Population productivity and properties of early super hybrid cultivars under nitrogen fertilization

Based on the concept of the highest population productivity of rice under N fertilization, field experiments were carried out to investigate the highest productivity and its formation mechanism of the early-season super hybrid rice. Four early-season super hybrids（Jinyou 458, Chunguang 1, 03you 66 and Ganxin203） and two ordinary hybrids （Rongyou 9 and Wufengyou 623） which are widely grown by rice farmers in China were selected. There were five nitrogen levels, 0, 135, 165, 195 and 225 kg/ha. The main results show that the maximum outputs and the highest nitrogen population productivities of the early-season super hybrids are reached in the nitrogen level of 195 kg/ha, except that the early-season super hybrid, 03you 66 and ordinary early-season hybrid rice are in 165 kg/ha level. The yields of early-season super hybrids are increased by an average of 8.42% compared to that of the ordinary hybrid rice, and the number of effective panicles is increased by 10.67% and the number of grains per spike is increased by 13.96%, which suggests that the higher total spikelet number is the main reason for the increase of productivity of the early-season super hybrids. Under their respective nitrogen levels of the highest population productivity, compared with the early-season ordinary hybrid, the leaf area indexes at growth stages of the early-season super hybrids are significantly improved except for at the stage of critical leaf age for effective tillers, and a slower decreasing process in leaf area index after the heading. The population photosynthetic potential is increased by 9.00%, which could increase dry matter accumulation in super hybrid rice, especially after the middle growth stage. Total nitrogen uptakes of super hybrids are from 167.53 to 191.72 kg/ha with an average of 182.62 kg/ha, and increased by 20.02%, among which increased by 30.09% from jointing to heading, and 28.92% from heading to maturity. The agronomic N use efficiency and physiological N use efficiency of the super hybrids are higher than those of the early-season ordinary hybrid, while the situation is quite the reverse for physiological N use efficiency and partial factor productivity of applied N.