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Jessica L. Boldt, James E. Barrett and David G. Clark

Petunia × hybrida `Electric Purple' plants, genetically transformed (Selecta Klemm Co.) via Agrobacterium tumefaciens to constitutively express the Cauliflower Mosaic Virus 35S promoter (CaMV35S) fused to two separate Arabidopsis c-repeat binding factor cDNAs (CBF3 & CBF4), were utilized to evaluate water relations. Non-stressed plants followed a classical stomatal conductance pattern, with maximum conductance between 1000 hr and 1400 hr. CBF3 and CBF4 plants showed an increase in transpiration rates and a decrease in stomatal resistance at 1230 hr, compared to `Electric Purple'. Transpiration rates (per unit leaf area) were similar in CBF3 and `Electric Purple' plants, but CBF4 plants were 12% less than `Electric Purple'. Xylem water potentials at visible wilt were between –1.4 and –1.5 MPa and there were no significant differences between line or irrigation treatment. A fourth experiment observed differential plant responses to stress cycles. Under non-stress irrigation conditions, CBF4 plants showed an increase in stomatal resistance and a decrease in transpiration rate compared to `Electric Purple' plants. There were no differences in the xylem water potential at visible wilt for the first and third stress cycles, but, for the second cycle, xylem water potentials at wilt were –1.9, –1.7 and –1.4 Mpa for CBF4, `Electric Purple' and CBF3 plants, respectively. CBF3 and CBF4 plants showed small differences in performance as compared to `Electric Purple' and under mild stress conditions as imposed in these experiments apparent heterologous overexpression of the Arabidopsis CBF3 & 4 transgenes may not be sufficient for conferring drought tolerance in petunia.

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C.C. Shock, E.B.G. Feibert and L.D. Saunders

Onion (Allium cepa L., `Great Scott') was grown on silt loam soils and submitted to four irrigation thresholds (-25, -50, -75, and -100 kPa) in 1992 and six irrigation thresholds (-12.5, -25, -37.5, -50, -75, and -100 kPa) in 1993 and 1994. Irrigation thresholds (soil water potential measured at 0.2-m depth) were used as criteria to initiate furrow irrigations. Onions were evaluated for yield and grade after 70 days of storage. In 1992 and 1994, total yield, marketable yield, and profit increased with increasing irrigation threshold. In 1993, total yield increased with increasing irrigation threshold, but marketable yield and profit were maximized by a calculated threshold of -27 kPa due to a substantial increase of decomposition during storage with increasing threshold.

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Allan Fulton, Richard Buchner, Cyndi Gilles, Bill Olson, Nick Bertagna, Jed Walton, Larry Schwankl and Ken Shackel

Covering a plant leaf with a reflective, water impervious bag ensures that equilibrium is reached between the nontranspiring leaf and the stem, and appears to improve the accuracy of determining plant water status under field conditions. However, the inconvenience of covering the leaf for 1 to 2 hours before measuring stem water potential (SWP) has constrained on-farm adoption of this irrigation management technique. A second constraint has been that the requirement of midafternoon determinations limits the area that can be monitored by one person with a pressure chamber. This paper reports findings from field studies in almonds (Prunus dulcis),prunes (P. domestica), and walnuts (Juglans regia) demonstrating modified procedures to measure midday SWP, making it a more convenient and practical tool for irrigation management. For routine monitoring and irrigation scheduling, an equilibration period of 10 min or longer appears to be suitable to provide accurate SWP measurements. Based on the large sample sizes in this study, we estimate that measurement error related to equilibration time for SWP can be reduced to an acceptable level [0.05 MPa (0.5 bar)] with a sample size of about 10 leaves when using a 10-min equilibration period. Under orchard conditions where tree growth and health appears uniform, a sample of one leaf per tree and 10 trees per irrigation management unit should give an accurate mean indicator of orchard water status. Under more variable orchard conditions a larger sample size may be needed. Midmorning and midday SWP both exhibited similar seasonal patterns and responded alike to irrigation events. On some occasions, midday SWP was accurately predicted from midmorning SWP and the change in air vapor pressure deficit (VPD) from midmorning to midday, but both over- and underestimate errors [to 0.3 MPa (3.0 bar)] appeared to be associated with unusually low or high diurnal changes in VPD, respectively. Hence, direct measurement of SWP under midday conditions (about 1300 to 1500 hr) is still recommended.

