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John M. Nelson, David A. Palzkill, and Paul G. Bartels

Flower bud injury resulting from freezing temperatures has been a major problem in jojoba [Simmondsia chinensis (Link) C. Schneid.] production. A 3-year field study, which began with 4-year-old plants, evaluated the effect of three irrigation treatments on growth, flower bud survival, seed yield, seed weight, and seed wax concentration of six clones. After 3 years, irrigation cut-off dates of late May (dry treatment) and early September (medium treatment) resulted in reduced plant height and width compared to irrigating through November (wet treatment). Flower bud survival and seed yields were very low in the first year for all treatments. In the second and third years, bud survival for most clones, even at -8C, was greatly improved by withholding water in the fall. In December of the second and third years, plants in the medium and dry plots had lower leaf water potential than those in the wet plot. In the second year, plants in the medium and dry plots had seed yields that were 3.5 times higher and wax yields that were were 2.3 times higher than plants in the wet plot. In the third year, the medium treatment had the highest seed and wax yields. Average seed weight and seed wax concentration were generally highest for plants in the wet plot where seed yields were low. Withholding irrigation from jojoba in the fall appears to improve flower bud survival and seed and wax yields following cold winters.

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Agnes A. Flores-Nimedez, Paul H. Li, and Charles C. Shin

GLK-8903, an experimental product whose main ingredient is produced by hydrogenation of a primary alcohol extracted from plants, showed significant potential in protecting bean (Phaseolus vulgaris L.) plants from chilling injury. The GLK-8903 protection mechanism was assessed by examining several physiological and biochemical responses. The decline in leaf water potential and the increase in osmotic potential caused by chilling exposure to 4C (day/night) were minimized by the application of GLK-8903. Chilling causes an increase in electrolyte leakage, an indication of chilling injury of the plasma membrane. Increased electrolyte leakage was reduced significantly in the GLK-8903-treated plants during chilling. This minimized leakage may be due to less damage of the plasma membrane. Plasmolysis and deplasmolysis studies of the epidermal cells suggest that GLK-8903 is able to reduce the plasma membrane perturbation in the chilling environment, as evident by: 1) the lower permeability coefficient to urea at 4C, and 2) the swelling of protoplasts in the cells of untreated tissues after chilling exposure with no swelling of the protoplast being observed in the GLK-8903-treated cells. Malondialdehyde (MDA), a product of lipid peroxidation, increased more in untreated controls than in treated plants exposed to 4C. Plasma membrane ATPase activity decreased less in GLK-8903-treated plants than in untreated controls after 3 days at 4C. The mechanism of GLK-8903-alleviated chilling injury is discussed.

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David Goldhamer, Mario Viveros, and Ken Shackel

Previously well irrigated mature `Nonpareil' almond trees (Prunus dulcis) were subjected to varying periods of water deprivation prior to harvest and then to either full or no postharvest irrigation. Eight preharvest water deprivation (PWD) lengths ranging from 14 to 63 days were evaluated on a sandy loam soil with a rooting depth of about 1.5 m.

Development of tree water deficits occurred rapidly following PWD. Predawn leaf water potential decreased to about -1.8 and -3.1 MPa after 10 and 20 days, respectively. Defoliation began about 30 days after PWD and trees subjected to more than 50 days completely defoliated. The rate of hull split was directly related to the PWD duration. With early cutoffs, the size of the hull split-arrested nuts at harvest was large compared with the same nut type in later cutoffs suggesting that as nuts develop, large nuts are preferential sinks for assimilates. Kernel size was only mildly reduced by PWD during the first study year. There was a trend toward lower total kernel yield with longer PWD as a result of smaller kernel girth but yield differences were not significant. The number of nuts remaining in the tree after shaking was not related to PWD. Bark strength increased after PWD with 10 to 14 days required to prevent shaker damage. Postharvest irrigation resulted in late season defoliation but no rebloom. Bloom density reductions in 1990 were related more to the lack of 1989 postharvest irrigation than to early PWD.

