in average daily temperatures above 22C associated with the formation of abnormal fruit shapes. Heat lamps were used to increase spur temperatures 5-7C above ambient during the July through September period, High percentages of abnormal flowers were produced in the season after 2 July, but not after 21 August heating, Strategies to lower high summer canopy temperatures helped to reduce abnormal fruit shapes.

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Lack of pollen dispersal was noted in various sites and cultivars of sweet cherry (Prunus avium) following one of California's warmest recorded winters (≈550 hours @ 7°C in the Central Valley). `Bing' cherry is thought to require 850 to 880 hours for adequate budbreak and bloom development. Cross pollination is required by most sweet cherry cultivars for fruit set, including `Bing'. Complete anther dehiscence averaged 13% in `Bing' trees sampled, compared to 52% in `Rainier', 65% in `Brooks', 84.5% in `Burlat', 33% in Van, 23% in `Larian', and 86% in `Black Tartarian'. A range of degree of dehiscence from none to half-open was widely apparent, again by cultivar. Many partially dehiscent anthers did not shed pollen normally but appeared to have the mass of pollen completely adherent inside the pollen sacs. `Black Tartarian', `Larian', and `Burlat' shed pollen readily, however, pollen from dehiscent anthers of other cultivars generally appeared to stick together on the everted locule walls and required direct manipulation to be withdrawn from the pollen sac. Anther morphology ranged from normal size to half normal size, anthers appearing to be without pollen altogether that shriveled on drying, and lobes that were aborted. Pollen germination was low overall: 19% `Bing', 18% `Rainier', 20% `Brooks', 57% `Burlat', 14% `Van', 48% `Larian', and 48% `Black Tartarian'. Poor fruit set in low chill years is often attributed to lack of bloom overlap with pollenizers, however, inadequate chilling also may contribute to low fruit set by inhibiting anther and pollen growth and development. The implications of a critical chilling requirement for normal floral differentiation are that in cherry-growing areas where low chill years are common, pollen may not be viable or transferrable from pollenizers and female gametophytic development also may be impaired.

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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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During the fruit growing season, April through August 1990, 1991, and 1992, four sprays of 20-22 liters/tree of KNO3 were applied to `French' prune trees (Prunus domestica L. syn. `Petite d'Agen). Spray applications of KNO3 were compared to single annual soil applications of KCl (1.4-2.3 kg/tree) and sprays of urea + KNO3 with respect to leaf K and N, fruit size, drying ratio, and dry yield. Potassium nitrate sprays were as effective, or better, than soil-applied K in maintaining adequate levels of leaf K throughout the season. Treatment effects were not carried over into the next year. Lowest leaf K was found in trees where no K had been applied. Those values were below the adequate level of 1.3% K and the untreated group developed K deficiency symptoms. Consistent effects on leaf K were not obtained when urea was applied and no negative effect on leaf K was demonstrated. Equivalent dry yields per tree were obtained by foliar and soil K applications. There was no best time for KNO3 sprays. Yield per tree was not enhanced when foliar K-N sprays were applied to trees that had levels of 1.3% K or more as of 15 Apr. 1992. Trees that were below optimum K in April tended toward improved dry yields after four K-N sprays. Trees that had no applied K were lowest yielding. Drying ratios and fruit size (number of fruit per kilogram) were not different among K treatments. Dry yields per tree were increased without a decrease in fruit size or an increase in drying ratio with either soil or foliar K application. These results suggest that foliar KNO3 sprays applied four times throughout the growing season can be used to correct incipient K deficiency in `French' prune and to obtain dry yields equivalent to those obtained with soil applications of KCl.

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A 2-ac block of `Bing'/Colt, Mahaleb or GM61/1 was planted 8×15′, Feb 1991 so that 7 treatments were arranged in 6 blocks. We imposed water stress during 2 distinct periods, June or July in 1992. Once trees had stopped growing water was added (estimated @ 50% ETc) to maintain leaf turgor, but not to initiate further shoot growth. Well-watered (100% ETc) trees were summer pruned to check growth. Water stress was monitored by measuring water potential of basal bagged leaves with a pressure chamber at midday. Shoot growth was stopped by water stress. Shoot growth ceased at a trunk water potential of approximately -17 bars in both Colt and Mahaleb rootstocks although Colt rootstock went into stress more rapidly than Mahaleb. Colt may be more sensitive to water deprivation at this site than Mahaleb rootstock. Well-watered GM 61/1 trees had tree sizes much like those of water stressed Colt trees. A combination of early season stress and summer pruning (heading) may be the best combination used to control growth. Flower bud formation was noticed by season's end in these 2nd leaf trees. Water stress can be used to control tree growth, save water and encourage fruitfulness when integrated into an appropriate orchard system.

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