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S.I. Shibairo, M.K. Upadhyaya and P.M.A. Toivonen

Studies were carried out to understand the effects of moisture loss on water potential and root deterioration in carrot (Daucus carota L. `Eagle') roots during short-term storage. The roots were stored at various temperatures and relative humidities (RH) to provide 0.7 (low), 3 (medium), and 9 mbars (high) of water vapor pressure deficit (WVPD). Carrots at high WVPD lost the most weight, followed by those at medium and lowest WVPD. Water potential and osmotic potential of the carrot tissue at high WVPD did not change significantly up to 6 days, but decreased thereafter. There was no change in water potential and osmotic potential for carrots at medium and low WVPD. A significant quadratic relationship (P = 0.05, r = –0.764) between water potential and carrot root weight loss was observed. Relative electrolyte leakage increased over time in carrots at the high WVPD. At medium WVPD, relative electrolyte leakage did not change up to 6 days, but increased significantly thereafter. Carrots at the low WVPD did not change in relative electrolyte leakage. Relative electrolyte leakage and weight loss correlated positively (P = 0.05, r = 0.789). The results suggest that water stress during short-term storage causes tissue deterioration that may further increase rate of moisture loss and hence reduce the shelf life of carrots.

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Richard P. Buchner*, Allan Fulton, Bruce Lampinen, Ken Shackel, Terry Prichard, Larry Schwankl, Sam Metcalf and Cayle Little

Ninth leaf California Chandler Walnuts (Juglans regia) on Northern California Black (Juglans hindsii) or Paradox (English/black hybrid) rootstock were irrigated to achieve three levels of Midday Stem Water Potential (MSWP). Target potentials were: 1) low water stress (average MSWP of -3.2 bars); 2) mild water stress (average MSWP of -6.2 bars); and 3) moderate water stress (average MSWP of -7.3 bars). Stem Water Potential was measured midday (12-4 pm) by placing leaves inside water impervious, light blocking foil bags. Leaves remained bagged for at least ten minutes to achieve equilibrium. Bagged leaves were removed, placed inside a pressure chamber and stem water potential was measured at endpoint. Data are presented for the 2002 and 2003 seasons. Withholding irrigation water had a significant impact on `Chandler' growth, productivity, and profitability particularly on young, vigorously growing trees. Chandler/Black appears to be more tolerant to water stress compared to Chandler/Paradox For Chandler on Paradox, water stress significantly reduced growth, yield, price per pound, percent edible kernel, and resulted in darker kernels. In addition, water stress significantly increased the total percent offgrade. Withholding irrigation does not appear to be a good strategy in young, vigorously growing `Chandler' orchards. Mature trees and trees grafted onto Northern California black rootstock may be more tolerant of moisture stress.

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David C. Percival, John T.A. Proctor and J.P. Privé

Rubus idaeus L. cv. Heritage raspberries were placed in controlled environment chambers (25°C, 14-hour photoperiod, 2.0 kPa vapor pressure deficit, CO2 concentration of 380 mol·m-2·s-1) to study the effects of drought stress on leaf gas exchange and stem water potential. Whole-plant photosynthesis (Pn) and transpiration were sensitive to drought stress and gradually decreased from the second day of the study until rehydration. Stomatal aperture feed-back regulation was present during the initial 48 hours of the study with transpiration rates dropping in response to a decrease in stem water potential. Spatial differences were also present with leaf Pn, and stomatal and CO2 conductance values of the younger, distal (i.e., closer to the apex) leaves decreasing at a faster rate than the older, proximal leaves (i.e., close to crown). Evidence of increased mesophyll resistance to drought stress was apparent with ci either remaining constant or increasing, while Pn and carboxylation efficiency simultaneously decreased. Protection of the underlying photochemistry was evident with parahelionastic leaf movements which resulted in a reduction in the effective leaf area and subsequent heat load. Therefore, an optimum balance between water loss and ci existed, and an alteration in these rates represented a stomatal conductance adjustment to match the intrinsic photosynthetic capacity rather than just a causal relationship.

