Mi. Chaudhary et K. Fujita, RESPONSES OF DINITROGEN FIXATION AND PHOTOSYNTHETIC ACTIVITY IN MASHBEAN (VIGNA-ACONITIFOLIA) CULTIVARS UNDER LOW P CONDITIONS, Soil Science and Plant Nutrition, 44(1), 1998, pp. 31-41
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.