Experimental site.

Measurements of ψ_stem and ψ_leaf in 25- to 30-year-old ‘Western Schley’ pecan trees were carried out during June to Sept. 2010 and 2011 growing seasons at two orchards near Las Cruces, NM. A detailed description of these orchards can be found in Deb et al. (2011a, 2011b). Site 1 (0.9 ha) consisted of five rows of pecan trees in a diamond pattern with 15 trees in each row, 8 m between-row spacing, and 15 m in-row spacing. Site 2 (1 ha), located at New Mexico State University Leyendecker Plant Science Research Center (LPSRC), comprised seven rows of pecan trees in a rectangular pattern (7 m × 8 m) with 29 trees in each row. Root-zone soil water content and ψ_stem and ψ_leaf were monitored in three representative trees for each site (denoted as east, south, and north tree for Site 1 and north, south, and southwest tree for Site 2, respectively). Both orchards were flood-irrigated with a combination of surface water and groundwater. The climate of the study areas is semiarid, and average annual temperature and precipitation are 17.7 °C and 297 mm, respectively.

Soil physical properties at Sites 1 and 2 were reported in Deb et al. (2011a, 2011b). Soil texture at Site 1 is sandy loam, and the soil texture is silty clay loam at Site 2. In accordance with the soil texture, the saturated hydraulic conductivity (K_s) for Site 1 was generally higher, and field capacity (FC) and wilting point (WP) water contents at 30 kPa and 1500 kPa, respectively, were lower for Site 1 than Site 2. The K_s for Site 1 ranged from 0.78 to 1.87 cm·h⁻¹ above 40 cm soil depth and 3.72 to 3.84 cm·h⁻¹ below 40 cm. The ranges of K_s values at Site 2 were between 0.01 and 1.7 cm·h⁻¹ above 60 cm and 0.75 and 3.72 cm·h⁻¹ below 60-cm soil depth. The average FC and WP water contents were 0.25 and 0.10 cm³·cm⁻³ above 40 cm and 0.15 and 0.05 cm³·cm⁻³ below 40 cm at Site 1, whereas these parameter values were 0.38 and 0.20 cm³·cm⁻³ above 60 cm and 0.28 and 0.12 cm³·cm⁻³ below 60 cm at Site 2.

Measurement of stem water potential and leaf water potential.

Midday ψ_stem and ψ_leaf (between 1400 and 1500 hr Mountain Standard Time) were determined simultaneously using a pressure chamber (Model 1000 with digital gauge; PMS Instrument Company, Albany, OR) for each of the selected trees at both sites on a weekly basis over several irrigation cycles (i.e., every third and sixth day after irrigation) from June to Sept. 2010 and 2011. In 2010, for determining diurnal patterns, measurements of ψ_stem and ψ_leaf were carried out at each site throughout the day under typical dry and wet soil conditions. At each site, the diurnal ψ_stem and ψ_leaf measurements were made at 1-h intervals from 0800 to 1900 hr and thereafter measurements were made at 2100, 0000, 0300, 0600, and 0900 hr. For ψ_stem determinations during 2010, on each tree, shoot tips with several leaves on the sunlit sides of the canopy at tree heights of 2.5 m, 4.6 m, and 7.6 m above the soil surface, denoted as lower canopy, midcanopy, and upper canopy, respectively, were enclosed with foil-laminated plastic bags (Deb et al., 2011a). The effect of these bags on leaf temperature was also checked at both sites during a midday ψ_stem and ψ_leaf measurement day (between 1400 and 1500 hr). Test leaves together with HOBO H8 temperature sensors (Onset Computer Corp., Bourne, MA) were enclosed with bags at lower canopy, midcanopy, and upper canopy positions and temperature inside the bags were recorded at 1-s intervals, which showed no additional leaf heating even on sunlit side leaves (data not presented).

During ψ_stem measurements, the leaves were bagged for 30 min, which effectively stopped the natural transpiration from the leaves, allowing ψ_leaf to equilibrate with the ψ_stem (Fulton et al., 2001). Each shoot tip sealed in foil-laminated plastic bag was excised and then quickly placed into the pressure chamber. The exposed edge of the shoot was carefully observed for the appearance of a drop of water (sap). The rate of pressure increase in the chamber was kept low to avoid errors during each measurement. The time between bagged leaves excision and chamber pressurization was generally less than 30 s. As soon as the water appeared, the corresponding pressure (ψ_stem) was read from the chamber digital gauge.

On each tree, leaves, chosen for ψ_leaf determinations at lower, mid-, and upper canopy were fully expanded, mature leaves exposed to direct sunlight. For non-bagged leaves, both leaf transpiration effects on water potential values and desiccation of transpiring leaves after excision from the plant have been reported in previous studies (e.g., Fulton et al., 2001; Turner and Long, 1980). Therefore, the targeted leaves were covered with foil-laminated plastic bags and sealed immediately after excision to avoid any further transpiration, and within 5 to 10 s, midday ψ_leaf was determined using the same pressure chamber. During the period of June to Sept. 2011, midday ψ_stem and ψ_leaf at the lower, mid-, and upper canopy were determined simultaneously for each of the selected trees at both sites on both shaded and sunlit side leaves of the tree.

Measurement of soil water content.

