Effects Of Nitrogen Management On The Yield Formation Of Regenerated Tillers In Different Node Positions Under Different Cutting Heights And The Related Mechanism In Ratoon Rice

Ratoon rice,consisting of main crop（MC）and ratoon crop（RC）,is a major innovation of rice cropping systems in China.It is crucial to promote the wide adoption of rice ratooning technology to improve rice yield and farmers’profits,and promote sustainable rice production,which is effective in ensuring national food security.However,low and unstable grain yield of RC has limited the wide spreading of this technology.There is an urgent need to improve RC yield by optimizing crop management practices to promote large-scale ratoon rice production.Cutting height of MC and nitrogen（N）applications at15 d after heading of MC for promoting bud initiation（N_B）and at 1 d-3 d after harvest of MC for promoting tiller growth（N_T）had profound effects on RC yield.It is unknown whether cutting height of MC could affect the yield increases of RC from N_B and N_T,although their effects on RC yield had been intensively studied.Furthermore,RC produces regenerated tillers in different node positions on the stubble of MC,whereas limited information is available on the N responses of regenerated tillers in different node positions in ratoon rice.Therefore,this study was conducted at the Ratoon Rice Research Experimental Station of Huazhong Agricultural University,Hubei Province,China in 2019and 2020.Field experiments were arranged in a split-split plot design with N_B(0 and 75 kg N ha^-1)as main plot,cutting height（15,30,and 45 cm）as subplot,and N_T(0 and 75 kg N ha^-1)as sub-subplot with four replications.Huanghuazhan was used in this study,which is a widely planted cultivar as ratoon rice in central China.Grain yield and yield-related traits,dynamics of survival number and length,total dry weight,and total N uptake of regenerated tillers in different node positions,and the concentrations of non-structural carbohydrate concentration and N concentration of stubble were measured to examine the effects of cutting height,N_B,N_T,and their interactions on the tiller growth and yield formation of regenerated tillers in different node positions and the related mechanisms,which will provide an insight to optimize crop management strategies for high-yielding ratoon rice production.The main results are as follows:（1）Grain yield of MC ranged from 9.20 t ha^-1 to 9.42 t ha^-1 across different treatments,and N_B had no significant impact on grain yield of MC in both years.Grain yield of RC ranged from 3.03 t ha^-1 to 6.05 t ha^-1 across different treatments.Both of cutting height and N treatments significantly impacted RC yield,whereas the interactions between cutting height and N treatments had no significant impact on RC yield.Overall,the effects of cutting height on RC yield were much lower than that of N treatments.The yield variation was 0.05 t ha^-1-0.26 t ha^-1 across different cutting heights,whereas it was 0.60 t ha^-1-1.46 t ha^-1 across different N treatments.（2）Applications of N_B and N_T significantly increased RC yield.Compared with treatment without N application in RC（N₀）,N_B,N_T,and application of both N_B and N_T(N_B+T)increased RC yield by 10.4%-25.5%,35.4%-48.1%,and 37.4%-40.2%,respectively.N_B and N_T consistently increased RC yield across different cutting heights and years,whereas their mechanism in yield improvement of RC yield were different.Yield improvement was mainly attributed to increased panicles m^-2 for N_B and both panicles m^-2and spikelets panicle^-1 for N_T.Furthermore,N_B and N_T also increased total dry weight and total N uptake in RC,but N_T was more effective than N_B.On average,N_B and N_T increased total dry weight during ratoon season by 19.9%and 37.4%,respectively,and total N uptake during ratoon season by 20.2%and 57.1%,respectively.（3）Different mechanisms in yield improvement of N_B and N_T were confirmed by the differences in the survival number and growth of regenerated tillers among N treatments.N_B and N_T significantly increased the survival number of regenerated tillers after harvest of MC,but N_T was more effective than N_B.Furthermore,only N_T significantly increased the length of regenerated tillers after harvest of MC.Compared with N₀,N_B and N_Tincreased the survival number of regenerated tillers at 21d after harvest of MC by 9.2%-23.2%and 32.8%-49.8%,respectively.Furthermore,N_T significantly increased the length of regenerated tillers at 21 days after harvest of MC by 13.9%-20.2%,but N_B had small impacts on the length of regenerated tillers.Moreover,the survival number of regenerated tillers showed a downward trend from 15 days after heading of MC to heading of RC,in which the period of 7 days after harvest of MC had the highest death rate of regenerated tillers,whereas it generally maintained stability after heading of RC.Therefore,the period of 7 days after harvest of MC was the period that the survival and growth of regenerated tillers were more sensitive to N application compared with other growth periods.These results had also explained that N_T application at 1 d-3 d after harvest of MC was more effective in promoting the survival and growth of regenerated tillers and increasing the grain yield of RC.（4）Across different cutting heights and years,N applications had no significant impacts on the grain yield of regenerated tillers in upper nodes,whereas N_B and N_Tsignificantly improved the grain yield of regenerated tillers in lower nodes by 25.3%-47.9%and 31.4%-155.3%,respectively.The mechanism in yield improvement of regenerated tillers in lower nodes was different between N_B and N_T.Yield improvement of regenerated tilers in lower nodes were mainly due to the increase in the survival number of regenerated tillers in lower nodes for N_B resulted from the increases in stubble non-structure carbohydrate concentration at maturity of MC and stubble N concentration at heading in RC.However,N_T application not only increased the survival number of regenerated tillers in lower nodes more effective than N_B but also increased spikelets panicle^-1,dry weight per stem,and N uptake per stem,which were attributed to increased stubble N concentration in RC.Different N responses of growth and yield formation of regenerated tillers in lower nodes further explained that N_T was more effective than N_B in improving RC yield.Overall,cutting height had no impacts on the yield responses of RC to N_B and N_T.Regenerated tillers in lower nodes were more responsive to N application compared with that in upper nodes,and N_B and N_T increased the grain yield of regenerated tiller in lower nodes not in upper nodes,and N_T was more effective than N_B.Furthermore.The survival and growth of regenerated tiller were highly dependent on stubble N concentration at 7 d after harvest of MC.Therefore,it is essential to increase stubble N concentration after harvest of MC to improve the N response of regenerated tillers in lower nodes in the optimization of N management in ratoon rice.