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A. Naor, H. Hupert, Y. Greenblat, M. Peres, A. Kaufman, and I. Klein

The interactions between irrigation and crop level with respect to fruit size distribution and midday stem water potential were investigated for 3 years in a nectarine (Prunus persica L. `Fairlane') orchard located in a semi-arid zone. Wide ranges of crop loads and irrigation rates in stage III were employed, extending from practically nonlimiting to severely limiting levels. Irrigation during stage III of fruit growth ranged from 0.63 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (300 to 2000) fruit/tree in the 555-tree/ha orchard. The yields and stem water potentials from 1996, 1997 and 1998 were combined together and the interrelations among yield, crop load and stem water potential were examined. Fruit <55 mm in diameter growing at 400 fruit per tree were the only ones not affected by irrigation level. The yield of fruit of 60 to 75 mm in diameter increased with irrigation level, but only a slight increase was observed when the irrigation rate rose above 1.01 ETp. A significant decrease in the yields of 60 to 65, 65 to 70, and 70 to 75-mm size grades occurred at crop levels greater than 1000, 800, and 400 fruit per tree, respectively. Midday stem water potential decreased with increasing crop level, and it is suggested that midday stem water potential responds to crop load rather than crop level. Relative yields of the various size grades were highly correlated with midday stem water potential. It was suggested that the midday stem water potential integrates the combined effects of water stress and crop load on nectarine fruit size.

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Christine Schumann, Henrik Jürgen Schlegel, Eckhard Grimm, Moritz Knoche, and Alexander Lang

Many soft and fleshy fruit crack on exposure of the fruit to surface water. Osmotic water uptake that occurs along a gradient in water potential between the surface water on the fruit and the water inside the fruit is thought to be causal. The water

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A. Naor, I. Klein, H. Hupert, Y. Grinblat, M. Peres, and A. Kaufman

The interactions between irrigation and crop level with respect to fruit size distribution and soil and stem water potentials were investigated in a nectarine (Prunus persica (L.) Batsch. `Fairlane') orchard located in a semiarid zone. Irrigation treatments during stage III of fruit growth ranged from 0.62 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (200 to 1200 fruit/tree in the 555-tree/ha orchard). Total yield did not increase with increasing irrigation rate above 0.92 ETp in 1996 and maximum yield was found at 1.06 ETp in 1997. Fruit size distribution was shifted towards larger fruit with increasing irrigation level and with decreasing crop level. The two highest irrigation treatments had similar midday stem water potentials. Our findings indicate that highest yields and highest water use efficiency (yield/water consumption) are not always related to minimum water stress. Total yield and large fruit yield were highly and better correlated with midday stem water potential than with soil water potential. This confirms other reports that midday stem water potential is an accurate indicator of tree water stress and may have utility in irrigation scheduling.

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Yahia Othman, Dawn VanLeeuwen, Richard Heerema, and Rolston St. Hilaire

% relative to control ( Garrot et al., 1993 ). Root growth and number of growing roots of black walnut ( Juglans nigra ) approached zero as soil water potential ranged from –0.5 to –1.0 MPa ( Kuhns et al., 1985 ). Yield of almond ( Prunus dulcis ) trees

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Enoc Barrera-Aguilar, Luis A. Valdez-Aguilar, Ana M. Castillo-González, Andrew D. Cartmill, Donita L. Cartmill, Edilberto Avitia-García, and Luis Ibarra-Jímenez

research we also assessed the relationship of K nutrition with leaf water potential (ψ w ), photosynthetic rate, leaf anatomy, and plant nutritional status. Materials and Methods Cultural conditions and plant material. The study was conducted under

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Amos Naor, Isaac Klein, and Israel Doron

The sensitivity of leaf (ψleaf) and stem (ψstem) water potential and stomatal conductance (gs) to soil moisture availability in apple (Malus domestics Borkh.) trees and their correlation with yield components were studied in a field experiment. Two drip irrigation treatments, 440 mm (H) and 210 mm (L), were applied to a `Golden Delicious' apple orchard during cell enlargement stage (55-173 days after full bloom). Data collected included ψstem, y leaf, gs, and soil water potential at 25 (ψsoil-25) and 50 cm (ψsoil-50). No differences in midday ψleaf's were found between irrigation treatments. Stem water potential was higher in the H treatment than in the L treatment in diurnal measurements, and at midday throughout the season. Stomatal conductance of the H treatment was higher than the L treatment throughout the day. Stomatal conductance between 0930 and 1530 hr were highly correlated with ψstem. The H treatment increased the percentage of fruit >65 mm, and increased the proportion of earlier harvested fruit reaching marketable size compared to the L treatment. Fruit size in the first harvest and the total yield were highly correlated with ψstem. The degree of correlation between plant water stress indicators and yield component decreased in the following order: ψstemsoil-25,soil-50leaf. The data suggest that midday ψstem may serve as a preferable plant water stress indicator with respect to fruit size.

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Harold McCutchan and K.A. Shackel

Abbreviations: ET, evapotranspiration; ψ, water potential; VPD, vapor pressure deficit. 1 UC Cooperative Extension, 733 County Center III Court, Modesto, CA 95355. This work supported in part by a grant from the California Prune Board. The cost of

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Sanjit K. Deb, Manoj K. Shukla, and John G. Mexal

discrimination because of a closer relationship to soil water potential and transpiration. In contrast, Intrigliolo and Castel (2006a) reported that midday ψ stem values could not discriminate between irrigation treatments, which were shown to differ based on

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Lenny Wells

to improve pecan production. The objectives of this study were to compare the effects of hedge pruning on pecan nut quality, yield, and midday stem water potential of pecan trees in the temperate climate of the southeastern United States and to

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Daniel Drost and Darlene Wilcox-Lee

Asparagus is considered a relatively drought tolerant plant, but few studies are available on the gas exchange response to soil moisture stress. Seedlings were grown in the greenhouse for six months before initiation of the water stress treatments. Soils were allowed to dry to matric potentials of -0.05, -0.3 and -0.5 MPa before rewatering to pot capacity. Gas exchange and fern water potentials were measured diurnally on asparagus plants when soil matric potentials reached their minima. Decreasing soil matric potentials decreased net carbon dioxide assimilation, stomatal conductance and fern water potential. Assimilation rates (6 am) were between 3 and 5 umols m-2 s-1 for all soil moisture treatments. Carbon assimilation rates of 10, 8, and 7 umols m-2 s-1 were recorded at 10 am for the -0.05, -0.3 and -0.5 MPa soil matric potentials, respectively. Assimilation rates decreased sharply over the remainder of the day. The diurnal pattern for conductance were similar to the assimilation rates. Fern water potentials were greater in the -0.05 MPa than in the -0.5 MPa treatment for all measurement periods with an intermediate response for soil matric potentials of -0.3 MPa. Fern water potentials were highest at 6 am (-0.2 to -0.6 MPa) before declining to their minima (-1.5 to -1.8 MPa) at 10 am. Water potentials remained at these low levels throughout the day before recovering slightly at 6 pm.