Effects Of Sun Light,Planting Density,Nitrogen Rate And Its Interaction On Yield Formation And Nitrogen Use Efficiency In Hybrid Rice

Largely planted hybrid rice contributes greatly to improve grain yield per unit land area as well as ensure national food security in China.However,two problems have restricted hybrid rice to attain high and stable yield.One is the excessive nitrogen fertilizer input in rice paddy field and the other is frequent occurrence of low-light stress in rice grain filling phase.In addition,there is a reduced adoption of hybrid rice by farmer in recent years because the need of higher seed rate is essential to simplified and mechanized crop managements,which are more and more popular in China due to less labor input.Therefore,several field experiments were conducted with seedling rate,nitrogen rate,planting density in Changsha,Hunan province,and Chengmai,Hainan province?2012-2013?,and with light rate,nitrogen rate and planting density in Changsha,Hunan province?2014-2015?,as well as with sowing method and seedling rate in Changsha,Hunan province and Zhaoqing,Guangdong province?2015?.The present study aimed to identify whether hybrid rice may attain higher and stable yield with increased nitrogen use efficiency under?i?reduced seedling rate with high planting density and?ii?reduced nitrogen rate with high planting density.The study also explored that hybrid rice may mitigate yield loss and has higher nitrogen use efficiency at reduced nitrogen rate with high planting density under low light intensity,and understand yield increase mechanism under mechanized transplanting at high hill density with single seedling per hill in hybrid rice.The results are as follows:1.Planting density and seedling rate had significant influence on yield,yield components and biomass of super hybrid rice in Changsha and Chengmai,but no consistent interaction of planting density by seedling rate was observed.Dense planting(26.5-40.1hills m^–2)did not significantly improve grain yield in Changsha whereas significantly did in Chengmai compared with conventional density(18.8 hills m^–2).Conversely,low planting density(14.0 hills m^–2)significantly reduced yield compared with conventional density in two sites.On the other hand,Chengmai significantly produced higher yield by 6.43%-8.00%,whereas Changsha did lower yield by 4.35%-4.56%under 2-3 seedlings per hill than under single seedling per hill.Yield has a positive relationship to spikelets m^–2 in Changsha which decreases with increasing seedling per hill and increases with increasing planting density except for the highest density.In Chengmai,similarly,yield has positive relationships to biomass at heading and maturity,panicles m^–2,spikelets m^–2 and spikelets filling rate which increase with both increasing seedling per hill and planting density.Overall,super hybrid rice produces more grain yield due to more spikelets m^–2 under 18.8-26.5 hills m^–2 with single seedling per hill in Changsha and due to higher panicles m^–2,spikelets m^–2,spikelets filling rate and biomass under 26.5-40.1 hills m^–2 with 2 seedlings per hill in Chengmai.2.Nitrogen rate and planting density had significant difference in yield,yield components,biomass,N uptake and partial factor productivity of applied nitrogen fertilizer?PFP_N?for super hybrid rice in Changsha and Chengmai.Also significant influence of nitrogen rate by planting density on yield components and PFP_N was observed.Two sites improved yield by 5.83%-6.97%,N uptake by 25.88%-26.91%,whereas reduced PFP_N by46.52%-47.10%under 150 kg N hm^–2 compared with 75 kg N hm^–2.On the other hand,high planting density(26.5-40.1 hills m^–2)increased yield by 2.03%-7.45%,N uptake by2.12%-10.62%and PFP_N by 2.57%-9.38%as compared with conventional density(18.8hills m^–2).There was no significant increase in yield when nitrogen rate and planting density were more than 150 kg N hm^–2 and 40.1 hills m^–2,respectively.Response of super hybrid rice to N rate and planting density were the same in Changsha and Chengmai,which panicles m^–2,spikelets m^–2,biomass at heading and maturity as well as N uptake are increased with increasing N rate and planting density and have positive correlations with grain yield.The adoption of low-moderate nitrogen rate(75-150 kg N hm^–2)and high planting density(26.5-40.1 hills m^–2)could attain high yield and nitrogen use efficiency for super hybrid rice because of more panicles m^–2,spikelets m^–2,biomass and N uptake.3.Sun light,nitrogen rate and planting density had significant effect on yield,yield components,biomass,harvest index,N uptake and N use efficiency for two hybrid rice varieties.Significant interactions of sun light by nitrogen rate and sun light by planting density were observed on yield and yield components.Under 30%of natural light after heading,significant decreases in spikelets filling rate,1000-grain weight,biomass,harvest index and N uptake were observed,which resulted in marked reduce in yield,recovery efficiency rate of applied nitrogen fertilizer?RE_N?and PFP_N in both varieties by46.19%-47.18%,16.90%-24.94%and 45.56%-46.31%,respectively.High nitrogen rate(240 kg N hm^–2)increased N uptake,but significantly decreased spikelets filling rate and1000-grain weight,which leaded to reduce in yield,RE_N and PFP_N by 10.23%-12.43%,22.99%?only for LY9348?and 45.61%-46.93%,respectively,as compared with 145.3 kg N hm^–2 under 30%of natural light.In contrast,high planting density(26.5-40.1 hills m^–2)significantly increased spikelets filling rate and 1000-grain weight as well as improved biomass,harvest index and N uptake,which contributed to improved yield,RE_N and PFP_N by 10.33%-14.02%,5.42%-35.58%and 8.02%-18.17%,respectively,as compared with14.0 hill m^–2.Shading significantly reduced nitrogen absorption from indigenous nitrogen,whereas increasing planting density improved nitrogen absorption from nitrogen fertilizer.Yield has positive relationships to spikelets filling rate,1000-grain weight and biomass which improve with reducing N rate and increasing planting density under 30%of natural light.Briefly,reduced nitrogen rate with dense planting could mitigate yield loss and improve nitrogen use efficiency for hybrid rice due to higher spikelets filling rate,1000-grain weight and biomass under low-light stress after heading.4.Leaf age,white root number,total root number,basal width of shoot,dry weight of shoot and dry weight of root of the printing sowing were higher than conventional sowing at Changsha and Zhaoqing.Mechanized transplanting at high hill density with single seedling per hill?MTHS?had more spikelets per panicle and spikelets per m²,higher spikelets filling rate and 1000-grain weight,as well as significantly increased grain yield by 10.28-13.96%compared to conventional mechanized transplanting?CMT?.On the other hand,The MTHS had more primary branch number and secondary branch number,longer panicle length,higher grain density,panicle weight,harvest index and radiation use efficiency than CMT,with significant differences.Therefore,the MTHS could not only produce high grain yield for hybrid rice due to the formation of large panicle but also reduce sowing rate and improve seedling quality.