Root Growth and Yield of Differing Alfalfa Rooting Populations under Increasing Salinity and Zero Leaching

stand of alfalfa under irrigation extended to 2.5 m (Dudley et al., 1994). It may be advantageous for deep-rooted Accumulation of salinity in the root zone can be detrimental to crops such as alfalfa to exploit the lower average salinity sustained crop production. Irrigation, even with moderately saline water, pushes accumulated salts deeper into the root zone, allowing of the upper root zone preferentially as salinity increases roots to proliferate in regions of relatively low salinity. Two alfalfa (Minhas and Gupta, 1993). (Medicago sativa L.) subpopulations with lowand high-fibrous rootSelection in the field for root traits in alfalfa under ing characteristics, MnPL-9-LF and MnPL-9-HF, were used to test varying environmental conditions has had a positive the effectiveness of increased rooting on yield when plants were irrieffect on yield. Typically, nondormant varieties of algated with saline water but without leaching. Treatments were three falfa are strongly tap rooted (Smith, 1993), while inlevels of heterogeneous root zone salinity predicted by the SOWACH creased dormancy is associated with greater branching model to represent 10, 20, and 30 yr of irrigation with saline water. of the tap root and greater fibrous root mass (Barnes Plants were grown for five successive harvests in 10-cm-diam., 130et al., 1988). To improve winter survival, Saindon et al. cm-deep cylinders. The treatments were constructed with NaCl and (1991) selected for root yield within two alfalfa cultivars. gypsum. As soil became depleted to 50% extractable water, irrigation water with an electrical conductivity (EC) of 2.8 dS m 1 was applied. This resulted in increased root branching and was correBy the fifth harvest, soil solution EC from the top to the bottom of lated with higher forage yield. After evaluating the root the profile ranged from 3 to 12 dS m 1 for the control and from 3 to growth of several genotypes differing in yield, a positive 23 dS m 1 for the highest salinity treatment. Root production of the correlation was shown between shoot growth and highly high-fibrous root type was stimulated more at low and medium salinity branched root architecture. A larger root system or one than that of the low-fibrous root type. Across salinity treatments, final that has an architecture better suited for soil resource root length density (cm root length per cm3 soil volume) was 24% acquisition was also proposed for improving the yield higher for the high-fibrous root type, and herbage yield of the highof beans (Lynch and van Beem, 1993). fibrous root type was 14% higher than that of the low-fibrous root To improve nodulation and thus nitrogen fixation, type. Differential rooting was greatest in the upper half of the root which occurs primarily on fibrous roots, Viands et al. zone. High fibrous rooting in alfalfa is a trait with potential usefulness as a salinity stress avoidance mechanism. (1981) produced two subpopulations of alfalfa that differed significantly in their rooting characteristics, a lowfibrous (MnPL-9-LF) and high-fibrous (MnPL-9-HF) subpopulation. It was our hypothesis that as salts accuA problem in irrigated agriculture is the gradmulated in the root zone, the alfalfa subpopulation with ual buildup of salts in the root zone. Periodic leachgreater fibrous rooting would have higher forage yield ing with low-saline water can greatly reduce the concenunder increasing heterogeneous salinity, because these tration of soluble salts, but may have undesirable plants would generate more root mass in the less-saline consequences for users of downstream drainage water. regions of the upper root zone. The objective of this One strategy for reducing downstream impacts of low study was to compare the forage yield and root growth quality water is to forego leaching and store salt in of subpopulations with different rooting characteristics the lower portion of the root zone. Irrigating in small under a range of heterogeneous salinity levels. amounts with increased frequency keeps the water content high and salinity low in the upper root zone. A root zone of heterogeneous salinity develops under irrigation MATERIALS AND METHODS without leaching, and yield reductions occur as salt accuConstruction of Rooting Cylinders mulates first in the lower then the upper root zone, which is particularly salt sensitive (van Schilfgaarde et Randomly selected plants from two near-isogenic subpopulations of alfalfa selected for low(MnPL-9-LF) and highal., 1974; Jame et al., 1984; Smith, 1993). fibrous (MnPL-9-HF) rooting characteristics (Viands et al., For long-term productivity, perennial crops such as 1981) were grown in cylinders constructed from polyvinylchloalfalfa must be able to adapt to increasing heterogeride (PVC) pipe. The design of the cylinders was adapted neous root zone salinity. The root zone of an established from LeNoble et al. (1996). Each cylinder was 10 cm diam. and 130 cm deep, with a wall thickness of 3 mm. One side L.V. Vaughan, Abgenix, Inc., 6701 Kaiser Drive, MS 41, Fremont, was replaced with an 8-cm-wide, flat, 3.2-mm-thick, clear PVC CA 94555; J.W. MacAdam and L.M. Dudley, Dep. of Plants, Soils, window the length of the cylinder, bonded in place with Weldand Biometeorology, Utah State Univ., Logan, UT 84322-4820; and On epoxy (Industrial Polychemical, Gardena, CA). Caps made S.E. Smith, Dep. Rangeland and Forest Resources, School of Renewof PVC with an inside diameter of 10 cm were bonded to the able Natural Resources, Univ. of Arizona, Tucson, AZ 85721. Partial funding was provided by Pacificorp and funding for a preliminary study was provided by San Diego State Univ. Foundation. ContribuAbbreviations: EC, electrical conductivity; ECe, electrical conductivity tion of the Utah Agric. Exp. Stn., Journal Series no. 7429. Received of saturation extract of soil paste; ESW, extractable soil water; PVC, 2 Oct. 2001. *Corresponding author ([email protected]). polyvinylchloride; RLD, root length density; TRI, traced root intensity. Published in Crop Sci. 42:2064–2071 (2002).