ACCUMULATION AND PARTITIONING OF NITROGEN AND DRY-MATTER BY CONTRASTING GENOTYPES OF MUNGBEAN (VIGNA-RADIATA (L) WILCZEK)

The accumulation and partitioning of nitrogen (N) and dry matter (DM) by mungbean (Vigna radiata (L.) Wilczek) were examined in a glasshouse experiment using three contrasting genotypes: a wild accession, a landrace cultivar and a recent commercial cultivar developed for mechanized agriculture. There was genotypic variation for all characteristics investigated. The landrace line accumulated N and DM at greater rates and to greater amounts than either of the other genotypes. However, yields of seed DM and N depended more on the harvest index (HI) and nitrogen harvest index (NHI) than on total biomass and total N accumulation, and both HI and NHI ranked commercial cultivar > wild genotype > landrace line. The three genotypes exhibited differing strategies for the partitioning of assimilates to seed. Seed production in the commercial cultivar occurred in two distinct flushes, with the immediate products of fixation and photosynthesis adequate to provide most of the requirements for N and DM for seed from the first flush. Seed from the second flush derived c. 37% of the N and 31% of the DM from remobilization of vegetative reserves. By contrast, 90% and 42% of seed N and DM respectively in the landrace line came from remobilization. The wild genotype was intermediate in that seed assimilates were concurrently derived from both N2 fixation, photosynthesis and remobilization. The implications for adaptation and further improvement in productivity through breeding are discussed.