Irrigation is one of the most important management tools used in pecan production and results in increased nut size, yield, nut quality, and precocity (Alben, 1957; Brison, 1974; Daniell, et al., 1979; Stein et al., 1989; Worley, 1982). Despite variations in available water resources among pecan producing regions, the question of irrigation frequency and the amount of water applied to pecan is a common concern. Much of the irrigation research regarding water use of pecan has occurred in arid and semiarid climates (Garrott et al., 1993; Miyamoto, 1983; Sammis et al., 2004; Wang et al., 2007). Georgia is the largest pecan producer in the United States with over 56,000 ha of mature commercial pecan orchards (USDA, 2012). This region of the humid southern United States receives an average of 127 cm or more rainfall annually; however, periods of moisture stress occur during the growing season, particularly during the months of August and September when pecans are in the kernel-filling stage and water demand is at its peak. Thus, irrigation has been proven to markedly enhance pecan production in the region (Worley, 1982). Yet, irrigation scheduling and management of pecans in humid climates is not well established. Daniell (1985) suggested an irrigation application rate of 22,440 L·ha−1·d−1 for mature pecans during the kernel-filling stage under drip irrigation based on 70% evaporation from a class A evaporation pan in Georgia. However, the study was conducted on a heavy clay soil and may underestimate the water requirements of pecan on the lighter textured soils in most of the southeastern U.S. pecan production region.
Current recommended irrigation schedules for pecan (Wells, 2007) in the region are based largely on a study by Daniell (1985) and data related to plant water stress, evapotranspiration, and soil water depletion generated in more arid climates. These schedules often fail to take into account that supplemental rainfall may reduce the need for irrigation water before the nut sizing period in humid climates. Trees within the pecan producing region of Georgia are commonly irrigated at rates and frequencies based mainly on grower experience and observation of crop growth and yield rather than on quantitative scientific information. With increasing agricultural water use, a growing population, and declining groundwater levels, irrigation efficiency in the region is necessary for sustainability.
A plant-based measurement, such as ψ, should be a straightforward indicator of plant water stress and, hence, of the need for irrigation (Peretz et al., 1984), because it measures the integrated effect of soil, plant, and atmospheric conditions on water availability within the plant itself. Midday stem ψ measurement can be used as a tool to indicate plant water stress and assist with irrigation management decisions. The relation of leaf ψ to leaf conductance in transpiring leaves may be obscured by the occurrence of a within-leaf ψ gradient, which is positively associated with the rate of leaf transpiration (Shackel and Brinckmann, 1985). Postexcision errors in leaf ψ (Turner and Long, 1980) have a similar obscuring effect on the relation between leaf ψ and leaf conductance, since excised leaves with a high conductance would desiccate and decline more rapidly in ψ than leaves with a low conductance. Excision artifacts and within-leaf ψ gradients can be eliminated by stopping leaf transpiration with a plastic or foil bag before leaf ψ is measured (Begg and Turner, 1970; Garnier and Berger, 1985; Meyer and Green, 1981), allowing leaf ψ to equilibrate with the ψ of the stem, and, therefore, would be a measure of stem ψ. Stem ψ is less influenced by short-term environmental variability than is transpiring leaf ψ (Meyer and Green, 1981) and, under some conditions, has been found to be more clearly related to soil-water conditions.
The purpose of this study was to compare a proposed reduced early season irrigation schedule for pecans in humid climates with the currently recommended schedule (Wells, 2007) using midday stem ψ as measured by the pressure chamber. Stem ψ was also correlated with volumetric soil moisture in the non-irrigated trees to establish a baseline relationship between the two on Tifton loamy sand, a common agricultural soil in the southeastern United States.
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