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Thomas J. Tworkoski, Michael E. Engle, and Peter T. Kujawski

A polypropylene fabric containing control-release pellets of the herbicide, trifluralin, can be oriented in the soil to regulate the distribution of plant roots. In 1990, trenches were dug near 10-year-old red oak (Quercus rubra L.) and 10-year-old yellow poplar (Liriodendron tulipifera L.) and fabric containing trifluralin control-release pellets and polypropylene fabric alone were installed vertically to redirect root growth. Roots grew alongside trifluralin fabric and fabric alone and did not penetrate either fabric 38 months after installation. Shoot growth of yellow poplar was reduced about 47% each year by the trifluralin fabric treatment compared to control. Red oak shoot growth was not affected by trifluralin fabric. Leaf water potential was not affected by treatment in either species. Trifluralin residues in trifluralin fabric decreased from 23.3% to 22.0% from July 1990 to October 1993. During this time, trifluralin levels increased from 0.4 to 3.6 mg·kg-1 in soil sampled 0 to 15 cm below trifluralin fabric. These results suggest that controlled-release trifluralin will provide persistent inhibition of root and shoot growth of some species and will not migrate significantly in the soil. Chemical names used: α,α,α-trifluoro-2,6-dinitro-N-N-dipropyl-p-toluidine (trifluralin).

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Todd C. Einhorn, Horst W. Caspari, and Steve Green

Approach-grafted 1-year-old `Gala'/M7 apple trees were grown with both tops for the remainder of the 2003 season in a greenhouse. Trees were supplied with >100% (control, PRD100) or 50% (PRD50, DI50) of daily ETc either applied to one root compartment only (PRD100, PRD50) or divided evenly across both root compartments (control and DI50). ETc was estimated from gravimetric measurements, and irrigation was switched between wet and dry root compartments several times throughout the experiment. Soil moisture was measured both gravimetrically (tripod) and volumetrically (time-domain reflectometry). Predawn leaf water potential (υpd) and single leaf gas exchange (photosynthesis, stomatal conductance, and transpiration) were recorded daily, and sap flow in stems and roots was monitored continuously using the heat-pulse technique. Leaves were collected for abscisic acid (ABA) determination following gas exchange measurements. Regardless of irrigation placement (i.e., PRD or DI), both 50% ETc treatments experienced similar declines in υpd and single leaf gas exchange relative to control levels. In addition, ABA concentrations were similar for PRD50 and DI50, and were significantly higher than the control and PRD100 treatments. PRD100 trees had similar υpd as control trees; however, gas exchange was reduced >25% compared to the control. Bulk leaf ABA concentration did not differ significantly from control levels and does not by itself explain the down regulation of stomata with PRD100.

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William W. Inman and William L. Bauerle

Recent work has shown that stomatal conductance (gs) and net photosynthesis (Anet) are responsive to the hydraulic conductance of the soil to leaf pathway (Xp). Two tree species with differing xylem structures were used to study the effect of systematic manipulations in Xp that elevated xylem hydraulic resistance. Simultaneous measures of gs, Anet, bulk leaf abscisic acid concentration (ABAL), leaf water potential (L), and whole plant transpiration (Ew) were taken under controlled environment conditions. Quercus shumardii Buckl. (shumard oak), a ring porous species and Acer rubrum L. `Summer Red' (red maple), a diffuse porous species, were studied to investigate the short-term hydraulic and chemical messenger response to drought. Both species decreased Anet, gs, L, and Ew in response to an immediate substrate moisture alteration. Relative to initial well-watered values, red maple Anet, gs, and Ew declined more than shumard oak. However, gs and Anet vs. whole-plant leaf specific hydraulic resistance was greater in shumard oak. In addition, the larger hydraulic resistance in shumard oak was attributed to higher shoot, as opposed to root, system resistance. The results indicate hydraulic resistance differences that may be attributed to the disparate xylem anatomy between the two species. This study also provides evidence to support the short-term hydraulic signal negative feedback link hypothesis between gs and the cavitation threshold, as opposed to chemical signaling via rapid accumulation from root-synthesized ABA.

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G. S. Sibbett, D. Goldhamer, S. Southwick, R.C. Phene, J. Yeager, and D. Katayama

Variable lengths of water deprivation immediately prior to harvest were imposed on mature French prune trees for four consecutive years. Irrigation cutoff durations were about 45, 37, 30, 22, 17 and 12 days prior to harvest during 1986-89.

Predawn leaf water potential best reflected water deprivation length and reached minimum values of about -1.5 MPa with the most severe cutoff. Magnitude of peak stomatal conductance was reduced and occurred earlier in the day with longer cutoff regimes.

