| Large area planting of hybrid rice has made great contribution to food security in China.In recent years,With the acceleration of agricultural mechanization and the expansion of crop production scale,the application of rice machine-transplanted is more and more extensive.However,some factors limit the development of machine-transplanted hybrid rice in using conventional machine-transplanted cultivation methods,such as high seed rates,short elasticity at seedling age,poor seedling quality,and unmatching double cropping rice cultivars growth period.To address the above problems,less seedlings machine-transplanted hybrid rice under dense planting conditions was established for our research group.Preliminary demonstrations shown that high grain yield can be achieved in less seedlings machine-transplanted hybrid rice under dense planting conditions in some cities and counties for Hunan,Anhui,Henan,and Guangdong provinces.However,its high grain yield mechanism needs further study.Filed experiments were done to found out the physiological mechanisms for high yield in double-season hybrid rice machine-transplanted with less seedlings per hill,in Yong,an Town,Liuyang City,Hunan Province,China in 2017 and 2018.Seedling quality,grain yield and yield components,aboveground biomass accumulation,and nitrogen absorption and utilization were measured with different hill density and seedlings per hill.The results were showed as following:1.Both in the early seasons（ES）and late seasons（LS）,less seedlings machine-transplanted hybrid rice under dense planting conditions had a larger seedling quality.Seedlings had higher leaf chlorophyll,shoot dry weight and root dry weight per hill than did seedlings under MSH by 58%,21%and 78%in the ES,and 53%,15%and 50%in the LS,respectively.2.The higher grain yield was achieved with less seedlings machine-transplanted hybrid rice under dense planting conditions and averagely reached to 8.40 t ha-1 and 7.67 t ha-1 and increased averaged by 12%compared with other conditions both in the ES and LS.From the perspective of yield components,panicle number per m2 was decreased by 20%～23%,spikelets per panicle increased significantly,with an average increase of 27%～38%.Compared with other treatments,the average growth rate of large and small vascular bundles per stem was 12%～22%and 9.4%～24%,respectively.Panicle traits of hybrid rice with less seedlings per hill were better,panicle length,spikeltets per cm of panicle length,primary branches per panicle,and secondary branches per panicle was greater by 3.1%～7.5%,23%～25%,8.6%～21%,and 23%～25%,respectively.3.Aboveground biomass was lower aboveground biomass production at early growth stage with less seedlings machine-transplanted hybrid rice under dense planting conditions.However,canopy photosynthetic advantage was obvious in the late growth stage.Leaf area from flag to 3rd leaf rate,specific leaf weigh,SPAD value,photosynthetic rate of flag leaf and radiation use efficiency（average increased by 33%and 26%in the ES and LS,respectively）was higher at full heading,which increased the crop growth rate average by16%and 53%in the ES and LS,respectively.Higher dry weight per stem and aboveground biomass（average increased by 14%and 12%in the ES and LS,respectively）was achieved with less seedlings machine-transplanted hybrid rice under dense planting conditions.4.Higher total N accumulation was achieved with less seedlings machine-transplanted hybrid rice under dense planting conditions and averagely reached to 144 kg ha-1 and 210 kg ha-1 and increased averaged by 23%and 24%compared with other conditions in the ES and LS at heading and maturity,respectively,which resulting in more N accumulation from heading to maturity.From the distribution of N in organs,N content percentage and total N accumulation was higher in leaf and straw at mid-tillering and heading,however,N content percentage was lower in straw（including leaves）.In addition,partial factor productivity of applied N was higher with less seedlings machine-transplanted hybrid rice under dense planting conditions.In conclusion,the higher grain yields with less seedlings machine-transplanted hybrid rice under dense planting conditions were mainly due to higher seedling quality,larger spikelets per panicle,and higher nitrogen accumulation and photosynthetic capacity at late growth period. |
