Relative Efficacy of Different Alfalfa Cultivar- Rhizobium meliloti Strain Combinations for Symbiotic Nitrogen Fixation

Publisher Website
Google Scholar
Abstract
Five R. meliloti isolates with different capabilities for expression of nitrogenase activity under symbiotic conditions were inoculated onto 4 representative Medicago sativa cultivars under aseptic conditions. Nitrogenase activities and respiratory activity were measured for whole plants and excised nodules. Dry weights and nodule numbers were also recorded after 4 wk of growth in plastic pouches on a N-free nutrient medium. Hydrogen evolution and acetylene reduction rates were used to calculate the fraction of reducing power allocated to dinitrogen reduction. Although the efficiency of the system defined in this way was poorly correlated with plant yield, a high linear correlation was obtained between yield and the algebraic product of nitrogenase activity and efficiency. High correlation (r > 0.78) was obtained between respiration and nitrogenase activity for whole plants as well as for excised nodules. Nodular respiration accounted for 10-20% of the total plant dark respiration. The 4 test cultivars exhibited significantly different symbiotic responses to the inocula, although trends in potential for expression of the nitrogenase system by the 5 R. meliloti strains were evident. There was significant interaction between the host plant and symbiont in determining nitrogenase activity and yield. This screening method allows quantitative discrimination between effective and ineffective host-inoculum combinations.