THE N-2-FIXING CAPACITY OF PEANUT CULTIVARS WITH DIFFERING ASSIMILATEPARTITIONING CHARACTERISTICS

The contribution of symbiotic N-2 fixation to the total N budget of ir rigated crops of peanut (Arachis hypogaea L.) during vegetative and re productive growth was investigated using four peanut cultivars with di ffering patterns of dry matter (DM) partitioning to developing pods. E stimates of N-2 fixation were obtained with the N-15 natural abundance procedure by using a non-nodulating peanut genotype as a non-N-2-fixi ng reference plant. Partitioning was assessed on the basis of vegetati ve DM equivalents, with adjustments to pod DM based on relative synthe sis costs of vegetative and pod DM. Cultivars differed in crop duratio n, DM production and yield of pods and kernels. Despite large differen ces in derived DM partitioning coefficients among cultivars (0.68 to 1 .03), both total crop N and fixed N increased as a constant proportion of accumulated, energy-adjusted DM. Crop duration was the primary fac tor determining both total crop N and fixed N. In addition to fixation , all cultivars continued to accumulate soil mineral N throughout the season. However, in all cultivars except TMV-2, crop N uptake during r eproductive growth was insufficient to meet the demands of developing pods and N was remobilized from vegetative plant parts. Remobilized N was almost exclusively N derived originally from N-2 fixation. Despite relatively high levels of N-2 fixation (from 140 to 210 kg N ha(-1), depending on crop duration), all cultivars except Virginia Bunch showe d a negative apparent N balance when the amounts of N-2 fixed were com pared to N removed in pods at final harvest. This was primarily due to high N harvest indices (0.62 to 0.73), and is likely to be a feature of many recently released, high yielding cultivars.