Cultivar differences in assimilate partitioning and capacity to maintain N2 fixation rate in pea during pod-filling

Effects of plant development on the rate of N₂ fixation and assimilate partitioning in pea were investigated. In growth cabinets, N₂ fixation declined with the onset of pod-filling in a small, determinant cultivar of field pea (cv. Express). In contrast, in a larger, indeterminant variety (cv. Century) N₂ fixation rate did not peak until several weeks into the pod-filling period. The smaller cultivar, Express, fixed 66% less nitrogen than the cultivar Century. Dry matter and nitrogen content increased during pod-filling in nodules but declined, or held steady in leaves, stems, and roots for Century. This indicates that nodules could compete successfully with pods for assimilates during pod-filling. In contrast, dry matter and nitrogen content did not increase in all non-reproductive plant parts (including nodules) for the smaller cultivar, Express. Under field conditions, rates of N₂ fixation declined severely for cv. Century with the onset of pod-filling. It is proposed that maintenance of the rate of N₂ fixation with the onset of pod-filling is dependent on genetic and environmental factors which influence the source-to-sink ratio of carbon in the plant at the start of pod-filling. This hypothesis is incorporated into a proposed scheme of how to maximize nitrogen accumulation by a legume in a growing season.