Estimating Midday Leaf and Stem Water Potentials of Mature Pecan Trees from Soil Water Content and Climatic Parameters

Diurnal and temporal patterns of stem water potential (ψ_stem) and leaf water potential (ψ_leaf) were determined during June to Sept. 2010 and 2011 at lower (2.5 m tree height), mid- (4.6 m), and upper (7.6 m) canopy positions for two flood-irrigated, mature pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards near Las Cruces, NM. Diurnal measurements of ψstem and ψleaf at three canopy heights were correlated under both dry and wet soil conditions. However, although soil water contents at Site 2 (silty clay loam texture) remained higher compared with Site 1 (sandy loam), ψ_stem and ψ_leaf values, particularly under dry soil conditions at Site 2, were consistently lower, showing the effect of clayey soil texture on pecan water stress. Diurnal patterns of ψ_stem and ψ_leaf indicated that measurements of ψ_stem and ψ_leaf should be made close to early afternoon (between 1400 and 1500 hr Mountain Standard Time) to evaluate mature pecan water stress, which also corresponded to maximum climatic stress conditions. Midday ψstem and ψ_leaf measured at three canopy heights over several irrigation cycles during the 2010 season were correlated with one another, midday soil water content at different depths, and atmospheric vapor pressure deficit (VPD). Multiple regression analysis [between midday ψ_stem or ψ_leaf and midday θ_avg (soil water content at 0 to 40 cm), air temperature (Tmd), and relative humidity (RH_md)] during the 2010 season revealed that two-parameter regression models [ψ_stem or ψ_leaf = f (midday θ_avg and T_md)] were the most significant for the interpretation of midday ψstem or ψ_leaf at both sites. Using the two-parameter model, predictions of ψ_stem and ψ_leaf measured on the both shaded and sunlit sides of trees at three canopy heights for 2011 showed good agreement between measured and predicted ψ_stem and ψ_leaf (R² ranged from 0.70 to 0.98). Two-parameter models derived in an earlier study generally underpredicted ψ_stem both in 2010 and 2011, which further supported the importance of the time of midday ψ_stem and ψ_leaf measurements suggested in this study.