Physiological Mechanisms Of Genotypic Differences In Nitrogen Use Efficiency By Rice Genotypes

China accounts for more than one quarter of the total nitrogen (N) fertilizer used in the world. However, the recovery of N fertilizer by crops has been generally low because of excessive N application. The problems resulting from excessive mineral N fertilizer application in Chinese intensive agricultural regions can be exemplified by the rice production in Eastern China. The century-old double-cropping (irrigated summer rice / upland winter wheat) systems have been transformed from a N -limited to a N -saturated state, with mean N application reaching up to 335 kg N hm^-2 year^-1. The alternating water regime causes high N losses during the N transformations and low crop N uptake efficiencies of 28%-41%. This is leading to large-scale, non-point source pollution of aquifers. Low N use efficiency with high environmental impact is a worldwide problem. It may be resolved by reducing target yield and N input in developed countries with surplus arable land and small population. This is, however, impossible in China since the country is still facing the food unsecurity. Different approachs should be made in order to reduce the environmental impact while the crop yield and productivity can be maintained or even increased at the same time. Therefore, the N input should be optimized to reach the harmonious point between high yield and sound environment. Field experiments have shown that genetic variability for N use efficiency exists in rice plants. There is a possibility of improving N utilization efficiency in rice through genotype selection. Therefore, this study was conducted to select rice cultivars that could exploit N more efficiently in order to minimize loss of N from the soil and make more economic use of the absorbed N.1. Identification of the traits of N uptake and utilization at the seedling stage is considered to be an effective approach for selecting N -efficient rice genotypes. Different levels of N concentration (4, 10, 40 mg N L^-1) were supplied to three rice genotypes (4007, Nanguang and ELIO) that had shown different N use efficiencies. Biomass, N uptake, total root length, root surface, N influx rate and uptake kinetics were determined in all the plants. N deficiency reduced the dry matter accumulation in all the genotypes. ELIO accumulated more dry matter than other two rice genotypes, and the rate of reduction of dry...