Estimating Nodule Activity of Sainfoin, Alfalfa, and Cicer Milkvetch Seedlings

Legume cultivars with improved symbiotic dinitrogenfixing capabilities greatly benefit agriculture. Limited research on several forage legume species has suggested that breeding for improved dinitrogen fixation should be possible, but effective screening techniques are needed if significant breeding progress is to be made. This paper describes a study conducted on three forage legumes to determine the relationship of several plant characteristics to dinitrogen fixation, with a view to developing a nondestructive method of identifying plants with superior dinitrogen-fixing ability. Growth of 500 seedlings of sainfoin (Onobrychis viciaefolia Scop.), 425 of alfalfa (Medicago sativa L.), and 417 of cicer milkvetch (Astragalus cicer L.) was measured in a N-free rooting medium. At harvest, seedlings were clipped above the cotyledonary node, nodules were removed and dry weights of shoot, root, and nodules were determined. The amount of acetylene reduced by nodules and the nitrogen content of shoots were determined on one-fifth of the plants before harvest. Large variations were observed for all of the variables measured. For all three species, variation in nodule dry weight accounted for 60 to 71% of variation in shoot dry weight. Variation in nodule activity, as measured by the acetylene reduction assay, accounted for 18 to 45% of variation in shoot dry weight. These results suggested that shoot dry weight was a good estimator for nodule dry weight and nodule activity in a N-free rooting medium. The amount of acetylene reduced by alfalfa nodules was not as closely related to shoot dry weight as it was for sainfoin or ucer milkvetch. A poor relationship between shoot percent N and shoot growth in sainfoin and alfalfa suggested that all available N in the shoot was used for top growth by these species, whereas in ucer milkvetch, a portion of the N in the shoot was not used for growth.