At each site, time domain reflectometry (TDR) CS616 sensors (Campbell Scientific, Inc., Logan, UT) were installed horizontally at depths of 5, 10, 20, 40, 60, and 80 cm to continuously record volumetric water content (θ) once every 10 min at under- and outside-canopy locations of selected three trees during the 2010 and 2011 growing seasons. Sensor data were calibrated in situ using gravimetric water content data (Deb et al., 2011a). During the 2009 season, soil cores (11.4 cm in diameter) near the middle of the canopy and just inside the tree dripline were collected up to 100-cm depth for each tree and site to determine rooting depth and root length density (RLD) (Deb et al., 2011a, 2011b). At both sites, according to Deb et al. (2011a, 2011b), total RLD was much higher in the shallow depths (0 to 40 cm) than in the deeper depths (40 to 80 cm). Deb et al. (2011a) also reported that soil water depletion within the root zone (0 to 80 cm) was higher in the shallow depths (0 to 40 cm). As suggested by Deb et al. (2011a), soil water contents within the shallow depths (i.e., TDR data at depths of 5, 10, 20, and 40 cm), where RLD was also higher than in the deeper depths, were used to evaluate correlations with the midday ψ_stem and ψ_leaf at each tree and site. At each site, soil water content at shallow depth (0 to 40 cm) (θ_avg) was obtained by averaging θ at soil depths of 5, 10, 20, and 40 cm among selected trees at the time of midday ψ_stem and ψ_leaf measurements.

Estimation of atmospheric vapor pressure deficit.

Air temperature, wind speed, relative humidity, and solar radiation at 2 m height above the ground were continuously recorded by a HOBO U30-NRC weather station (Onset Computer Corporation, Bourne, MA) at Site 1. Rainfall was measured at Site 1 using a tipping bucket measuring at 10-min intervals. Hourly meteorological variables were also obtained from the New Mexico State University Fabian Garcia Science Center (FGSC) weather station, ≈6 km southeast of Site 1, to facilitate weather data comparison with the HOBO weather station. For Site 2, hourly rainfall, air temperature, solar radiation, wind speed, and relative humidity measured at 2-m height were collected from the LPSRC weather station, which is located ≈0.1 km from Site 2. To account for diurnal changes in evaporative demand at both sites, atmospheric VPD values were calculated using Murray’s (1967) equations separately for Sites 1 and 2 using the average hourly air temperature and average hourly relative humidity data. For each measurement of midday ψ_stem and midday ψ_leaf, midday conditions of VPD at both sites were calculated using T_md and RH_md values between 1400 hr and 1500 hr. The diurnal and midday VPD were used to evaluate correlations with the diurnal variations of ψ_stem and ψ_leaf and midday ψ_stem and ψ_leaf, respectively.

Data analysis.

Linear regression analysis was used to explore relationships between dependent and independent variables at both sites, namely, diurnal and midday ψ_stem and ψ_leaf values at lower, mid-, and upper canopy; midday VPD; T_md; RH_md; midday θ at different soil depths; and midday θ_avg. The effect that midday soil water content at shallow depth (θ_avg) and midday VPD parameters (climatic parameters T_md, and RH_md) had on midday ψ_stem and ψ_leaf was evaluated through multiple regression analysis for the 2010 season. For each tree, one-parameter (dependent variables ψ_stem or ψ_leaf were regressed against independent variables θ_avg or T_md), two-parameter (against independent variables θ_avg and T_md or RH_md and T_md), and three-parameter (against independent variables θ_avg, T_md, and RH_md) regression models were derived at both sites. F-test was performed to determine whether a regression model with more parameters provided a significant improvement over the regression models with a smaller number of independent parameters (Deb et al., 2011a). The null hypothesis was that a model with higher number of independent parameters did not provide a significantly better fit than models with fewer parameters, and the null hypothesis was rejected if the F calculated from the data was greater than the critical value of the F distribution at 0.05 significant level.

Diurnal pattern of stem water potential and leaf water potential in pecan trees.

The diurnal variations of ψ_stem and ψ_leaf of lower canopy, midcanopy, and upper canopy, θ at soil depths of 10, 20, and 40 cm, and microclimate variables at both Sites 1 and 2 are plotted for dry and wet soil conditions (Figs. 1 and 2). The ψ_stem and ψ_leaf of the both wet and relatively dry soil conditions at both sites decreased rapidly at sunrise (from 0600 hr), reached a minimum in early afternoon (between 1400 and 1500 hr), and rapidly increased in late afternoon (after 1800 hr).

