| Improving grain yield,resource use efficiency,and grain quality is a major challenge in rice production.The increase of nitrogen（N）fertilizer has contributed greatly to the grain yield of rice.However,in recent years,excessive application of N fertilizer is common,which leads to the decrease of N use efficiency（NUE）and the deterioration of rice quality.In addition,it also brings about a series of environmental problems.Therefore,reducing N input may be an effective way to solve the above problems,but reducing N input increases the risk of rice yield loss,so it is very important to explore reasonable cultivation measures to ensure rice yield under reducing N input.Increasing planting density can improve rice yield by increasing the panicle number,however,whether dense planting with reduced N input can achieve high yield,good quality and high efficiency in rice production remains to be seen,on the other hand,the compensation effects of increasing planting density on yield loss of hybrid and inbred cultivars caused by reducing N rate are still unclear.To this end,we conducted field experiments in the High-tech Park of Yangtze University in 2019 and 2020.A split-split plot design was used in this experiment.Indica/japonica hybrid rice（Yongyou 4949）,super hybrid indica rice（Yangliangyou 6）and inbred indica rice（Huanghuazhan）were used as experimental materials.Two N application levels and two planting density treatments were set:the normal N(180 kg N ha-1)and the reduced N(90 kg N ha-1),and the normal density（30 cm×15 cm）and increasing density（20cm×15 cm）.The results are as follows:（1）Reducing N input significantly reduced panicle number per unit area,total spikelet number and total dry weight,thus leading to a significant yield loss.Increasing planting density significantly improved grain yield,mainly because of increasing panicle number per unit area.The study also showed that the effect of increasing planting density on yield increase of hybrid rice was significantly higher than that of inbred rice under both normal and low nitrogen treatments,indicating that hybrid rice still had higher yield under dense planting.In addition,the yield increase effect of increasing planting density under reduced N input was 6.4-22.2%higher than that under normal N treatment.In addition,the study also showed that there was little difference between the yield of dense planting with reduced N input treatment and that of normal nitrogen and normal density treatment.Thus,dense planting with reduced N input cultivation could achieve high and stable yield of rice.（2）The yield loss of Yongyou 4949,Yangliangyou 6 and Huanghuazhan caused by reducing N input was 1.54,1.25 and 1.34 t ha-1,while increasing planting density under reduced N input increased the yield of the three varieties by 1.27,1.47,and 0.58 t ha-1,respectively.Therefore,the compensation effect of increasing planting density on yield loss caused by reducing N input in Yongyou 4949,Yangliangyou 6 and Huanghuazhan was 82.7%,117.6%and 43.1%,respectively.Therefore,the compensation effects were higher in hybrid rice than inbred rice.In addition,the study also showed that there was a significant positive correlation between the yield of dense planting with reduced N input treatment and that of normal nitrogen and normal density treatment（R2=0.61）,indicating that the rice cultivars with good performance under general high yield conditions can also obtain higher yield under dense planting with reduced N cultivation.（3）Total N uptake,accumulated temperature and radiation use efficiency were decreased by 22.2%,8.7%and 9.0%,respectively,but partial factor productivity of applied N（76.1%）and NUE for grain production（13.2%）were significantly increased by reducing N treatment.Increasing planting density increased partial factor productivity of applied N,NUE for grain production,accumulated temperature and radiation use efficiency by 11.5%,17.5%,10.8%and 9.9%,respectively.Compared with normal N application and normal density treatment,dense planting with reduced N input treatment increased partial factor productivity of applied N,NUE for grain production,accumulated temperature and radiation use efficiency of Yongyou 4949 by94.7%,21.9%,1.8%and 0.8%,respectively.The corresponding increases in Yangliangyou 6 were 104.9%,33.8%,5.4%and 4.0%,respectively and those in Huanghuazhan were 81.7%,17.4%,-8.5%and-7.9%,respectively.Therefore,dense planting with reduced N input cultivation is beneficial to increase the resource use efficiency of rice,especially for hybrid rice.（4）The chalkiness grain percentage and chalkiness degree of rice were significantly decreased by 16.4%and 12.1%under reduced N input,which was beneficial to improve the appearance quality of rice.Increasing planting density decreased the rate of milled rice and chalkiness degree,but increased the chalkiness grain percentage.In addition,increasing planting density under reduced N input significantly reduced amylose content of Yongyou 4949,Yangliangyou 6 and Huanghuazhan by 9.5%,6.9%and12.6%,respectively.However,increasing planting density under normal N treatment increased amylose content of rice.In addition,both increasing planting density and reducing N input can effectively reduce grain protein content,and the latter has a greater effect on grain protein content than the former.Compared with normal N input and normal density treatment,the chalkiness degree,grain protein content and amylose content of the three varieties were significantly decreased by dense planting with reduced N input treatment,which was beneficial to improve the appearance quality and cooking and eating quality of rice.In conclusion,dense planting with reduced N input cultivation is beneficial to synergistic improvement of rice yield,resource use efficiency and rice quality,and hybrid rice performs better under dense planting with reduced N input cultivation than inbred rice,suggesting that hybrid rice requiring more space to develop yield potential is also suitable for dense planting with reduced N input cultivation. |
