The N₂-fixing capacity of peanut cultivars with differing assimilate partitioning characteristics

Abstract

The contribution of symbiotic N2 fixation to the total N budget of irrigated crops of peanut (Arachis hypogaea L.) during vegetative and reproductive growth was investigated using four peanut cultivars with differing patterns of dry matter (DM) partitioning to developing pods. Estimates of NZ fixation were obtained with the 15N natural abundance procedure by using a non-nodulating peanut genotype as a non-N2-fixing reference plant. Partitioning was assessed on the basis of vegetative DM equivalents, with adjustments to pod DM based on relative synthesis costs of vegetative and pod DM. Cultivars differed in crop duration, DM production and yield of pods and kernels. Despite large differences 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 factor 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 reproductive growth was insufficient to meet the demands of developing pods and N was renlobilized from vegetative plant parts. Remobilized N was almost exclusively N derived originally from N2 fixation. Despite relatively high levels of N2 fixation (from 140 to 210 kg N ha-1, depending on crop duration), all cultivars except Virginia Bunch showed a negative apparent N balance when the amounts of N2 fixed were compared 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.