Rate and time-course development of preharvest fruit drop was variable from year-to-year, but there were no significant differences in total drop between cutoff treatments. Only in the fourth year, following three years of no difference were tree fruit load and yield significantly reduced but then only with the most severe cutoff. Soluble solids were higher and drying ratios lower with the longer cutoffs. Fruit size was significantly reduced in the third year of the experiment. Trunk circumferences were significantly lower only with trees subjected to the longer cutoff regimes.

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Albert Liptay, Peter Sikkema, and William Fonteno

The theme of this review is modulation of extension growth in transplant production through restraint of watering of the seedlings. The purpose of the modulation is to produce transplants of 1) appropriate height for ease of field setting and 2) adequate stress tolerance to withstand outdoor environmental conditions. Physiological responses of the plant are discussed in relation to the degree of water deficit stress and are related to the degree of hardening or stress tolerance development in the transplants. Optimal stress tolerance or techniques for measuring same have not been fully defined in the literature. However, stress tolerance in seedlings is necessary to withstand environmental forces such as wind and sand-blasting after the seedlings are transplanted in the field. It is also imperative that the seedlings undertake a rapid and sustained rate of growth after outdoor transplanting. Water deficit stress applied to plants elicits many different physiological responses. For example, as leaf water potential begins to decrease, leaf enlargement is inhibited before photosynthesis or respiration is affected, with the result of a higher rate of dry matter accumulation per unit leaf area. The cause of the reduced leaf area may be a result of reduced K uptake by the roots with a concomitant reduction in cell expansion. Severe water deficits however, result in overstressed seedlings with stunted growth and poor establishment when transplanted into the field. In transplant production systems, appropriate levels of water deficit stress can be used as a management tool to produce seedlings conducive to the transplanting process.

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Jeffrey Melkonian and David W. Wolfe

Cucumber (Cucumis sativus L. cv. Marketmore 80) plants were exposed to a soil water deficit and subsequently rewatered. Maximum stress intensity was -1.5 MPa midday leaf water potential compared to -0.6 to -0.8 MPa in the well watered control, eight days after withholding water. Midday stomatal conductance {ks), leaf turgor potential and water potential decreased in the stress treatment compared to the control beginning at the first sampling, two days after withholding water. The decrease in all three was approximately linear with time over the stress. Decreased leaf elongation was observed at the second sampling, three days after the initial decline in ks and five days after withholding water. At similar relative water content {RWC), osmotic potentials of the stress and control treatments were the same throughout most of the stress. Further, there was no difference in osmotic potential, at the same RWC, between the stress and control treatments 12 - 16 hours after rewatering. Split-root experiments were also conducted to examine a possible role of a non-hydraulic signal from roots in drying soil in the regulation of ks and leaf elongation in cucumber. No conclusive evidence of a signal was found despite significant decreases in soil water potential of one-half of the root system of the stress plants. However, fluctuating vapor pressure gradients (vpg) may have obscured evidence of a signal.

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William R. Graves, Robert J. Joly, and Michael N. Dana

Honey locust (Gleditsia triacanthos var. inermis Wind.) and tree-of-heaven Ailanthus altissima (Mill.) Swingle] sometimes are exposed to high root-zone temperatures in urban microclimates. The objective of this study was to test the hypothesis that seedlings of these species differ in how elevated root-zone temperature affects growth, leaf water relations, and root hydraulic properties. Shoot extension, leaf area, root: shoot ratio, and root and shoot dry weights were less for tree-of-heaven grown with the root zone at 34C than for those with root zones at 24C. Tree-of-heaven with roots at 34C had a lower mean transpiration rate (E) than those grown at 24C, but leaf water potential (ψ1) was similar at both temperatures. In contrast, shoot extension of seedlings of honey locust grown with roots at 34C was greater than honey locust at 24C, E was similar at both temperatures, and ψ1 was reduced at 34C. Hydraulic properties of root systems grown at both temperatures were determined during exposure to pressure in solution held at 24 or 34C. For each species at both solution temperatures, water flux through root systems (Jv) grown at 34C was less than for roots grown at 24C. Roots of tree-of-heaven grown at 34C had lower hydraulic conductivity coefficients (Lp) than those grown at 24C, but Lp of roots of honey locust grown at the two temperatures was similar.