Physiological Mechanism And Crop Management For High Grain Yield In Direct-seeded,Double-season Rice System

Double-season rice?DSR?that allows rice to be harvested twice a year is an effective measure to increase cropping intensity in central China.Recently,massive labor input and low planting benefit have reduced farmers'enthusiasm to grow DSR,leading to DSR was widely changed to single-season rice cultivation.Given transplanting is replaced by direct seeding to complete crop establishment twice a year,labor input for DSR will be largely reduced.Only rice varieties that mature in around90 days can be used in direct-seeded DSR under subtropical conditions in central China where the thermal time is limited.However,there is no early-maturing variety with high yield potential to be successfully used to direct-seeded DSR at present.Besides,limited information is available about physiological mechanism associated with high yield of early-maturing rice,and greenhouse gas emission from direct-seeded DSR.Therefore,field experiments were conducted at Wuxue county,Hubei province,China in 2015 and2016.Twelve early-maturing varieties were used in both the early and late seasons of2015,and the objective is to initially screen suitable varieties for direct-seeded DSR in terms of growth duration,yield,lodging resistance.Six screened varieties were used in the early season of 2016,and the same varieties and three planting times?July 20,July25,and July 30?were set as two-factor treatments in the late season of 2016.The objectives are to determine suitable variety and Planting date for direct-seeded,double-season rice,and to uncover the desired plant traits and physiological mechanism associated with high yield of early-maturing rice.Field experiments were also conducted in 2017 and repeated in the same field in2018.Experiments were arranged in a split plot design with the planting systems(direct-seeded DSR:DDR_A and DDR_B;traditional transplanting DSR:TDR_B and TDR_HY)as the main plot,and N application rates(N₀:0 kg ha^-1;N₁:60 kg N ha^-1;and N₂:weekly 15 kg N ha^-1 application)as subplot with four replications.Early-maturing rice varieties with shortest growth duration or relatively high yield potential were used for DDR_Aand DDR_B,respectively.TDR_B used the same variety with DDR_B.TDR_HYused local high yield variety with medium growth duration for both the early and late seasons.The main purposes were to evaluate the differences between double-seeded and transplanted DSR in annual yield,N use efficiency,and greenhouse gas emissions;and to uncover the physiological mechanism underlying these differences.?1?Total six varieties were suitable for direct-seeded DSR according to the performance of growth duration,yield,lodging resistance.Early rice should be seeded around April 10,and late rice should be seeded before July 25 to ensure safe production.Otherwise,late rice encountered cold stress during heading and grain filling period,leading to the decrease in grain filling percentage of 16.8%-22.8%and in yield of 9.1%-11.9%.Among tested varieties,Xiangzaoxian6 showed strong resistance to cold stress that there was nonsignificant difference among three planting times in grain filling percentage and yield.Therefore,a total of suitable 12 variety and planting time combinations were screened out of which annual growth duration was 186 to 198 d,and annual yield was 13.3 to 14.7 t ha^-1.?2?The grain yield of five tested varieties ranged from 6.4 to 9.9 t ha^-1 with a total growth duration of 89 to 105 d across varieties,seasons,and years.The wide difference in grain yield was attributed to the spikelets per panicle and biomass production,rather than growth duration,other yield components,or harvest index.Within such short growth duration,high-yielding variety mainly depended on rapid tillering emergence and leaf expansion at vegetative stage,high net assimilate rate at reproductive stage,and slow leaf senescence at ripening stage to increase biomass accumulation at maturity.?3?The average annual growth duration of direct-seeded DSR was 28 days shorter than that of transplanted DSR in 2017 and 2018.DDR_B yields were significantly higher than TDR_B across N treatments,indicating that the transformation of establishment method from transplanting to direct-seeding would not result in yield penalty in DSR.The average yield of DDR_A and DDR_B was 11.7%lower than TDR_HY under N0 and N1;However,the average yield of DDR_A and DDR_B came to 7.5 t ha^-1 under N2 which was just 4.3%lower than TDR_HY.In addition,the increase of N application rate significantly increased rice yield.Grain yields of N1 and N2 increased by 6.4%and 7.4%compared to N0,respectively.The annual yield of DDR_A and DDR_B was 15.0 t ha^-1 under N2,which was 1.2 t ha^-1 higher than TDR_B and only 0.6 t ha^-1 lower than TDR_HY.?4?The yield variation across planting systems and N managements was mainly attributed to spikelets per m² and biomass at maturity.The intercepted radiation before heading in DDR_A and DDR_B was 28%higher than that of TDR_B and TDR_HY due to the high tiller number and leaf area index,which caused significant increase in crop growth rate and biomass accumulation compared to transplanted DSR.However,DDR_A and DDR_B significantly decreased biomass accumulation during grain filling period compared to TDR_B and TDR_HY.This was mainly because RUE during this period was16.7%lower than that of TDR_B and TDR_HY on average.?5?N recovery use efficiency was higher than 50%,and N agronomic efficiency exceeded 17 kg kg^-1 across planting systems and N treatments.Recovery use efficiency of DDR_A and DDR_B was 25.1%higher than that of TDR_B and TDR_HY on average.However,its physiological use efficiency was 27.6%lower than that of TDR_B and TDR_HY.Overall,there was no significant difference in N agronomic efficiency among different planting systems.?6?Annual CH₄ emission of DDR_A and DDR_B were 39%lower than those of TDR_Band TDR_HY.However,there were no significant differences in the emissions of N₂O and CO₂ among planting systems.Generally,GWP per unit area in direct-seeded DSR was 29%lower than traditional transplanted DSR,and its GWP per unit grain yield was31%lower than traditional transplanted DSR.In conclusion,direct-seeded DSR can be successfully carried out in central China through choosing suitable early-maturing variety and planting time.Direct-seeded DSR could obtain the same or even higher annual yield and N use efficiency compared to transplanted DSR.Meanwhile,GWP are supposed to be significantly reduced in this way.Therefore,direct-seeded DSR can be used as a simplified planting method to replace traditional transplanted DSR.