Yield Performance And Associated Physiological Basis Of Historical Rice Varieties Evolved In The Middle Of Yangtze River Basin Of China

Rice is main food crops in the human diet of worldwide.It is the foremost staple food for more than half of population in China.Contribution of rice production accounted for 33.5% of grain production in the country and ensured food security.In the past half centry gross rice production of China has maintained a rising trend.The trend in increase of grain production was attributed to optimization achieved in cultivation techniques,crop protection and cultivar evolution.Among them,the contribution of cultivar evolution has been the most decisive factor for improvement in grain production.There were two significant achievements made in revision of yield potential of rice.The first achievement occurred through the introduction of IR8-parented semi-dwarf cultivars,and the second by exploitation of hybrid vigor.The yield variations and progression of growth related features associated with yield were investigated with cultivar evolution by many workers.However,scant attention was paid to the phenotypic characteristics related to growth,nutrition uptake,utilization and transportation,dry matter accumulation and allocation,light interception and use efficiency,yield formation,lodging related traits and grain quality of historical rice varieties in the mixed cropping region between single and double cropping rice in the mid reaches of Yangtze River basin.This area is prominent for intensive rice farming,but embodies a fragile and fluctuating ecosystem varying with incidence of solar radiation,precipitation and ambient temperature.Plant responses to these environmental factors have changed with rice cultivar evolution.But there are several disputes in the literature in ascertaining the relationships existing between physiological factors like radiation use efficiency,fertilizer use efficiency and growth related factors and grain yield.In addition,the influence of lodging related traits on grain yield that occurred through the years,has not been explained.So in the present study,we determined the annual increase in the grain yield and other associated features in 15 rice varieties that were released from 1936 to 2005 in Middle Reaches of Yangtze River under current cultivation technology and management.The field experiments were conducted in 2013 and 2014 in the rice growing season from June to October.The cultivars were arranged in a randomized complete block design with four replications.The individual plot size was 5 m × 6 m.The objective of this study was to evaluate the changes in yield performance in relation to the characters determining grain yield that occurred in the cultivars over the years for making strategies for future rice breeding.The study intends to assess the current status of breeding,guide breeding for super high yield rice variety,improve technology of crop cultivation and management,and to ensure sustainable rice production and food security.The results showed that:(1)The grain yield gradually improved with evolvement process of rice cultivars during the 70-year period in the middle reaches of Yangtze river with annual gains of 61.9 and 75.3 kg ha-1 in 2013 and 2014,respectively,that corresponded to an annual increase of 1.18 and 1.16% in grain yields,respectively.On the average,the grain yield of modern varieties was 24.6 and 35.7% higher than old varieties released before 1970 s in 2013 and 2014,respectively.The yield increase in new cultivars was attributed to improvements in spikelets per panicle,total spikelets per m2,grain filling percentage and biomass.Spikelets per panicle,total spikelets per m2,grain filling percentage and biomass significanly increased with the year of release.However,panicles per unit area were often found decreasing with year of release in the cultivars studied.Decrease in panicle number per unit area led to the formation the larger panicles.In addition,less variation was seen in harvest index(HI)and grain weight.(2)It was noticed that biomass accumulation was greatly increased in modern varieties because both radiation transformation efficiency and crop growth rate enhanced with prolonged growth duration and improved plant type.The optimized plant type of modern variety contributed to increase in area of radiation interception,leaf thickness and tenacity,duration of light interception and photosynthesis that facilitated greater biomass accumulation and translocation of assimilates for reproductive growth for maintenance of a well co-ordinate source-sink relationship.(3)At the same time,N uptake,N harvest index,N use efficiency for grain production and partial factor productivity of applied nitrogen increased in the new varieties.The modern varieties had stronger ability in N uptake and remobilization of the nutrient from vegetative to reproductive structures.(4)Further,lodging resistance and grain quality of modern varieties also improved because of genotype selection.It significantly improved diameter,fresh weight and breaking resistance of lower internodes in modern rice,as indicated in decreased the lodging index.Cultivar improvement had very little effect on milling quality of grains although it improved appearance quality.The percentage of brown rice,milled rice and head rice remained relatively stable,but chalky rice percent and chalkiness decreased with year of release.Furthermore,grain length increased and conversely grain width decreased with year of release.These results suggest that:(1)The yield potential of old rice varieties is greatly increased with year of release in cultivar evolution,which is attributed to increase in spikelets per panicle(form the big panicles)and dry matter accumulation.The stagnation in grain filling percentage of modern variety can be broken by enhancing dry matter accumulation and translocation efficiency in future genetic breeding.(2)Radiation use efficiency(RUE)increased significantly with year of release and is closely related to increase in biomass and grain yield.The increase in biomass production and grain yield is driven by enhancing the RUE.Hence,enhancement in RUE is one of the most promising traits for further improvements in the rice.(3)Nitrogen uptake and use efficiency were greatly advanced by cultivar evolution.But they need further improvement in modern varieties.(4)Finally,lodging-resistance of modern rice cultivars should be improved by scaling down the present level of lodging index to cope with the enlargement of sink size.For this diameter,dry weight and dry weight per unit length of internode of basal internodes should be enhanced for strengthening lodging-resistance.