Nitrogen-efficient rice cultivars can reduce nitrate pollution

Environmental pollution by un-utilized nitrogenous fertilizer at the agricultural field is one of the key issues of the day. Rice-based cropping system, the mainstay of Indian agriculture, is one of the main sources of unused N-fertilizer since rice utilizes only 30-40% of total applied N, and the rest goes to waste and creates environmental as well as economic loss. Identification of rice genotypes that can grow and yield well at low nitrogen levels is highly desirable for enhancement of nitrogen use efficiency (NUE). In the present study, we have identified large variability in the NUE of rice cultivars on the basis of plant with low, medium, and high levels of N in nutrient solution. To establish the basis of this wide variability in NUE, nitrate uptake kinetics and enzymes of nitrate assimilation were studied. The data of nitrate uptake kinetics revealed that the nitrate uptake is mediated by low-affinity transporter system (LATS) in N-inefficient rice cultivars and by both LATS and high-affinity transporter systems (HATS) in N-efficient genotypes. Activities of nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, and the soluble protein content were found to be increased in moderately N-efficient and low N-efficient cultivars with increase in external supply of nitrogen. However, a non-significant decrease in these enzymes was recorded in high N-efficient cultivars with the increase in N supply. This study suggests that the HATS, high NR, and glutamine synthetase activity and the soluble protein content distribution have a key role in N efficiency of rice genotypes. These parameters may be considered in breeding and genetic engineering programs for improving the NUE of rice, which might be helpful in reducing the fertilizer loss, hence decreasing environmental degradation and improving crop productivity through improvement of nitrogen utilization efficiency in rice.