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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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Jason McAfee and Curt Rom

Pesticides and alternative fruit thinners are needed for certified organic fruit growers. Transient reductions in photosynthesis (Pn) have proven an effective technique for fruit thinning. Pesticides can be detrimental to plant growth by Pn reduction. This study was developed to measure plant response to foliar applications of essential oils at 2% concentrations. Treatments were applied to vegetative apple trees grown under controlled environment conditions to study photosynthetic effects. There was no significant effect on Pn for treatments; however, clove oil was very phytotoxic and defoliated all trees in this study. Cinnamon oil and cedarwood oil significantly decreased evapotranspiration and stomotal conductance 1 day after treatment. Differences in plant growth were not significantly different for all treatments excluding clove oil. Studies on concentration effects may determine horticultural usefulness of these compounds.

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Paolo Sabbatini and James A. Flore

The naturally occurring carbon isotope composition (or 13C: 12C ratio, expressed with the notation d13C) of plant tissue may be used as an indicator of water use efficiency during plant growth. d13C has been shown to be an effective tool to study physiological response of plant to environmental conditions, especially water stress. The objective of this work was to test if d13C could be an indicator of carbon limitations or a low source: sink ratio. Trees of `Imperial Gala'/Bud 9 (n = 12), 6-years-old, field grown at the Clarksville Horticultural Research Station (Clarksville, Miss.), were assessed with different crop load (LCL = Low Crop Load, 0.76 ± 0.44 fruit per trunk sectional area (TCA); NCL = Normal Crop Load, 7.25 ± 1.83 fruit/TCA; HCL = High Crop Load, 15.83 ± 1.76 fruit/TCA) and leaf: fruit ratio (LCL: 52.78 ± 8.55, NCL: 13.33 ± 3.06, HCL; 4.31 ± 0.68) immediately following June drop. Net photosynthetic rate of leaves were monitored during the season and elevated rates were observed in NCL and HCL and correlated with the fruiting process. Photosynthesis was inhibited in LCL more in the afternoon (from 20% to 42% in relation to NCL) than in the morning (from 5% to 20%) and this was positively correlated with crop sink strength. Variations of the stable carbon isotope composition of roots (fine and coarse), fruit, leaves, and current-year stems were examined. The d13C varied by tissue (fruit > shoot and leaf > root) and in relation to the level of crop load (d13C‰ in fruit: LCL –23.513 ± 0.248, NCL –24.891 ± 0.594; and HCL –24.935 ± 0.375). These results may have implications for analysis of isotopic signals in carbohydrate stress and fractionation steps will be discussed.

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D. Joseph Eakes, Robert D. Wright, and John R. Seiler

Abbreviations: CA, leaf chamber CO 2 concentration; CI, leaf internal CO 2 concentration; E, transpiration; g L , leaf conductance; MSC, moisture stress conditioning; Pn, net photosynthesis; r m , mesophyll resistance to CO 2 ; SI, stomatal

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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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Madhulika Sagaram, Leonardo Lombardini, and L.J. Grauke

, A.J. Davies, W.J. 1998 The coupled response of stomatal conductance to photosynthesis and transpiration J. Expt. Bot. 49 399 406 Johnson, H.B. 1975 Plant pubescence: An ecological perspective Bot. Rev

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Vahid Rahimi Eichi, Stephen D. Tyerman, and Michelle G. Wirthensohn

content ( Guerfel et al., 2009 ), and diffusive limitations, including mesophyll conductance (g m ) ( Ethier and Livingston, 2004 ; Grassi and Magnani, 2005 ). Under non-stress conditions, non-stomatal limitations are dependent more on diffusional rather

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Hector R. Valenzuela, Stephen K. O'Hair, and Bruce Schaffer

Abbreviations: A, net CO assimilation; Ci, substomatal CO concentration; E, transpiration; g, stomatal conductance; WUE, water use efficiency. Florida Agricultural Experiment Stations Journal Series no. R-00461. We thank R. Fethiere of the Forage

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Kirk D. Larson, Bruce Schaffer, and Frederick S. Davies

Abbreviations: A, net CO 2 assimilation; C i , internal CO 2 concentration; E, transpiration; g s , stomatal conductance. Florida Agricultural Experiment Station Journal Series no. R-00568. This research was funded, in part, by U.S. Dept. of