Salinity stress is an ever-present environmental constraint to crop productivity in arid and semiarid regions. The quality of irrigation water remains a primary factor influencing soil salinity. In the arid and semiarid southwestern United States, the occurrence of salt-affected soils in pecan orchards is common in upland soils irrigated with moderately saline water (1 to 2 dS·m−1) and in alluvial soils with poor drainage or inadequate permeability (Miyamoto and Storey, 1995). High evapotranspiration rates exceeding rainfall as well as poor-quality irrigation water containing considerable amounts of salts have accentuated salt problems across pecan-producing areas of this region. Salts present in the soil and irrigation water are expected to increase as the competition for freshwater uses among domestic, agriculture, and industrial sectors intensifies; the expectation of optimized consumptive water use of pecans and enhanced irrigation efficiency grows; and the increased emphasis on conservation of water resources in conflict with the necessity of large amounts of good-quality irrigation water for pecan production continues.
Irrigation water in pecan-producing areas of the southwestern United States, a leading pecan-producing region in the nation, is considerably saline and less sodic (Miyamoto and Storey, 1995). The supply of good-quality water from river irrigation projects has become increasingly limited in many areas. The shortfall is commonly supplemented with groundwater that may have elevated salinity. Most irrigation water from groundwater sources in the pecan-producing areas of the southwestern region contains 500 to 1500 ppm of dissolved salts, resulting in 5 to 15 tons of salts per hectare carried into the orchard annually (Miyamoto et al., 1995). Picchioni et al. (2000) surveyed 15 commercial pecan orchards along a 120-km stretch of the middle Rio Grande basin in southern New Mexico to evaluate whether pecan cultivation in this region was threatened by high soil salinity. Ten of the 15 sites were found to be on soils considered too saline for pecan trees with the soil saturated paste electrical conductivity (EC) values between 2 and 3 dS·m−1 at the upper 0 to 60 cm of soil depth. Water availability is considered one of the major constraints to pecan productivity in the southwestern United States (Deb et al., 2011, 2013), but the decline in irrigation water quality, particularly salinity, continues to be a major challenge to pecan production in this region.
Pecans are among the most salt-sensitive tree crops, yet there is a paucity of research concerning the effects of soil and irrigation water salinity on the growth of pecans. Miyamoto and Gobran (1983) reported that symptoms of salt damage to mature pecan trees ranged from marginal leaf tip burn to mortality of trees. The threshold soil salinity of the saturation extract in the main root zone for tree growth appears to be ≈2.0 dS·m−1 when sodium (Na+) is the dominant cation (Miyamoto et al., 1986; Miyamoto and Nesbitt, 2011). Tree growth decreases when soil salinity of the saturation extract reaches 2 to 3 dS·m−1 and ceases at soil salinity of 4 to 5 dS·m−1, and branch dieback begins at soil salinity of ≈5 to 6 dS·m−1 in the soil saturation extract (Miyamoto et al., 1986; Miyamoto and Nesbitt, 2011). High soil salinity also causes reduction in nut size and yield (Miyamoto et al., 1986).
The responses of pecans to high salinity may be expected to vary with growth stage. There have been only a few studies that have examined the responses of pecan seedlings to irrigation water salinity levels. In particular, it is difficult to assess the effects of increased salinity of irrigation water or quality of irrigation water on budbreak or growth of pecan seedlings. In a greenhouse experiment where healthy seedlings were subjected to short-term exposure to saline solutions, Faruque (1968) reported that leaf injury of pecan seedlings (cv. Riverside) was related to chloride (Cl–) but not Na+ or SO42– ions. Miyamoto et al. (1985) studied the effects of Na+ and Cl– on growth and ion uptake of three rootstock cultivars, Apache, Burkett, and Riverside, which were grown in outdoor lysimeters and irrigated with eight different qualities. Seedling growth, evaluated in terms of leaf, stem, and root weights, was inversely related to Na+ concentrations in both soil solutions and seedling leaves. Because the Riverside cultivar absorbed substantially lesser Na+, Miyamoto et al. (1985) recommended that ‘Riverside’ was a better suited rootstock for saline areas. However, no studies have examined whether increased salinity of irrigation water suppresses budbreak of grafted seedlings or whether salt-affected buds adjust to salt stress under higher irrigation water salinity treatment levels. The objective of this study was to compare salinity responses and salinity-related suppression of budbreak of drip-irrigated pecan seedlings of rootstock ‘Riverside’ grafted with ‘Western Schley’ scions under different irrigation water salinity treatment levels.
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