Diurnal variation of average air temperature, solar radiation, and relative humidity (A and D), volumetric water content (θ) at soil depths of 10, 20, and 40 cm (B and E), and measured stem water potential (ψ_stem) and leaf water potential (ψ_leaf) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface and atmospheric vapor pressure deficit (VPD) (C and F) for pecan trees at Site 1 after an irrigation event on 4 Aug. 2010: under wet soil conditions during 5 to 6 Aug. 2010 (left; A through C), and under relatively dry soil conditions during 9 to 10 Aug. 2010 (right; D through F). The ψ_stem and ψ_leaf values at each tree height represent the averages of three measurements on the selected east, south, and north pecan trees of Site 1.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Diurnal variation of average air temperature, solar radiation, and relative humidity (A and D), volumetric water content (θ) at soil depths of 10, 20, and 40 cm (B and E), and measured stem water potential (ψ_stem) and leaf water potential (ψ_leaf) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface and atmospheric vapor pressure deficit (VPD) (C and F) for pecan trees at Site 1 after an irrigation event on 4 Aug. 2010: under wet soil conditions during 5 to 6 Aug. 2010 (left; A through C), and under relatively dry soil conditions during 9 to 10 Aug. 2010 (right; D through F). The ψ_stem and ψ_leaf values at each tree height represent the averages of three measurements on the selected east, south, and north pecan trees of Site 1.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Diurnal variation of average air temperature, solar radiation, and relative humidity (A and D), volumetric water content (θ) at soil depths of 10, 20, and 40 cm (B and E), and measured stem water potential (ψ_stem) and leaf water potential (ψ_leaf) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface and atmospheric vapor pressure deficit (VPD) (C and F) for pecan trees at Site 1 after an irrigation event on 4 Aug. 2010: under wet soil conditions during 5 to 6 Aug. 2010 (left; A through C), and under relatively dry soil conditions during 9 to 10 Aug. 2010 (right; D through F). The ψ_stem and ψ_leaf values at each tree height represent the averages of three measurements on the selected east, south, and north pecan trees of Site 1.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Diurnal variation of average air temperature, solar radiation, and relative humidity (A and D), volumetric water content (θ) at soil depths of 10, 20, and 40 cm (B and E), and measured stem water potential (ψ_stem) and leaf water potential (ψ_leaf) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface and atmospheric vapor pressure deficit (VPD) (C and F) for pecan trees at Site 2: under wet soil conditions during 28 to 29 Aug. 2010 after an irrigation event on 25 Aug. 2010 (left; A through C), and under dry soil conditions during 7 to 8 Aug. 2010 after the previous irrigation applied on 19 July 2010 (right; D through F). The ψ_stem and ψ_leaf values at each tree height represent the averages of three measurements on the selected north, south, and southwest pecan trees of Site 2.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Diurnal variation of average air temperature, solar radiation, and relative humidity (A and D), volumetric water content (θ) at soil depths of 10, 20, and 40 cm (B and E), and measured stem water potential (ψ_stem) and leaf water potential (ψ_leaf) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface and atmospheric vapor pressure deficit (VPD) (C and F) for pecan trees at Site 2: under wet soil conditions during 28 to 29 Aug. 2010 after an irrigation event on 25 Aug. 2010 (left; A through C), and under dry soil conditions during 7 to 8 Aug. 2010 after the previous irrigation applied on 19 July 2010 (right; D through F). The ψ_stem and ψ_leaf values at each tree height represent the averages of three measurements on the selected north, south, and southwest pecan trees of Site 2.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Diurnal variation of average air temperature, solar radiation, and relative humidity (A and D), volumetric water content (θ) at soil depths of 10, 20, and 40 cm (B and E), and measured stem water potential (ψ_stem) and leaf water potential (ψ_leaf) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface and atmospheric vapor pressure deficit (VPD) (C and F) for pecan trees at Site 2: under wet soil conditions during 28 to 29 Aug. 2010 after an irrigation event on 25 Aug. 2010 (left; A through C), and under dry soil conditions during 7 to 8 Aug. 2010 after the previous irrigation applied on 19 July 2010 (right; D through F). The ψ_stem and ψ_leaf values at each tree height represent the averages of three measurements on the selected north, south, and southwest pecan trees of Site 2.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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The differences of both ψ_stem and ψ_leaf values between wet and relatively dry soil conditions at both Sites 1 and 2 were straightforward, i.e., under dry soil conditions, ψ_stem and ψ_leaf values were more negative, particularly at early afternoon (between 1400 and 1500 hr) for both sites. For example, for 5 to 6 Aug. 2010 (Fig. 1C), after an irrigation event on 4 Aug. at Site 1, ψ_stem measured at midcanopy ranged from –0.76 (at 1500 hr) to –0.23 MPa (at 0300 hr) under wet soil conditions, whereas ψ_leaf at midcanopy varied from –1.03 (1500 hr) to –0.25 MPa (0300 hr). In contrast, for 9 to 10 Aug. 2010 (Fig. 1F) 5 d after the irrigation event on 4 Aug. at Site 1, the values of ψ_stem and ψ_leaf at midcanopy under relatively dry conditions ranged from –1.12 (1400 to 1500 hr) to –0.31 (0300 hr) and –1.46 (1300 to 1500 hr) to –0.32 MPa (0300 hr), respectively. Similar diurnal trends in ψ_stem and ψ_leaf under soil wet and dry conditions (Figs. 2C and Fig. 2F, respectively) were observed at Site 2. However, an important aspect to be noted in Figures 1F and 2F is that ψ_stem and ψ_leaf values under dry soil conditions exhibited relatively larger amplitude in the silty clay loam Site 2 compared with the sandy loam Site 1 with diurnal variations in ψ_stem and ψ_leaf values at midcanopy ranging from –1.31 to –0.39 MPa and –1.72 to –0.41 MPa, respectively, under dry soil conditions at Site 2 (Fig. 2F). On the other hand, soil water content at Site 2 remained higher than Site 1 (Figs. 1B, 1E, 2B, and 2E), which could be explained by higher clay content and therefore higher field capacity and water-holding capacity within the root zone at Site 2. Obviously, the relatively lower ψ_stem and ψ_leaf values at Site 2 indicated that soil texture has an important effect on pecan water stress because capillary conductivity of soil influences flow of water from soil to roots. Previous work reported that root-zone soil water depletion at silty clay loam Site 2 was much lower than Site 1 (Deb et al., 2011a), indicating that reduced water adsorption rate by the root system might be related in part to the clayey soil type at Site 2.

