Effect of ratio of nitrate to ammonium on physiological characteristics of rice genotypes at seedling stage under moderate water stress

Rice genotypes includingGangyou 527,Yangdao 6,Zhonghan 3 and Nongken 57 were grown under differentwater stress conditions and treated with ammonium and nitrate at various ratios to investigate the effects of water stress and nitrogen forms on growth,nitrogen absorption and physiological characteristics of rice.The results indicated that,under normal water condition,moderate decrease in the ratio of ammonium/nitrate(50∶50)could increase nitrate content in the leaves and rootsof rice,while the netphotosynthetic rate(Pn)was notaffected.However,when the proportion of nitrate Nwas higher than 50%,the physiological and metabolic indexes decreased significantly and the growth of rice seedlingswas inhibited.Undermoderate water stress condition,the ratio of ammonium/nitrate at50∶50 wasmore beneficial to increasePnof top three leaves,to promote the accumulation of compatible osmolytes,to improve the nitrogen uptake and to boostthe growthof rice seedlings,as comparedwith thattreatedwith normalwaterand 100%ammoniumN,Moreover,undermoderate water stress condition,the responses of different rice genotypes to increasing nitrate N differed from each other significantly.In terms ofPnand nitrogen use efficiency,indica rice was superior to japonica rice;similarly,hybrid indica rice was superior to inbred indica rice,and japonica upland rice was better than japonica paddy rice.No difference was observed in drought tolerance within the same rice genotype.