COMPARISON OF PHOSPHORUS DEFICIENCY EFFECTS ON THE GROWTH-PARAMETERS OF MASHBEAN, MUNGBEAN, AND SOYBEAN

Experiments were conducted in order to examine the inter-specific diff erence in low P tolerance in terms of biomass production, photosynthet ic rate per unit leaf area (Po) and biological nitrogen fixation (BNF) . Three legume species: mashbean (Vigna aconitifolia cv. Mash-88), mun gbean (Vigna radiata cv. Moong-6601), and soybean (Glycine max L. cv. Tamahomare) were grown at different levels of P applied in 1993 and 19 94. Dry matter production increased in all the species with the increa sing P levels. At low P levels up to 25 kg P ha(-1), soybean displayed a relatively higher biomass production than mashbean and mungbean, wh ich was mainly caused by the promotion of root growth and leaf area ex pansion. In low P mashbean plants leaf area development was severely i nhibited while Po remained relatively stable in all the species irresp ective of the P levels except for extremely low Po at later growth sta ges without application of P. A positive correlation between the amoun t of N accumulation and acetylene reduction activity (ARA) was found i n all the species (r=0.77). At low P levels, soybean absorbed a large r P amount than other species and the amount of P absorbed was positiv ely correlated with biomass production (r=0.99*). At low P levels, th e P concentration decreased in all the plant parts and such a decrease was pronounced in the nodules. These results suggest that soybean has a higher tolerance under suboptimal conditions of P supply compared w ith mashbean and mungbean, mainly due to a higher ability of leaf area expansion and absorption activity of P through expansion of root inte rception.