At Nh <sub> 4 </ Sub> ~ + Different Rice Genotypes At Seedling Stage No <sub> 3 </ Sub> ~ - Absorption And In Vivo To Restore

The nitrate nitrogen (NCV-N) uptake characteristics of different rice genotypes and the nitrate reductase activity (NRA) in rice leaves were studied by water culture methods. Moreover, the mechanics of the effects of ammonium nitrogen (NH4+-N) on NCV-N uptake of rice seedlings was explored.If NH4+-N and NCV-N were supplied simultaneously in the medium NH4+-N was absorbed more rapidly than NCV-N by the roots of rice seedlings (14 d). Japonica rice responsed better to NH4+-N than to NCV-N, while Indica rice had a more effective response to NO3--N than to NH4+-N. NO3--N slightly affected the uptake of NH4+-N. The effect of NH4+-N on the uptake of NCV-N was different among rice genotypes. The presence of NH4+-N in the medium considerably repressed the NCV-N uptake of Japonica rice seedlings (14 d), while little influenced that of Indica rice seedlings (14 d), and even exerted marginal stimulation at the low level of NH4+-N (< 1.0 mmol L-1). Since the inhibition of NH4+-N occurred obviously within one hour after adding NH/-N, the rapidity of the effect reasonably led to the presumption that the inhibition resulted from the direct effects of NH4-N at the plama membrane.The relation between NCV-N concentration at the root surface and the uptake of NCV-N can be characterized by using Michaelis-Menten kinetics. The equation considers two parameters, Vmax and Km, which are respectively measures of the maximum rate of uptake and the affinity of the uptake sites for NO3--N. The results of our experiments showed that the kinetic parameters were apparently different among the rice genotypes and the differences were clearer with the existence of NH4+-N. As comparing Indica rice with Japonica rice, hybrid rice with conventional rice, upland rice with hydro rice, NCV-N showed more effective effect on the former than NH4+-N. The superiority, mainly reflected on the Vmax for the NCV-N uptake at the early seedlingstage (20 d). If NH4+-N and NO3--N were presented together the Km for the NO3--N uptake was affected very little, while the Vmax of different rice genotypes was reduced to different extent. This inhibition appeared to be noncompetitive. The Vmax of Japonica rice seedlings (20 d) for the NO3--N uptake sharply decreased with the presence of NH4+-N, while the Vmax of Indica rice seedlings (20 d) little changed and even was slightly stimulated. The Vmax of hybrid rice seedlings (20 d) was inhibited more obviously than that of conventional rice seedlings (20 d). In addition, the inhibition of NH4+-N to upland rice seedlings (20 d) was rather little.The rate of NO3--N uptake showed a descending tendency with the passage of time and the dynamic parameters changed. The Vmax for NO3--N uptake decreased and the Km increased, viz. the uptake of NO3--N slowed down and the affinity of the uptake sites for NO3--N reduced. The differences among the genotypes at the late seedlings stage (50 d) were mainly reflected on the Km for the NO3--N uptake and this was not as well as that at the early seedling stage (20 d).The rice seedlings (7 d) without induction had low NRA hi the leaves, and the variation among the genotypes was not clear. The leaf NRA enhanced with the induction of NO3--N, while reduced with the presence of NH4+-N. NH4+-N affected the leaf NRA of Japonica rice seedlings (7 d) more apparently than that of Indica rice seedlings (7 d). Though the leaf NRA of rice seedlings was very sensitive to NH4+-N, the phenomenon did not mean that to affect NRA was the only means to affect the uptake of NO3--N. The leaf NRA lowered with the passage of time, and the tendency among the genotypes at the seedling stage (20 d) was similar.