Root growth and yield of differing alfalfa rooting populations under increasing salinity and zero leaching

Accumulation of salinity in the root zone can be detrimental to sustained crop production. Irrigation, even with moderately saline water, pushes accumulated salts deeper into the root zone, allowing roots to proliferate in regions of relatively low salinity. Two alfalfa (Medicago sativa L.) subpopulations with low- and high-fibrous rooting characteristics, MnPL-9-LF and MnPL-9-HF, were used to test the effectiveness of increased rooting on yield when plants were irrigated with saline water but without leaching. Treatments were three levels of heterogeneous root zone salinity predicted by the SOWACH model to represent 10, 20, and 30 yr of irrigation with saline water. Plants were grown for five successive harvests in 10-cm-diam., 130-cm-deep cylinders. The treatments were constructed with NaCl and gypsum. As soil became depleted to 50% extractable water, irrigation water with an electrical conductivity (EC) of 2.8 dS m(-1) was applied. By the fifth harvest, soil solution EC from the top to the bottom of the profile ranged from 3 to 12 dS m(-1) for the control and from 3 to 23 dS m(-1) for the highest salinity treatment. Root production of the high-fibrous root type was stimulated more at low and medium salinity than that of the low-fibrous root type. Across salinity treatments, final root length density (cm root length per cm(3) soil volume) was 24% higher for the high-fibrous root type, and herbage yield of the high-fibrous root type was 14% higher than that of the low-fibrous root type. Differential rooting was greatest in the upper half of the root zone. High fibrous rooting in alfalfa is a trait with potential usefulness as a salinity stress avoidance mechanism.