Study On Mechanism Of Difference Nitrate Reductase Activity In Different Wheat Cultivars

This paper is performed on regulation of nitrate reductase activity in vivo by nitrate availability in wheat cultivars with different protein content (PH82-2-2,PH85-16,Taishan l,Lumai 14); and then joined ammonium, to study the effect of it on the physiology and biochemiisty metabolism of nitrate reductase. The main results are as follows:1 Varietal differences in nitrate uptake and accumulation by wheat and relation with nitrate reductase activityIn light of low intensity, nitrate uptake rate was low in wheat seedlings and showed no difference between different varieties, while for the uptake in light of high intensity, significant varietal differences were observed. In seedlings pretreated with nitrate, and then allowed to absorb in light of high intensity prior to uptake studies, remarkable differences existed between varieties. The difference in rate of uptake as following: PH82-2-2 > PH85-16, Taishan 1>Lumai 14. The uptake kinetics suggested that Taishan 1 and Lumai 14 are the more efficient at absorbing nitrate than that of PH82-2-2 and PH85-16 from low concentration and are represented by relatively low Km(0.30 and 0.25mmol/L). PH82-2-2 and PH85-16 are considered to be less efficient in acquiring nitrate from low concentration solution, as reflected by the Km's for nitrate uptake of 0.59 and 0.84 mmol/L, respectively. But the Vmax's of PH82-2-2 and PH85-16 arehigher than that of Taishan 1 and Lumai 14. Differences in nitrate accumulation in leaf segment of the first seedling leaf existed between varieties. The capacity of accumulation in PH82-2-2 was larger than that of PH85-16.2 The metabolic pool of nitrate reduction in wheat leaves and relation with nitrate reductase activityThe total content of nitrate in wheat leaves, the nitrate reductase (NRA) in the absence of nitrate in assay medium, and the metabolic pool size (MPS) of nitrate increased with the increase of nitrate concentration in nutrient solution. NRA and MPS changed synchronously and reached their steady values at 2.0mmol/L of nitrate. When the nitrate concentration increased to 2.5mmol/L, most of the absorbed nitrate was accumulated in vacuoles, with little in cytoplasm, and the ratio of MPS to total nitrate decreased to some extent. This indicated that the content of nitrate in the metabolic pool regulated NRA in vivo in wheat seedling leaves. The nitrate reductase activity in seedling leaves of four wheat cultivars grown in nutrient solution with l.Ommol/L nitrate was assayed in vivo. The NRAs in leaves of cultivard studies were different in the absence of nitrate in assay medium, but such difference was not observed in the presence of 50 mmol/L nitrate in the assay medium. The NRAs of cultivars PH82-2-2 and PH85-16 were higher than those of Taishan 1 and Lumai 14, and their nitrate contents in metabolic pool and ratios of MPS to total nitrate content in leaves were also higher. Less effects of nitrate in assay medium on NRA of PH82-2-2 and PH85-16 were observed, and NRA decreased more slowly during the incubation peroid. The differences of NRA among wheat cultivars were due to the different nitrate content in metabolicpool of leaves.3 Regulation of nitrate reductase activity by upward transport and vacuolar efflux of nitrate in wheat leavesThe role of nitrate transport from root to shoot in the regulation of nitrate reductase activity (NRA) in vivo was investigated with seedlings of wheat PH82-2-2. When the seedlings were deprived of nitrate, both NRA and nitrate content in leaves decreased, but the former decreased more slowly than the latter. Once nitrate was supplemented to the culture solution, the rates of nitrate uptake and transport to shoot became higher, and the NRA increased. With an increase of the nitrate concentration in nutrient solution, the rates of nitrate uptake and transport increased with the rise of the NRA. The seedlings with higher rate of nitrate transport had higher NRA and nitrate contents in metabolic pool (MPS). This suggested that the rate of nitrate transport from root to shoot could regulate the NRA in leavea by changing the MPS of nitrate. During the incubation period of leaf tissues, the volume of metabolic pool grew larger after 40 min. This indicated that the nitrate in vacuoles (storage pool) could leak to cytoplasm for reduction when the contents of nitrate in cytoplasm decreased. When wheat leaf slices were incubated at 30°C in darkness, nitrate production ceased after 3 h.4 The physiology and biochemistry mechanism of the effect of ammonium to nitrate reductaseIn wheat seedlings with ammonium, nitrate uptake rate was low, the Vmax of nitrate uptake decreased and the Km had no significant differences. The total content of nitrate in wheat leaves, the nitrate reductase (NRA) in the absence of nitrate in assay medium, and the metabolic pool size (MPS) of nitrate decreasedwith the increase of ammonium concentration in nutrient solution, NRA and MPS changed synchronously. When the seedlings were treated with ammonium, the rate of nitrate transport to shoot was 1 Decreased and no stimulated the efflux of nitrate from vacuole. Active staining of nitrate reductase on polyacrylamide gel electrophoresis shows: Ammonium could repress NRA in leaves and decrease nitrate reductase protein (NRP) contents.