Selection and evaluation of high-nitrogen efficiency of early rice cultivars in red soil agro-ecosystem in South China

It is one of the effective ways to reduce the amount of nitrogen fertilizer by using rice varieties with adaptation to low nitrogen environment or high efficient utilization of soil nitrogen. Red soil agro-ecosystem of South China is an important production base of double-cropped paddy rice. However, the rice yield and N use rate in this area have been at a low level especially for early rice, due to the unreasonable use of nitrogen fertilizer and the acid characteristic of red soil. To screen early rice cultivars with high nitrogen efficiency and reveal their physiological characterizations, twenty one rice cultivars widely grown in red soil area of South China were tested in randomized block field experiments under normal and low nitrogen supplies. The biological characteristics of early rice varieties with high nitrogen efficiency were identified by analyzing parameters related to yield and its components, the aboveground nitrogen accumulation, the leaf SPAD value and photosynthetic rate, and nitrate Reductase activity. Based on rice yield and nitrogen efficiency analysis, Zhongxuan972, Zhongjiazao17, Zhongzu 7 and zhongyouzao13 were selected as high nitrogen efficient cultivars due to their more than 6000 kg ha-1yields and more than 0.81 nitrogen efficiency, and Zhongjiazao2, Xiangzaoxian32, Jiayu948 and Zhongxuan 181 as low nitrogen efficient cultivars due to their lower yield and less than 0.70 nitrogen efficiency. Comparison of yield components revealed that the significant difference between two type cultivars was effective grain numbers of per unit area (panicle number xspikelet number per panicle). The aboveground nitrogen accumulation and net photosynthetic rate in high nitrogen efficient cultivars were significantly higher than those in low ones at the heading stage under nitrogen-free condition, and there was not a significantly difference in leaf SPAD between the high and low nitrogen efficient cultivars. However, leaf SPAD values of high nitrogen efficient cultivars declined more slowly than that of the other four low nitrogen efficient cultivars under nitrogen-free condition. Nitrate Reductase activities of high nitrogen efficiency varieties under nitrogen-free condition were 38% to 53% activities under normal nitrogen condition, and nitrate Reductase activities of low nitrogen efficiency varieties were 22% to 35% activities. It suggested that the high nitrogen efficient rice cultivars had more grain numbers of per unit area under low nitrogen supply condition, their functional leaves maintained a higher level of photosynthetic rate and nitrate Reductase activity and lower chlorophyll decomposition rate especially during heading stage, which might be the reason for enhancing nitrogen assimilation and acquiring the higher yields.