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

Diurnal and temporal patterns of stem water potential (ψstem) and leaf water potential (ψleaf) 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), ψstem and ψleaf 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 ψstem and ψleaf indicated that measurements of ψstem and ψleaf 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 ψleaf 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 ψstem or ψleaf and midday θavg (soil water content at 0 to 40 cm), air temperature (Tmd), and relative humidity (RHmd)] during the 2010 season revealed that two-parameter regression models [ψstem or ψleaf = f (midday θavg and Tmd)] were the most significant for the interpretation of midday ψstem or ψleaf at both sites. Using the two-parameter model, predictions of ψstem and ψleaf measured on the both shaded and sunlit sides of trees at three canopy heights for 2011 showed good agreement between measured and predicted ψstem and ψleaf (R 2 ranged from 0.70 to 0.98). Two-parameter models derived in an earlier study generally underpredicted ψstem both in 2010 and 2011, which further supported the importance of the time of midday ψstem and ψleaf measurements suggested in this study.

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Darby S. Kellum, Manoj K. Shukla, John Mexal and Sanjit Deb

Greenhouse gas (GHG) emissions are fueling global climate change, with methane and nitrous oxide being the primary agricultural gases emitted. It has been shown that N2O emissions correlate to moisture content fluctuations; however, emissions from agricultural fields in the semiarid regions of the Southwest where rewetting events occur regularly are not well established. The scope of this study was to quantify GHG emissions in correlation to soil moisture fluctuations and fertilizer application. The study was conducted continuously in two pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards between Aug. 2010 and Aug. 2011 on a sandy loam soil (La Mancha) and a silty clay loam soil (Leyendecker), both under normal management practices. The small chamber technique was used to measure GHGs. Emissions varied greatly throughout the year. The largest flux of CO2 at La Mancha and Leyendecker both occurred during a drying event immediately following an irrigation event: 84,642.49 μg·m−2·h−1 and 30,338.24 μg·m−2·h−1, respectively. The net CH4 flux at Leyendecker and La Mancha was close to zero with the largest emissions occurring during wetting events. Results showed that N2O emissions were maintained near the baseline except for the few days following an irrigation event. The largest emission peak at La Mancha occurred after irrigation and nitrogen application: 322.06 μg·m−2·h−1. The largest emission peaks of 26.37 and 1.13 μg·m−2·h−1 at Leyendecker and La Mancha, respectively, occurred after irrigation, nitrogen application, and tillage. Nitrogen application was the driving factor affecting N2O emissions at La Mancha, whereas soil moisture content was the driving factor at Leyendecker. Emission factors (EFs) at La Mancha and Leyendecker were 0.49% and 0.05%, respectively. A thorough accounting of GHG emissions is necessary for budgeting and identifying mitigation policy.

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Amir M. González-Delgado, Manoj K. Shukla and Brian Schutte

Appropriate soil management practices and correct use of agrochemicals for crop protection are essential to alleviate stresses that affect the quality and yield of pecans [Carya illinoinensis (Wangenh.) K. Koch]. A greenhouse study was conducted to evaluate the effect of soil surface manipulation and indaziflam application on evapotranspiration (ET) and gas exchange parameters of pecan trees, and phytotoxicity effects of indaziflam on pecan trees. Trees were planted in large pots with a homogeneous porous media (HM), including the controls (C), preferential flow channels open at the soil surface (PF), and preferential flow channels with surface soil manually tilled to 5 cm depth [shallow tillage (ST)]. Trees with HM, PF, and ST were treated with 50 g a.i./ha of indaziflam in 2014 and 2015, whereas an application rate of 150 g a.i./ha was used for trees with HM and ST in 2016. All trees were irrigated about every 14 days with 7 L of water in 2014 and 2015, and 5 L in 2016. A water balance analysis determined the ET in different treatments in 2014 and 2015. Gas exchange parameters were measured before and after irrigation in 2015 and 2016. Photosynthetic rates in C, HM, PF, and ST were consistently significantly lower before than after irrigation. PF and ST did not decrease the available water content of the soil because there was no significant difference in the volume of effluent, ET, and gas exchange parameters among the treatments. No herbicide injury symptoms and no influence on gas exchange parameters and ET were observed after using both application rates of indaziflam.

