Nitrate‐N and Rhizobium Strain Roles in Alfalfa Seedling Nodulation and Growth1

Alfalfa (Medicago sativa L.) seedlings grown for 14 day with initial levels of 0, 10, 25, and 50 µg N/ml as nitrate and inoculated with individual strains of Rhizobium meliloti or a mixture of these strains were investi. gated to learn how seedling growth and nodulation are mediated by nitrate N levels similar to those found in soils during alfalfa establishment. With a mixed Rhizobium culture, the proportion of the seedlings that nodulated declined from 84% at zero N to 53% at 50 µg N/ml. Significant (P = 0.05) differences in frequency of seedling nodulation occurred among individual Rhizobium strains at each N level, and significant (P = 0.05) strain × N interactions were observed. At 0 and 10 µg N/ml, number of nodules per seedling elicited by inoculation with the mixture of strains was significantly (P = 0.05) higher than with any of the five constituent strains. At 25 and 50 µg N/ml, number of nodules per seedling was similar with the strain mix. ture and with individual strains. At all N levels, seedlings inoculated with a strain mixture consistently developed more leaves and longer roots than uninoculated controis. Individual strains significantly (P = 0.05) differed in stimulation of leaf and root development at 0, 10, and 25 µg N/ml, but not at 50 µg N/ml. Variance analysis, leaf growth, and nodulation frequency experiments suggested that one of the five Rhizobium strains was less effective than the other four or than the mixture. The results suggest the possibility of improving alfalfa nodulation by selecting for increased nodule numbers in the presence of nitrate N, or by the identification of Rhizobium strains with maximum effectiveness at ambient N levels.