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 biological nitrogen fixation (BNF) and photosynthetic activity (Po), nine mashbean cultivars (three for each of early, medium, and late maturity 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 weight, leaf area, leaf number, Po, and acetylene reduction activity (ARA) were determined at the vegetative and flowering stages. At low P, the relative whole plant dry weight was reduced to more than 50% which was more significant in early and medium maturity groups than late maturity one. Low P application led to an increase in the dry matter distribution into roots in all the cultivars. Low P conditions decreased both leaf number and average leaf area, and those reductions were more pronounced in early maturity cultivars, which resulted in a greater reduction of photosynthesis on a whole plant basis than BNF at both growth stages. However, Po remained relatively stable at both P levels regardless of the maturity groups. The P content was significantly decreased in leaf blades and stem + petioles of all the cultivars. Nodules and roots showed a relatively higher P content than other plant parts, suggesting that under suboptimal P conditions, underground parts acquire P preferentially. Under the low P application conditions, all the cultivars 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, suggested that the early and medium cultivars are more susceptible to low P conditions in terms of biomass production than late maturity ones because of the larger reduction in photosynthesis due to the inhibition of leaf emergence and development than BNF.