Effects Of Nitrogen Management On Yield Formation Of Ratoon Rice And The Related Mechanism

Rice ratooning has become an important component of rice cropping system in central China.It plays an important role in reducing production input,increasing farmers'income,and guaranteeing nation's food security.However,the low grain yield and poor yield stability of the ratoon crop?RC?limits the wide adoption of this technology.In recent years,the planting area and the grain yield of ratoon rice increased rapidly,and RC yields of more than 5.5 t ha^-1 were achieved frequently in farmers'fields and field experiments in central China with better rice varieties and improved crop management.Nitrogen?N?fertilizer application plays a key role in yield improvement of rice.Numerous previous studies had elucidated the effects of N management on the grain yield and agronomic traits of RC.However,little knowledge of physiological responses,such as radiation capture and conversion,dry matter production and translocation,and N uptake and utilization,of ratoon rice to N management.Therefore,we conducted field experiments on ratoon rice with various N management treatments in Qichun county,Hubei province.Field experiments were arranged in a split-split-split-plot design with varieties as the main plot,N rate in the main season(N_main)as subplot,N rate for promoting bud development(N_bud)as sub-subplot,and N rate for promoting the development of regenerated tiller(N_tiller)as sub-sub-subplot with four replications.A hybrid variety,Liangyou6326,and an inbred variety,Huanghuazhan,were used in this study.For the main crop?MC?,100 or 200 kg N ha^-1applied as N_mian.For RC,0 or 100 kg N ha^-1 was applied as N_bud at 15 d after heading of MC.In addition,0 or 100 kg N ha^-1 was applied as N_tiller at 1-2 d after cutting of MC.Yield and its components,ratooning ability(the ratio of panicles m^-2 of RC to panicles m^-2 of MC),leaf area index?LAI?,radiation interception and use efficiency?RUE?,dry matter production and translocation,N uptake and use efficiency was measured and calculated in both MC and RC.With MC harvested manually and stubble height of 45 cm,the objectives of this study were to?1?elucidate the effects of N management treatments(N_main,N_bud,and N_tiller)on the grain yield of RC,and?2?to identify the underlying physiological mechanism of yield formation of ratoon rice in terms of radiation capture and conversion,dry matter production and transportation,and N uptake and utilization.The main results are as follows:?1?Total growth duration of RC,61-84 d,was 45%-58%of that of MC.RC produced grain yield,ranging from 2.96 to 6.49 t ha^-1,as much as 30%-95%of that of MC.N_mian had a relatively small effect on RC yield?6.3%?,although increased N_main improved MC yield by 13.7%averaged across years and treatments.Both N_bud and N_tiller had a significant effect on the grain yield of RC,and there was significant interaction between them.When N_tillerwas not applied,N_bud increased grain yield by 20.5%-55.4%.The effect of N_bud on grain yield was reduced to 12.7%-25.2%when N_tiller was applied.When N_bud was not applied,N_tiller increased grain yield by 11.5%-35.5%,whereas N_tiller had no significant effect on grain yield when N_bud was applied.Overall,N_bud had a larger effect on the grain yield of RC than N_tiller in our current study.?2?The increased sink size(spikelets per m^-2)mainly contributed to the improved grain yield of RC by application of N_bud and N_tiller,and the improvement in sink size by N_bud was greater than that by N_tiller.N_bud was more effective in increasing ratooning ability than N_tiller?22.9%vs.11.0%?,thereby achieving a higher increase in ratoon panicles per m^-2.On the other hand,the yield improvement in RC yield by N_bud was attributed to both improved aboveground total dry weight?TDW?and harvest index,while the enhanced grain yield of RC by N_tiller was mainly associated with increased TDW.In the ratoon season,N_bud had a greater effect?29.3%?on radiation use efficiency than N_tiller?16.7%?,and consequently N_budhad a higher improvement in dry matter production in the ratoon season than N_tiller.?3?In general,the yield of RC was determined by both post-heading dry matter production?W?and translocation of pre-heading accumulated dry matter to grain?T?.In the ratoon season,the poor W could not supply the requirement of carbohydrate for grain filling.Therefore,T also played a vital role in yield formation in the ratoon season,making contributions of 33.8%-53.3%to grain yield of RC.The improved RC yield by N_bud was attributed to increase in both W and T.Only enhanced W,however,rather than T,contributed to the increased grain yield of RC by N_tiller.?4?Aboveground total N uptake of RC?TN?was 60.4-143.6 kg ha^-1,and aboveground N content after the cutting of MC?AN?,36.1-78.0 kg ha^-1,accounted for 27%-90%of TN.N use efficiency for grain production ranged from 35.8-63.0 kg kg^-1across years and treatments.In the ratoon season,N_tiller had a greater improvement?33.9%?in TN than N_bud?27.1%?,but N_tiller decreased NUE_g of LY6326 by 18.2%.N_bud,however,had no significant effect on NUE_g of RC.In conclusion,under the conditions of our study,the increased N_main significantly improved MC yield but had a relatively small effect on the grain yield of RC.Both N_budand N_tiller were critical for improving the grain yield of RC.N_bud was more effective in increasing RC yield than N_tiller.Different with N_tiller,furthermore,N_bud had no negative effect on nitrogen use efficiency in the ratoon season.