The ψ_stem and ψ_leaf values of both the wet and relatively dry soil conditions at both sites responded to diurnal variations in incoming solar radiation, VPD, air temperature, and relative humidity (Figs. 1 and 2). The rapid decrease in ψ_stem and ψ_leaf values was closely associated with the increase in solar radiation. For example, under the peak solar radiation at 1400 hr, the values of ψ_stem and ψ_leaf at Site 1 decreased to –0.74 and –0.98 and to –1.12 and –1.46 MPa under wet and dry soil conditions, respectively (Fig. 1). At Site 2, the values of ψ_stem and ψ_leaf during peak solar radiation at 1200 hr decreased to –0.62 and –0.85 and to –1.29 and –1.70 MPa for wet and dry soil conditions, respectively (Fig. 2). The diurnal trends of ψ_stem, ψ_leaf, and VPD under dry and wet soil conditions were also similar at both sites with increases in the evaporative demand (expressed as atmospheric VPD, a function of the air temperature and relative humidity) inducing more negative ψ_stem and ψ_leaf (Figs. 1 and 2). Accordingly, increases in air temperature and decreases in relative humidity during the day induced more negative ψ_stem and ψ_leaf at both sites. Because air temperature and relative humidity are easy to determine and usually recorded at weather stations, air temperature or relative humidity might be directly used to interpret diurnal changes of ψ_stem and ψ_leaf in mature pecans under wet and dry soil conditions rather than estimating atmospheric VPD. The diurnal changes of ψ_stem and ψ_leaf and weather data for dry and wet soil conditions at both sites clearly suggest that, to evaluate water stress in mature pecans, measurements of ψ_stem and ψ_leaf should be made close to early afternoon (between 1400 and 1500 hr Mountain Standard Time) when the ψ_stem and ψ_leaf were minimum and responded to prevailing weather conditions that could impact water deficit on a given day. Although site-specific conditions including weather, soil, soil water depletion pattern, management conditions as well as plant properties affect daily dynamics of plant water status, there has been no general consensus regarding the most suitable time to measure midday ψ_stem and ψ_leaf, particularly for mature pecan. Time periods of midday ψ_stem measurements have been from 1100 to 1400 hr for grapevines (e.g., Choné et al., 2001), 1200 to 1500 hr for prunes (e.g., McCutchan and Shackel, 1992), and 1300 to 1500 hr for almonds, walnuts, and prunes (e.g., Fulton et al., 2001). Midday ψ_stem and ψ_leaf measurements were also taken only 0.5 h on either side of solar noon (from 1230 to 1330 hr) (Williams and Araujo, 2002).

Vertical differences in ψ_stem and ψ_leaf values among different tree heights, i.e., at lower, mid-, and upper canopy, developed soon after sunrise, indicating that a straightforward interpretation of ψ_stem and ψ_leaf determined at a single tree height within the canopy might not be representative. Both ψ_stem and ψ_leaf in the upper canopy were lower during the day at both sites than those in the lower canopy, suggesting that there was relatively more plant water stress at the upper canopy compared with the lower canopy at both sites. Consistently, the lower values of ψ_stem and ψ_leaf associated with tree heights within the canopy were most likely the result of resistances to liquid water flow in the longer pathway to the uppermost leaves (Begg and Turner, 1970; Hellkvist et al., 1974), differences in plant physiology, and flow against gravity. These vertical differences of ψ_stem and ψ_leaf values at lower, mid-, and upper canopy might also be related to the absorption and partitioning of radiation within the canopy, particularly the ψ_leaf might appear to reflect the degree of exposure of the leaves to solar radiation rather than its vertical position in the canopy above the soil surface. The differences in ψ_stem and ψ_leaf between the lower and upper canopy at Site 1 ranged from –0.01 to –0.2 MPa (Figs. 1C and 1F), whereas the differences were between –0.01 and –0.1 MPa at Site 2 (Figs. 2C and 2F). At both sites, under both wet and dry soil conditions (Figs. 1 and 2), although leaves were bagged just before excision, the ψ_leaf values were consistently more negative as compared with ψ_stem. This observation is consistent with studies of other crops (e.g., Williams and Araujo, 2002). However, diurnal measurements of ψ_stem and ψ_leaf of both the dry and wet soil conditions at three canopy heights (Figs. 1 and 2) were highly correlated with one another at Site 1 (R² > 0.94, P < 0.05) and Site 2 (R² > 0.95, P < 0.05).

Temporal pattern of midday stem water potential and leaf water potential in pecan trees.

The midday ψ_stem at three canopy heights and volumetric water content (θ) at soil depths of 10, 20, and 40 cm corresponding to midday ψ_stem measurements during June through Sept. 2010 for all trees at both sites are demonstrated in Figures 3 and 4, respectively. Simultaneous measurements of midday ψ_leaf at three canopy heights for each tree and site made throughout June to Sept. 2010 are not shown. The relationship between midday ψ_stem and midday ψ_leaf determined at midcanopy for selected trees at both sites are shown in Figure 5. As observed in diurnal trends of ψ_stem and ψ_leaf (Figs. 1 and 2), midday ψ_leaf exhibited relatively larger amplitude, i.e., more negative as compared with midday ψ_stem, because the midday ψ_leaf values depend partially on water loss from leaves or leaf transpiration rate at the time of measurement (Jones, 2004). Overall, the values of R² between midday ψ_stem and midday ψ_leaf ranged from 0.81 to 0.90 at the lower canopy (P < 0.05), 0.83 to 0.85 at midcanopy (P < 0.05), and 0.84 to 0.85 at the upper canopy (P < 0.05) at Site 1 (data not presented). At Site 2, the values of R² between midday ψ_stem and midday ψ_leaf were between 0.86 and 0.90 at the lower canopy (P < 0.05), 0.90 at midcanopy (P < 0.05), and between 0.90 and 0.91 at the upper canopy (P < 0.05) (data not presented).

