The Interactive Effects of Water Salinity and Management on Symbiotic Nitrogen Fixation in Alfalfa

A greenhouse study was conducted to assess the interactive effects of three irrigation water salinity levels (1.0, 3.0, and 9.0 mmho/cm) and three quantities of water app lied per irrigation (120, 240, 360 ml) on plant growth and nitrogen fixation by alfalfa (Medicago sativa L. cv. Resistador). Harvest dates corresponded to 10, 30, and 50 days after the initiation of salt and water treatments which were started after nodulation had been established in young plants. Alfalfa top growth was limited by both salt and water stresses. Irrigation water salinity had a greater effect on top growth than root growth while root distribution was unaffected by either the quantity of water applied or by water salinity. The effects of salinity on plant growth were reduced in the presence of limiting moisture. The specific nodule activity (mmol c2H4/hr/g) of water stressed alfalfa plants was enhanced by increasing the quantity of water applied at each irrigation and was adversely effected by increased irrigation water salinity. In contrast, both nodulation and nodule growth were insensitive to salt stress and sensitive only to severe moisture stress. Alfalfa plants continued to exhibit acetylene reducing capacity at the third harvest even under severe moisture and salt stress. The species apparently continues to fix nitrogen even though environmental stress is quite substantial