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Sanjit K. Deb, Parmodh Sharma, Manoj K. Shukla, Theodore W. Sammis and Jamshid Ashigh

Salinity responses and salinity-related suppression of budbreak of drip-irrigated pecan [Carya illinoinensis (Wangenh.) K. Koch] seedlings under different irrigation water salinity (ECIRR) levels were investigated in the pot-in-pot system. The 1-year-old pecan seedlings of rootstock ‘Riverside’ grafted with ‘Western Schley’ scions were transplanted in pots filled with sandy loam soil and grown for 2 years under the same amount of irrigation water but four irrigation ECIRR treatment levels consisting of 1.4 dS·m−1 (control), and three qualities of irrigation water obtained by using a solution of calcium chloride (CaCl2) and sodium chloride (NaCl) in a ratio of 2:1 (by weight) to reach the ECIRR levels of 3.5, 5.5, and 7.5 dS·m−1, respectively. The leachate electrical conductivity (ECd) was highly correlated with soil salinity (EC1:1) and was significantly higher when the irrigation ECIRR treatment levels increased from 1.4 (control) to 7.5 dS·m−1. However, both ECd and EC1:1 remained nearly constant within the same irrigation ECIRR treatment level during both years. Increasing salinity in irrigation water, particularly the ECIRR levels of 5.5 and 7.5 dS·m−1, showed significantly low seedling height and stem diameter growth and delayed or even inhibited budbreak in the seedlings. The EC1:1 that inhibited seedling heights, stem diameters, and budbreak was somewhere between 0.89 and 2.71 dS·m−1 (or ECIRR between 1.4 and 3.5 dS·m−1 and ECd between 2.10 and 4.86 dS·m−1), providing that soil water content was not a limiting factor in the root zone and irrigation water was uniformly distributed in the confined root zone to obtain uniform salt leaching. The visual symptoms of leaf scorch for irrigation ECIRR levels of 3.5, 5.5, and 7.5 dS·m−1 also indicated that somewhere between 0.89 and 2.71 dS·m−1 of the EC1:1, salt injury started to occur. Increasing salinity in irrigation water significantly increased chloride (Cl) accumulation but reduced nitrogen (N) content in the scorched leaves, particularly under the irrigation ECIRR levels of 5.5 and 7.5 dS·m−1. Leaf scorch symptoms in pecan seedlings were likely associated with Cl toxicity. No pecan seedlings under the irrigation ECIRR treatment levels of 5.5 and 7.5 dS·m−1 survived to the end of the 2-year growing period. Thus, threshold EC1:1 was somewhere between 0.89 and 2.71 dS·m−1 beyond which plant injury increases with increasing EC1:1 threatening the survival of pecan seedlings.

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Harmandeep Sharma, Manoj K. Shukla, Paul W. Bosland and Robert L. Steiner

Water saving, productivity, and quality of the chile pepper were evaluated under three irrigation treatments. Three drip irrigation treatments used were 1) control, where water was applied at the surface using two drip emitters; 2) partial root-zone drying vertically (PRDv), where subsurface irrigation was applied at 20 cm depth from soil surface; and 3) partial root-zone drying compartment (PRDc), where roots were divided into two compartments and irrigation was applied to one of the compartments on every alternate-day cycle for 15 days. Continuous measurements of soil water content were carried out during the growing seasons of 2013 and 2014, respectively. During both growing seasons, the stomatal conductance (g S) and net photosynthetic rates (Pn) were similar among all treatments including the control. In both PRD treatments, a higher rooting depth and root length density (RLD) than the control likely compensated for the water stress in dry soil zones by taking up more water from the water available parts of the root-soil system. In PRDc and PRDv treatments, 30% less water was applied than control without significant changes to plant stress expressed by stem water potential, plant height, capsaicinoid concentration, and yield. The increased irrigation water use efficiency (IWUE) demonstrated water saving potential of both PRD techniques for chile pepper production in water-limited arid environments.