Midday stem water potential (ψ_stem) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and volumetric soil water content (θ) at soil depths of 10, 20, and 40 cm corresponding to ψ_stem measurements taken at identical times during the period from June to Sept. 2010 for the selected east (top), south (middle), and north (bottom) pecan trees of Site 1. The predicted ψ_stem using two-parameter [Deb et al. (2011a); relationships between ψ_stem, and measured midday θ within the 0- to 40-cm soil depth (θ_avg) and midday air temperature (T_md) data during 2010] regression models for each tree are also presented. Estimated midday atmospheric vapor pressure deficit (midday VPD) for Site 1 is depicted in the east pecan’s (top) plot. Irrigation and rain events during the period of June to Sept. 2010 are shown in the east pecan’s (top) plot.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and volumetric soil water content (θ) at soil depths of 10, 20, and 40 cm corresponding to ψ_stem measurements taken at identical times during the period from June to Sept. 2010 for the selected east (top), south (middle), and north (bottom) pecan trees of Site 1. The predicted ψ_stem using two-parameter [Deb et al. (2011a); relationships between ψ_stem, and measured midday θ within the 0- to 40-cm soil depth (θ_avg) and midday air temperature (T_md) data during 2010] regression models for each tree are also presented. Estimated midday atmospheric vapor pressure deficit (midday VPD) for Site 1 is depicted in the east pecan’s (top) plot. Irrigation and rain events during the period of June to Sept. 2010 are shown in the east pecan’s (top) plot.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and volumetric soil water content (θ) at soil depths of 10, 20, and 40 cm corresponding to ψ_stem measurements taken at identical times during the period from June to Sept. 2010 for the selected east (top), south (middle), and north (bottom) pecan trees of Site 1. The predicted ψ_stem using two-parameter [Deb et al. (2011a); relationships between ψ_stem, and measured midday θ within the 0- to 40-cm soil depth (θ_avg) and midday air temperature (T_md) data during 2010] regression models for each tree are also presented. Estimated midday atmospheric vapor pressure deficit (midday VPD) for Site 1 is depicted in the east pecan’s (top) plot. Irrigation and rain events during the period of June to Sept. 2010 are shown in the east pecan’s (top) plot.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and volumetric soil water content (θ) at soil depths of 10, 20, and 40 cm corresponding to ψ_stem measurements taken at identical times during the period from June to Sept. 2010 for the selected north (top), south (middle), and southwest (bottom) pecan trees of Site 2. The predicted ψ_stem using two-parameter [Deb et al. (2011a); relationships between ψ_stem and measured midday θ within the 0- to 40-cm soil depth (θ_avg) and midday air temperature (T_md) data during 2010] regression models for each tree are also presented. Estimated midday atmospheric vapor pressure deficit (midday VPD) for Site 2 is depicted in the north pecan’s (top) plot. Irrigation and rain events during the period of June to Sept. 2010 are shown in the north pecan’s (top) plot.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and volumetric soil water content (θ) at soil depths of 10, 20, and 40 cm corresponding to ψ_stem measurements taken at identical times during the period from June to Sept. 2010 for the selected north (top), south (middle), and southwest (bottom) pecan trees of Site 2. The predicted ψ_stem using two-parameter [Deb et al. (2011a); relationships between ψ_stem and measured midday θ within the 0- to 40-cm soil depth (θ_avg) and midday air temperature (T_md) data during 2010] regression models for each tree are also presented. Estimated midday atmospheric vapor pressure deficit (midday VPD) for Site 2 is depicted in the north pecan’s (top) plot. Irrigation and rain events during the period of June to Sept. 2010 are shown in the north pecan’s (top) plot.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) at tree heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and volumetric soil water content (θ) at soil depths of 10, 20, and 40 cm corresponding to ψ_stem measurements taken at identical times during the period from June to Sept. 2010 for the selected north (top), south (middle), and southwest (bottom) pecan trees of Site 2. The predicted ψ_stem using two-parameter [Deb et al. (2011a); relationships between ψ_stem and measured midday θ within the 0- to 40-cm soil depth (θ_avg) and midday air temperature (T_md) data during 2010] regression models for each tree are also presented. Estimated midday atmospheric vapor pressure deficit (midday VPD) for Site 2 is depicted in the north pecan’s (top) plot. Irrigation and rain events during the period of June to Sept. 2010 are shown in the north pecan’s (top) plot.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Relationship between midday leaf water potential (ψ_leaf) and stem water potential (ψ_stem) measured at midcanopy (at 4.6 m tree height above the soil surface) at both Sites 1 and 2 during the period from June to Sept. 2010: (A) for east pecan tree at Site 1 and (B) for north pecan tree at Site 2. *Significant at P < 0.05.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Relationship between midday leaf water potential (ψ_leaf) and stem water potential (ψ_stem) measured at midcanopy (at 4.6 m tree height above the soil surface) at both Sites 1 and 2 during the period from June to Sept. 2010: (A) for east pecan tree at Site 1 and (B) for north pecan tree at Site 2. *Significant at P < 0.05.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Relationship between midday leaf water potential (ψ_leaf) and stem water potential (ψ_stem) measured at midcanopy (at 4.6 m tree height above the soil surface) at both Sites 1 and 2 during the period from June to Sept. 2010: (A) for east pecan tree at Site 1 and (B) for north pecan tree at Site 2. *Significant at P < 0.05.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Evaluation of the relationship between midday ψ_stem and midday ψ_leaf has yielded contrasting results in studies for other tree crops. For example, although midday ψ_stem and midday ψ_leaf were highly correlated with one another in grapevines (Stevens et al., 1995; Williams and Araujo, 2002) and peach (Selles and Berger, 1990), Naor et al. (1995) reported a weak correlation between ψ_stem and ψ_leaf for apple trees (R² of 0.35). The high correlation of the comparisons between midday ψ_stem and midday ψ_leaf at sandy loam Site 1 and silty clay loam Site 2 indicated that either measurement of midday ψ_stem and ψ_leaf could be a good indicator of the water status of mature pecans regardless of soil type. Moreover, both ψ_stem and ψ_leaf at lower, mid-, and upper canopy did not exhibit significant differences in magnitudes among trees, even under dry soil conditions at both sites. In a previous study, Deb et al. (2011a) found that root-zone soil water depletion within and outside the canopy locations of pecan trees for both sites did not vary spatially. In contrast, for other crops, ψ_stem has been shown to be better related to tree-to-tree differences in water status, particularly in drier soils (Choné et al., 2001; Shackel et al., 1997).

