Screening And Physiological Mechanism Of High Nitrogen Use Efficiency In Rice Cultivars

Rice is the most important food crop in China,and increasing the yield per unit area of rice is the main task of rice production.However,the high yields of rice in China are mostly obtained based on the application of high amounts of chemical fertilizers,which brings huge environmental issue.The absorption and utilization of nitrogen by rice are two important processes of the nitrogen(N)cycle in agricultural ecosystems.Fully exploiting the genetic potential of crops to absorb and utilize N is an important way to improve the utilization rate of N fertilizer.Therefore,it is valuable to select nitrogen-efficient rice from the rice varieties that are currently planted in a large area and to evaluate their potential for reducing N and increasing efficiency.Then,on the basis of analyzing the biological mechanism of rice’s efficient absorption and utilization of nitrogen,it is proposed how rice nitrogen-efficient genotypes reduce N input and enhance N efficiency.The utilization of nitrogen-efficient rice cultivars is of great importance to achieve a win-win goal for high-yield rice and environmental protection.In this study,34 japonica rice cultivars with a large area of popularization in Jiangsu Province in the past five years were selected to evaluate their grian yields and N use efficiency under two N rates in field trial.After nitrogen-efficient rice cultivars with high grain yields were screened,we further evaluated their potential to enhance N efficiency and reduce N fertilizer input.Furthermore,due to the contradiction between excessive application of basal tiller fertilizer and low N absorption peak of rice plants at seedling stage and the contradiction between rice tillers and nitrogen supply intensity,preliminary study was conducted to clarify biological mechanism about N efficient absorption at the seedling stage and the biological mechanism of reducing the need for ammonium in rice tillers,and then reducing the application of basal tillers to reduce N losses.The main results are obtained as follows:1.Using 34 japonica rice varieties largely cultivated in Jiangsu Province as the test subjects,the grain yield and N use efficiency of the tested rice were studied at two N levels supply in field trial.The results showed that rice yield and N absorption increased with the increase of N application,while the N efficiency was opposite.Under the same N supply level,the yield,biomass,N uptake and N efficiency were significantly different in different japonica rice varieties.Based on the average yield at two N levels,the 34 rice varieties are divided into 4 groups:double-N efficiency(N efficiency under two N rates),high-N efficiency(N efficiency under high N rate),double low efficiency(N low efficiency under two N rates)and low N efficiency(N efficiency under low N rate).2.Using the selected two N efficient and two N inefficient rice as test materials,five N application levels(0,180,216,270 and 350 kg ha-1)were set under field conditions,of which 216 kg ha-1 is based on the 54 kg ha-1 reduction of base and tillering fertilizer in the treatment of 270 kg N ha-1.The results showed that,under the five N supply levels,the yield of two N-efficient rice was higher than that two N-inefficient rice;by analyzing the composition of rice yield,we found that the panicle numbers of N-efficient rice varieties were significantly higher than that of N-inefficient rice.Compared with the 270 kg ha-1 treatment,the difference in tiller numbers of N-inefficient and N-efficient rice became greater after the application of basal tiller fertilizer was reduced by 20%(216 kg ha-1),indicating that the number of tillers in N-inefficient rice varieties are more sensitive to certain nutrient supplies.Further results showed that,compared with N-inefficient varieties,N-efficient rice varieties can efficiently absorb N in the early stages of growth,form more tillers and ears and achieve high yield.The maximum theoretical yield of rice varieties with different N efficiencies and the amount of N fertilizer required by each variety to reach its maximum theoretical yield are very different.Taking the regional test variety Wuyujing 3 as a reference,the N saving potential of the N-efficient rice variety Yangyujing 2 was 17.7%;the N saving potential of the N-efficient rice variety Yang 4038 was 31.1%.3.Hydroponic cultures were used to analyze tiller growth development of rice plants(Nipponbare)in response to two forms of N(NH4+ and NO3-).Compared with NO3nutrition,NH4+ can promote the growth of rice tillers mainly through inducing the elongation of tiller buds rahter than the occurrence of tiller buds.Compared with NO3-,the N concentration in NH4+-treated rice plants increased significantly.Under the low NH4+concentration(0.2 mM)treatment,the difference in tillering between N-inefficient and N-efficient rice was remarkedly higher than that in the high-NH4+concentration(3 mM)treatment.