Acetylene reduction activity in salt‐tolerant alfalfa/rhizobiumunder drought stress

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Abstract
Drought stress affects all stages of plant growth, particularly young seedlings. In this study, we sought to explore whether selected salt‐tolerant, efficient nitrogen‐fixing alfalfa, Medicago sativa L., could germinate, survive, and efficiently fix nitrogen under drought stress. Two‐day‐old seedlings were inoculated with an efficient salt‐tolerant strain of Rhizobium meliloti L. (1027 USDA‐Agricultural Research Service collection) and drought conditions simulated with polyethylene glycol‐6000 concentrations of 0.0, —0.4, —0.8, —1.0, or —1.2 MPa. Statistical analysis revealed no differences in the responses to drought stress by alfalfa that had previously shown salt tolerance. The number of nodules per seedling and rate of acetylene reduction activity were both decreased significantly, however, as drought stress increased. There also were statistically significant interactions between the water potentials and the six alfalfa populations. Alfalfa that had previously shown high salt tolerance behaved similarly with respect to seed germination and seedling growth, whether under salt or drought stresses. These results suggest that plant breeders who select alfalfa for salt tolerance may also, at the same time, be selecting for drought tolerance. Drought‐affected soils, normally deficient in rhizobia, can be improved when salt‐tolerant rhizobia that readily colonize salt‐tolerant alfalfa are introduced to such lands. We conclude that the osmotic potential induced by salinity stress also plays an important role in screening for drought stress, since drought often accompanies salinity.