For both sites, because midday ψ_stem and midday ψ_leaf values at lower, mid-, and upper canopy showed similar temporal trends between irrigation events, for ease of interpretation as well as comparison with the predictions of ψ_stem using the regression model (Deb et al., 2011a), temporal variations in midday ψ_stem for selected trees at both sites during the season 2010 are only presented (Figs. 3 and 4). As demonstrated in Figures 3 and 4, similar to diurnal trends (Figs. 1 and 2), temporal patterns of vertical differences in midday ψ_stem values from lower to upper canopy indicated that there was always relatively more plant water stress at the upper canopy compared with the lower canopy at both sites. In contrast, Deb et al. (2011a) reported that midday ψ_stem values at both Sites 1 and 2 during the growing season in 2009 were nearly the same at different heights after irrigation and a decrease in midday ψ_stem with canopy height was observed 10 to 14 d after the irrigation. Temporal trends of midday ψ_stem and midday ψ_leaf (data not shown) over several irrigation cycles in mature pecans at Sites 1 and 2 suggested that the differences in midday ψ_stem or midday ψ_leaf between measurements at different tree heights were not negligible.

Volumetric soil water content and midday stem water potential and leaf water potential.

At both sites, the midday ψ_stem followed the cyclic pattern of the θ between irrigations, and ψ_stem at different canopy heights decreased with decreasing θ at soil depths of 10, 20, and 40 cm (Figs. 3 and 4). For example, during the period of 26 June through 11 July 2010 at Site 1 (Fig. 3), after the irrigation on June 26, the θ at the time of midday ψ_stem measurements, on average, decreased from 0.40 to 0.23 cm³·cm⁻³ at 10 cm, 0.33 to 0.27 cm³·cm⁻³ at 20, and 0.33 to 0.25 cm³·cm⁻³ at 40 cm soil depth for the east, south, and north tree. These decreases in θ corresponded to the decreases in midday ψ_stem during the period of 26 June to 11 July 2010 with midday ψ_stem values measured at lower canopy decreasing from –0.63 to –1.25 MPa, –0.62 to –1.26 MPa, and –0.62 to –1.25 MPa for the east, south, and north trees of Site 1, respectively (Fig. 3). The respective decreases in midday ψ_stem values at midcanopy and upper canopy were from –0.68 to –1.31 MPa and –0.71 to –1.36 MPa, from –0.67 to –1.32 MPa and –0.72 to –1.40 MPa, and from –0.66 to –1.31 MPa and –0.69 to –1.35 MPa for the east, south, and north trees at Site 1, respectively (Fig. 3).

The volumetric soil water contents (θ) at soil depths of 5, 10, 20, 40, and 60 cm were compared with midday ψ_stem measurements taken at the same times for all trees. Regression analysis was used to evaluate correlations between midday ψ_stem at three canopy heights and θ at depths of 5, 10, 20, 40 and 60 cm for each tree at both sites. Soil water contents (θ) at depths of 5, 10, 20, and 40 cm, and 60 cm as well as average values of θ at the soil depth of 0 to 40 cm (θ_avg) were significantly correlated with midday ψ_stem at three canopy heights for each tree at both sites (Deb et al., 2011a). Overall, the values of R² between θ and midday ψ_stem at different canopies ranged from 0.66 (at 60-cm depth) to 0.78 (at 20 cm) (P < 0.05) and from 0.72 (at 40 cm) to 0.80 (at 5 cm) (at P < 0.05) among trees at Sites 1 and 2, respectively. The relationship between θ_avg and midday ψ_stem at different canopy heights provided the R² ranges of 0.73 to 0.78 (P < 0.05) and 0.80 to 0.82 (P < 0.05) among trees at Sites 1 and 2, respectively (data not presented).

