Rice is being increasingly cultivated in intermittently irrigated regious and also in aerobic soil in which Nitrate(NO3_3^-) plays important role in nutrition of plant.However,there is no information regarding the influence of nitrate on the overall growth and uptake of nitrogen(N)in rice plant.Solution culture experiments were carried out to study the effects of NO_3^- on the plant growth,uptake of N,and uptake kinetics of NH_4^+ in four typical rice(Oryza sativa L.)cultivars(conveutioual indica,conventional japonica,hybrid indica,and hybrid japonica),and on plasma membrane potential in roots of two conventional rice cultivars(indica and japonica)at the seedling stage.The results obtained indicated that a ratio of 50/50 NH_4^+-N/NO_3^--N increased the average biomass of rice shoots and roots by 20% when compared with that of 100/0 NH_4^+-N/NO_3^--N.In case of the 50/50 ratio,as compared with the 100/0 ratio,total N accumulated in shoots and roots of rice increased on an average by 42% and 57%,respectively.Conventional indica responds to NO_3^- more than any other cultivars that were tested.The NO_3^- supply increased the maximum uptake rate(V_(max))of NH_4^+ by rice but did not show any effect on the apparent Michaelis-Menten constant(K_m)value,with the average value of V_(max)for NH_4^+ among the four cultivars being increased by 31.5% in comparison with those in the absence of NO_3^-.This suggested that NO_3^- significantly increased the numbers of the ammonium transporters.However,the lack of effect on the K_m value also suggested that the presence of NO_3^- had no effect on the affinity of the transporters for NH_4^+.The plasma membrane potential in the roots of conventional indica and japonica were greatly increased by the addition of NO_3^-,suggesting that NO_3^- could improve the uptake of N by roots of the rice plant.In conclusion,the mechanisms by which NO_3^- enhances the growth and N uptake of rice plant was found by the increased value of V_^(max)of NH_4^+ and increased plasma membrane potential.Thus