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Timothy E. Elkner, J. A. Barden, M. M. Kushad and D. D. Wolf

Fruiting spurs (`Red Prince Delicious') (RD) and shoots (`Sundale Spur Golden Delicious') (CD) with three leaf:fruit ratios and comparable nonfruiting spurs and shoots were girdled on 7 September 1988. An interaction between fruiting status and time existed for most parameters measured on both cultivars while there was no effect of leaf:fruit ratio. At 1 day after treatment (DAT) few differences existed due to fruiting status on either cultivar. At 8 DAT with RD and at 4 and 8 DAT with GD, Pn, transpiration (Tr), leaf water potential (ψ L), and nonreducing sugars were greater on fruiting than nonfruiting spurs and shoots while leaf resistance (RL), SLW, and starch were lower on fruiting spurs. In nonfruiting spurs and shoots Pn, Tr, and ψL tended to decrease while RL and SLW increased with time whereas m fruiting spurs and shoots most parameters remained constant. Total nonstructural carbohydrates, reducing sugars, and starch were greater in nonfruiting than fruiting spurs and shoots.

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Jordi Marsal and Joan Girona

Relationships between midday (Ψmd) and predawn (Ψpd) leaf water potential, stomatal conductance (gs), and net CO2 assimilation rate (A) were determined at different fruit growth stages and for 2 years with different fruit loads in a `Sudanell' peach [Prunus persica (L) Batsch] plot subjected to two regulated deficit irrigation (RDI) strategies plus a control irrigation treatment. A postharvest RDI (PRDI) treatment was irrigated at 0.35 of the control after harvest. The second treatment (SPRDI) applied RDI during Stage II, the lag phase of the fruit growth curve, at 0.5 of the control and postharvest at 0.35 of the control. The control treatment and the PRDI and SPRDI when not receiving RDI were irrigated at 100% of a modified Penman crop water use calculation (ETo) in 1994, a full crop year, and 80% in 1995, a year of nearly zero crop. In 1995, with 80% of the 1994 irrigation rate and no crop, the Ψmd was higher, probably because of the lower crop load, while Ψpd was lower, probably because less water was applied to the soil. The relationship of gs and A with Ψmd during Stage II was steeper than during postharvest. Low Ψmd was not indicative of a depression in gs and A in Stage III. Osmotic leaf water potential at turgor loss (Ψπ 0) as derived from pressure-volume curves was more negative during Stage III and postharvest (about -2.9 MPa) than in Stage II (about -2.7 MPa). The Ψmd measurements together with Ψπ 0 determinations seemed to be more useful to characterize peach tree water status than Ψpd under soil water deficits because of their better relationship to midday stomatal closure.

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Bharat P. Singh, Kevin A. Tucker, James D. Sutton and Harbans L. Bhardwaj

Abbreviations: E, transpiration; g s , stomatal conductance; Pn, net photosynthesis; ψ, leaf water potential. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations

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Chuhe Chen, J. Scott Cameron and Ann Marie VanDerZanden

Leaf water potential (LWP). relative water content (RWC), gas exchange rates and 4th-derivative spectra were measured in water-stressed and normally Irrigated plank of Totem' strawberry (Fragaria × ananassa) grown in a growth chamber. CO2 assimilation rate (A) dropped sharply when LWP decreased from -0.5 to -1.2 MPa and almost ceased as LWP fell below -1.5 MPa. There was a significant but more gradual decline of A as RWC decreased form 90% to 55%. An exponential relationship with A was observed across a wide range of LWP and RWC (Rz= 0.64, 0.86, respectively). LWP was more closely related with transpiration and leaf and stomatal conductances than with A and water use efficiency. RWC was highly correlated with all gas exchange parameters.

Under moderate water stress, younger leaves maintain higher RWC and A than older leaves. There was no relationship between LWP and leaf age.

RWC and A were both negatively correlated with peak amplitudes of Ca 684 and Ca 697 and positively correlated with Ca 693 in their 4th-derivative spectra of chlorophyll. LWP had a negative correlation with Cb 640.