| China produce most rice grain in the world, the rice planting area is about 28-30 million hectares, which accounts for 22% of the total rice planting area in the world. The annual production of rice grain ranged between 1800～2000 hundred million kilograms, which accounts for about 30% of total rice production. Rice has a pivotal function in Chinese grain production. Chemical nitrogen fertilizer in rice production consumes probably more than 7 million tons, but the average N use efficiency (NUE) is about 28%. Low NUE not only waste nitrogen, but alao has serious effects on environmental condition. Nutrient absorption capacity or grain yield is major index that scientist usually used to evaluate the nutrient efficiency, but these indexes could be influenced easily by other factors. Therefore these factors could not really reflect the hereditary property of nutrient character. The ideal method is to seek the morphological and physiological target that closely related with nutrient characters. In the process of nitrogen absorption, the changes of rice root morphology parameters had a decisive function, and root growth and surface area influence the N validity. Several experiments with different rice cultivars (high and low NUE) were conducted with methods (i.e. root bags and sand culture). The studies investigated the differences of root morphology parameter and biological activity between rice cultivars, which can provide theoretic basis for screening high NUE rice cultivar.In the present study, hydroponic experiments were conducted to study the differences and mechanism of N absorption in seedling stage of 8 different genotype rice. The results showed that there were significant differences in N uptake capacity between different rice cultivars. Based on the N uptake accumulation of rice, eight cultivars could be divided into high response to N, such as Nanguang, Yujing 7, Qianyu 421 and Wuyunjing 7, and low response to N, such as Guidan 4, Elio, Yunjing 38 and 4007. The difference in N uptake response was related to N uptake rate and root characteristics. N uptake rate of high-responsing cultivars increased significantly with increasing N rate. N uptake rate exhibited much higher increase than that of total length of root. The linear regression between N uptake accumulation and root bulk and N uptake rate showed that N uptake rate was significantly correlated with N uptake across N levels, indicating N uptake rate plays a key role in studying of N uptake in seedling of rice.In the field experiment, two rice cultivars (Elio and Nanguang) which have similar development characteristic (growth stage, plant height, etc) were taken as experiment materials. Differences of absorbing nitrogen and root morphology were compared after seedling stage. The grain yields increased with N application rates. Under different N supplying level, the yields of the high NUE cultivar (Nanguang) were significant higher than that of the low NUE cultivar (Elio). There were differences in dry matter and biomass accumulation between two rice cultivars at each growth stage. Nanguang's root morphology parameters are obviously great than Elio. Root dry weight, total root length and root surface area of two cultivars are directly correlated with N accumulation. Therefore it is concluded that the production of rice root morphology parameter are the decisive factors to determine whether the rice take and use N efficiently.Sand culture was conducted to study the root physiological parameters of different NUE rice under the whole growth stages. From heading to maturity, increase of total dry matter and N accumulation of N absorption inefficient rice cultivar Elio was significantly less than N absorption efficient rice cultivar Nanguang. After heading stage, root system of Elio on high N supplying level senesced significantly, root dry weight, total root length and root surface area were significantly decreased than front growth stage. Root system index of Nanguang decreased little, but the root length increased. Root system physiology index of Nanguang on low N supplying level were not change obviously, but that of Elio completely showed a decreasing tendency, some index decreased significantly. Under this experiment condition, we concluded that relatively root system of N absorption efficient rice declined slowly, and still maintain the exuberant metabolism function after heading stage. This result is a strong foundation for getting higher grains yield.In the field experiment, another high NUE cultivar 4007 was compared with low NUE cultivar Elio to prove that root morphology parameter is the important index for N efficiency. The results showed that differences of 4007 and Elio were sameness with it of Nanguang and Elio.Increased with N level root activity of Nanguang was significantly larger than Elio, the former had 35% (2 mmol/L N level) and 15% (4 mmol/L N level) lager than the latter. The study of bleeding sap of rice on the middle-later period showed that root system activity was significantly different between Elio and Nanguang. Bleeding rate of Nanguang was significantly larger than Elio. In later growing stage the concentration and total of soluble sugar andamino acids in bleeding sap of Nanguang were significantly larger than that of Elio, and the kinds of free amino in bleeding sap of Nanguang were more than Elio. The results of MDA and SOD show that senescence of Nanguang'root was slower than Elio.
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