RESPONSES OF DINITROGEN FIXATION AND PHOTOSYNTHETIC ACTIVITY IN MASHBEAN (VIGNA-ACONITIFOLIA) CULTIVARS UNDER LOW P CONDITIONS

To examine the factor controlling the low P tolerance in the case of b iological nitrogen fixation (BNF) and photosynthetic activity (Po), ni ne mashbean cultivars (three for each of early, medium, and late matur ity groups) were selected for field experiments under two P levels of 10 kg P ha(-1) (low P) and 100 kg P ha(-1) (control) and dry matter we ight, leaf area, leaf number, Po, and acetylene reduction activity (AR A) were determined at the vegetative and flowering stages. At low P, t he relative whole plant dry weight was reduced to more than 50% which was more significant in early and medium maturity groups than late mat urity one. Low P application led to an increase in the dry matter dist ribution into roots in all the cultivars. Low P conditions decreased b oth leaf number and average leaf area, and those reductions were more pronounced in early maturity cultivars, which resulted in a greater re duction of photosynthesis on a whole plant basis than BNF at both grow th stages. However, Po remained relatively stable at both P levels reg ardless of the maturity groups. The P content was significantly decrea sed in leaf blades and stem + petioles of all the cultivars. Nodules a nd roots showed a relatively higher P content than other plant parts, suggesting that under suboptimal P conditions, underground parts acqui re P preferentially. Under the low P application conditions, all the c ultivars absorbed less than 50% of P amount relative to the respective control plants. Low P application conditions resulted in more than 50 % reduction in N accumulation than in the control. It is, therefore, s uggested that the early and medium cultivars are more susceptible to l ow P conditions in terms of biomass production than late maturity ones because of the larger reduction in photosynthesis due to the inhibiti on of leaf emergence and development than BNF.