The midday ψ_stem values were lower in trees at the silty clay loam Site 2 than at the sandy loam Site 1, particularly in dry soil conditions (Figs. 3 and 4), in accordance with previously reported data during the 2009 growing season (Deb et al., 2011a). For example, during the period from June to Sept. 2010, midday ψ_stem values of trees under wet soil conditions measured at midcanopy for both sites converged near –0.60 MPa, whereas under dry soil conditions, midday ψ_stem values at midcanopy were –1.88 MPa and –1.66 MPa for trees on silty clay loam (Site 2) and sandy loam soils (Site 1), respectively (Figs. 3 and 4). As shown in Figures 3 and 4, the decrease in midday ψ_stem was because of the increase in midday atmospheric VPD. When the midday ψ_stem and ψ_leaf at three canopy heights were regressed against midday atmospheric VPD, the values of R² ranged from 0.51 to 0.68 (P < 0.05) and 0.67 to 0.85 (P < 0.05) among trees at Sites 1 and 2, respectively (data not presented). The midday ψ_stem and/or ψ_leaf have also been reported to have a linear relationship with the VPD for various crops (Goldhamer and Fereres, 2001; McCutchan and Shackel, 1992; Stern et al., 1998; Stevens et al., 1995; Williams and Baeza, 2007), including mature pecans (Deb et al., 2011a).

Relationship between midday stem water potential and leaf water potential and soil water content and atmospheric vapor pressure deficit parameters.

For each tree at both sites during the 2010 season, one-parameter [ψ_stem or ψ_leaf = f (θ_avg or T_md)], two-parameter [ψ_stem or ψ_leaf = f (θ_avg and T_md or RH_md and T_md)], and three-parameter [ψ_stem or ψ_leaf = f (θ_avg, T_md, and RH_md)] regression models derived using the multiple regression analysis are presented in Tables 1 and 2. Introducing more independent variables to explain the variability of ψ_stem or ψ_leaf improved R² (P < 0.05) (Tables 1 and 2). F-test showed that in general the two-parameter regression model (between ψ_stem or ψ_leaf and θ_avg and T_md) provided a significantly better fit to the data than one- (θ_avg or T_md), two- (RH_md and T_md), and three-parameter (θ_avg, T_md, and RH_md) models for most of the trees at both sites, suggesting that θ_avg and T_md had an important influence on both midday ψ_stem and midday ψ_leaf variation at both sites. Therefore, measurements of θ_avg and T_md could be made as an adjunct for the simple interpretation of midday ψ_stem or midday ψ_leaf to help refine irrigation scheduling in mature pecan orchards.

Table 1.

Multiple regression analysis with midday stem water potential (ψ_stem, MPa) at midcanopy (i.e., at 4.6 m tree height above the soil surface) as the dependent variable and average values of volumetric water content at the shallow soil depth of 0 to 40 cm (θ_avg, cm³·cm⁻³), midday air temperature (T_md, °C), and midday relative humidity (RH_md, %) as the independent variables.

View Table

Table 2.

Multiple regression analysis with midday leaf water potential (ψ_leaf, MPa) at midcanopy (i.e., at 4.6 m tree height above the soil surface) as the dependent variable and average values of volumetric water content at the shallow soil depth of 0 to 40 cm (θ_avg, cm³·cm⁻³), midday air temperature (T_md, °C), and midday relative humidity (RH_md, %) as the independent variables.

View Table

In an earlier study on the same pecan orchards, two-parameter regression models similar to the models proposed in this study were developed only for ψ_stem using the measured ψ_stem data for 2009. Except for north and south pecan trees at Site 2 under dry soil conditions (Fig. 4), the predicted ψ_stem values using these two-parameter models [in Table 6 of Deb et al. (2011a)] for 2010 and 2011 were consistently higher than the measured ψ_stem values (Figs. 3, 4, 6A, and 7A). The most plausible explanation for this underprediction is that measurements in the previous study (Deb et al., 2011a) were made between 1200 and 1400 hr Mountain Standard Time, and ψ_stem was more variable and not minimum. In contrast, using two-parameter regression models [ψ_stem or ψ_leaf = f (midday θ_avg and T_md)] developed in this study from 2010 data for both sites (Tables 1 and 2) showed better agreement between the measured and predicted ψ_stem as well as ψ_leaf measured on the both shaded and sunlit sides of pecan trees (Figs. 6 and 7). The values of R² ranged from 0.70 to 0.98 (P < 0.05) among trees at Site 1, whereas the values of R² were in the range of 0.85 to 0.98 (P < 0.05) among trees at Site 2. These empirical relationships [ψ_stem or ψ_leaf = f (midday θ_avg and T_md)] were evaluated at each of the two pecan orchards, where irrigation scheduling and other management practices, and midday conditions of microclimate variables, particularly air temperature and relative humidity during the period from June to September (Fig. 8), were similar during the 2010 and 2011 seasons. Therefore, the further evaluation of these empirical relationships under different climatic conditions, irrigation, and other pecan management practices may improve their reliability to estimate the water status of mature pecans.

