Physiological and Molecular Analysis of Applied Nitrogen in Rice Genotypes

Ten genotypes of rice (Oryza sativa L.) were grown for 30 d in complete nutrient solution with 1 mmol/L (N-insufficient),4 mmol/L (N-moderate) and 10 mmol/L (N-high) nitrogen levels,and nitrogen efficiency (NE) was analyzed.Growth performance,measured in terms of fresh weight,dry weight and lengths of root and shoot,was higher in N-efficient than in N-inefficient rice genotypes at low N level.Of these 10 genotypes,Suraksha was identified as the most N-efficient,while Vivek Dhan the most N-inefficient.To find out the physiological basis of this difference,the nitrate uptake rate of root and the activities of nitrate assimilatory enzymes in leaves of N-efficient and N-inefficient rice genotypes were studied.Uptake experiments revealed the presence of two separate nitrate transporter systems mediating high-and low-affinity nitrate uptake.Interestingly,the nitrate uptake by the roots of Suraksha is mediated by both high-and low-affinity nitrate transporter systems,while that of Vivek Dhan by only low-affinity nitrate transporter system.Study of the activities and expression levels of nitrate assimilatory enzymes in N-efficient and N-inefficient rice genotypes showed that nitrate reductase (NR) and glutamine synthetase (GS) play important roles in N assimilation under low-nitrogen conditions.