Ontogenic variation in nitrogen assimilation,nodulation and nitrogen fixation in groundnut (Arachis hypogaea) genotypes*

Seasonal pattern of acetylene reduction (AR) and shoot nitrogen accumulation was studied in nine groundnut cultivars. Shoot N accumulation by all the cultivars was maintained until shortly before maturity and it occurred faster over the reproductíve growth phase than over the earlier phases. In all cultivars plant AR (PAR) did not reflect this pattern of N accumulation, being greater over the vegetative and pod initiation phases. This suggests that the commonly observed low PAR values for groundnut over the reproductive growth phase may be the result of factors other than sink competition. There were significant interactions of cultivar with stage of crop growth for PAR, nodule mass, and specific nitrogenase activity (SNA). Virginia types generally showed better nodulation, higher N₂-fixing capacity (both PAR and SNA) than valencias, and significant differences were observed between cultivars within a botanical type.