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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.

The greater susceptibility to desiccation of the Rhizobiurn legurninosarum group of rhizobia compared with the slow-growing rhizobia is related to the different amounts of water retained by these groups of bacteria at any relative vapour pressure, rather than differences in rates of water movement into or out of the bacteria. The higher retention of water by the R. legurninosarum group at any relative vapour pressure is related to greater availability of adsorptive surface area, and to higher surface energies resulting in a greater affinity for water. Although the internal osmotic pressure is greater in the slow-growing group than in the R. leguminosarum group, it is concluded that differences in internal solute concentrations cannot account for the different adsorption isotherms found for these groups.Montmorillonitic clay protects the fast-growing R. leguminosarum group from the effects of desiccation, but not the slow-growing group. Ca-montmorillonite retains greater quantities of water at any relative vapour pressure than either group of bacteria. Possible mechanisms of protection of the bacteria by the clay are discussed in terms of relative affinities for water under desiccation conditions.

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