Midday stem water potential (ψ_stem) and midday leaf water potential (ψ_stem) measured on the both shaded and sunlit sides of the tree at heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and the predicted ψ_stem using the two-parameter regression models at Site 1 during the period from June to Sept. 2011: (A) measured midday ψ_stem vs. predicted ψ_stem using two-parameter [multiple regression between ψ_stem, and θ_avg and T_md (Table 1); and Deb et al. (2011a)] regression models, and (B) measured midday ψ_leaf vs. predicted ψ_leaf using the two-parameter (multiple regression between ψ_leaf and θ_avg and T_md; Table 2) regression models. The measured ψ_stem and ψ_leaf values at each tree height represent the averages of six measurements on each of shaded and sunlit sides of the selected east, south, and north pecan trees at Site 1.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) and midday leaf water potential (ψ_stem) measured on the both shaded and sunlit sides of the tree at heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and the predicted ψ_stem using the two-parameter regression models at Site 1 during the period from June to Sept. 2011: (A) measured midday ψ_stem vs. predicted ψ_stem using two-parameter [multiple regression between ψ_stem, and θ_avg and T_md (Table 1); and Deb et al. (2011a)] regression models, and (B) measured midday ψ_leaf vs. predicted ψ_leaf using the two-parameter (multiple regression between ψ_leaf and θ_avg and T_md; Table 2) regression models. The measured ψ_stem and ψ_leaf values at each tree height represent the averages of six measurements on each of shaded and sunlit sides of the selected east, south, and north pecan trees at Site 1.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) and midday leaf water potential (ψ_stem) measured on the both shaded and sunlit sides of the tree at heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and the predicted ψ_stem using the two-parameter regression models at Site 1 during the period from June to Sept. 2011: (A) measured midday ψ_stem vs. predicted ψ_stem using two-parameter [multiple regression between ψ_stem, and θ_avg and T_md (Table 1); and Deb et al. (2011a)] regression models, and (B) measured midday ψ_leaf vs. predicted ψ_leaf using the two-parameter (multiple regression between ψ_leaf and θ_avg and T_md; Table 2) regression models. The measured ψ_stem and ψ_leaf values at each tree height represent the averages of six measurements on each of shaded and sunlit sides of the selected east, south, and north pecan trees at Site 1.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) and midday leaf water potential (ψ_stem) measured on the both shaded and sunlit sides of the tree at heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and the predicted ψ_stem using the two-parameter regression models at Site 2 during the period from June to Sept. 2011: (A) measured midday ψ_stem vs. predicted ψ_stem using three-parameter [multiple regression between ψ_stem, and θ_avg and T_md (Table 1); and Deb et al. (2011a)] regression models, and (B) measured midday ψ_leaf vs. predicted ψ_leaf using two-parameter (multiple regression between ψ_leaf and θ_avg and T_md; Table 2) regression models. The measured ψ_stem and ψ_leaf values at each tree height represent the averages of six measurements on each of shaded and sunlit sides of the selected north, south, and southwest pecan trees at Site 2.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) and midday leaf water potential (ψ_stem) measured on the both shaded and sunlit sides of the tree at heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and the predicted ψ_stem using the two-parameter regression models at Site 2 during the period from June to Sept. 2011: (A) measured midday ψ_stem vs. predicted ψ_stem using three-parameter [multiple regression between ψ_stem, and θ_avg and T_md (Table 1); and Deb et al. (2011a)] regression models, and (B) measured midday ψ_leaf vs. predicted ψ_leaf using two-parameter (multiple regression between ψ_leaf and θ_avg and T_md; Table 2) regression models. The measured ψ_stem and ψ_leaf values at each tree height represent the averages of six measurements on each of shaded and sunlit sides of the selected north, south, and southwest pecan trees at Site 2.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday stem water potential (ψ_stem) and midday leaf water potential (ψ_stem) measured on the both shaded and sunlit sides of the tree at heights of 2.5 m (lower canopy), 4.6 m (midcanopy), and 7.6 m (upper canopy) above the soil surface, and the predicted ψ_stem using the two-parameter regression models at Site 2 during the period from June to Sept. 2011: (A) measured midday ψ_stem vs. predicted ψ_stem using three-parameter [multiple regression between ψ_stem, and θ_avg and T_md (Table 1); and Deb et al. (2011a)] regression models, and (B) measured midday ψ_leaf vs. predicted ψ_leaf using two-parameter (multiple regression between ψ_leaf and θ_avg and T_md; Table 2) regression models. The measured ψ_stem and ψ_leaf values at each tree height represent the averages of six measurements on each of shaded and sunlit sides of the selected north, south, and southwest pecan trees at Site 2.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday air temperature (T_md) and relative humidity (RH_md) (1400 to 1500 hr Mountain Standard Time) at both Sites 1 and 2 during the 2010 and 2011 seasons.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday air temperature (T_md) and relative humidity (RH_md) (1400 to 1500 hr Mountain Standard Time) at both Sites 1 and 2 during the 2010 and 2011 seasons.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Midday air temperature (T_md) and relative humidity (RH_md) (1400 to 1500 hr Mountain Standard Time) at both Sites 1 and 2 during the 2010 and 2011 seasons.

Citation: HortScience horts 47, 7; 10.21273/HORTSCI.47.7.907

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Another aspect to be pointed out from Figures 6 and 7 is that the values of ψ_stem measured at different heights on both shaded and sunlit sides of the tree over several irrigation cycles during the 2011 season were almost the same (Fig. 6A and 7A), whereas the lower values of ψ_leaf of sunlit side leaves in comparison with their shaded counterparts were observed (Fig. 6B and 7B). From this perspective, it could be argued that when the midday parameters θ_avg and T_md were used as complementary factors for the interpretation of either midday ψ_stem or midday ψ_leaf at both sandy loam and silty clay loam sites, it might be more favorable to choose midday ψ_stem because it provided a relatively stable indicator of mature pecan water status on both shaded and sunlit sides of the pecan canopy.