Effects Of Different Nitrogen Application Rates On The Growth Of Regenerated Bud,Grain Filling Characteristic And Grain Yield Of Ratoon Rice

In recent years,ratoon rice has been developing rapidly in southern China,in which planting area and grain yield levels are increasing over the years.However,the ratoon rice yields are generally low and unstable,which may limit ratoon rice planting on a large-scale in China.Proper rice varieties as ratoon rice planting and optimized cultivation managements are prerequisites for high and stable ratoon rice yield,among which nitrogen fertilizer application is one of the important measures.Many studies have been conducted to investigate the effects of nitrogen fertilizer applications on the agronomic characteristics of ratoon rice,the application of panicle and bud-promoting nitrogen fertilizer is of great significance for ratoon rice yield formation.However,the combined treatments of different panicle nitrogen fertilizer application rates in main season and bud-promoting nitrogen fertilizer application rates on the dry matter accumulation and translocation,the growth of regenerated buds and grain-filling characteristics in ratoon season are relatively poor.Therefore,two widely grown rice varieties in Hubei province,Fengliangyouxiang1（FLYX1）and Liangyou6326（LY6326）were used to investigate the growth and development of regenerated buds,dry matter accumulation and allocation and grain-filling characteristics,and their relationships with ratoon rice yield under two panicle nitrogen topdressing rates(N1 and N2 were 30 and 60 kg N ha^-1,respectively)and three nitrogen application rates of bud-promoting(T1,T2,and T3 were 0,50 and 100 kg N ha^-1,respectively)under field conditions in 2020 and 2021.The main results are as follows:（1）Under the three bud-promoting fertilizer treatments,the high panicle fertilizer application rate had no significant effect on main rice yield for FLYX1,however,for LY6326,the main rice yield averagely increased by 12.3%（2020）and 10.8%（2021）.Under the three bud-promoting fertilizer treatments,the high panicle fertilizer application rate had no significant effect on ratoon rice yield for FLYX1,but for LY6326,the ratoon rice yield averagely increased by 21.9%（2020）.Under the two panicle fertilizer treatments,for FLYX1,the ratoon rice yield averagely increased by 27.0%（2020）and 26.1%（2021）under the two bud-promoting fertilizer treatments when compared with no application of bud-promoting fertilizer.Under the two panicle fertilizer treatments,for LY6326,the ratoon rice yield averagely increased by23.2%（2020）and 24.0%（2021）under the two bud-promoting nitrogen fertilizer treatments when compared with no application of bud-promoting fertilizer.The highest annual grain yield of FLYX1 and LY6326 were 14.46 under the N1T3(panicle and bud-promoting fertilizer was30 and 100 kg N ha^-1,respectively)treatment and 14.44 t ha^-1 under the N2T2(panicle and bud-promoting fertilizer was 60 and 50 kg N ha^-1,respectively)treatment,respectively.（2）In ratoon season,under the three bud-promoting fertilizer treatments,the high panicle fertilizer application rate averagely decreased the sink capacity by 9.1%（2020）and 2.7%（2021）for FLYX1,but for LY6326,the aboveground biomass at maturity averagely increased by16.1%（2020）and 10.9%（2021）,the sink capacity averagely increased by 12.5%（2020）and12.6%（2021）under the high panicle fertilizer treatment.In ratoon season,under the two panicle fertilizer treatments,compared with no application treatment of bud-promoting fertilizer,the aboveground biomass at maturity and sink capacity averagely increased by 37.2%（2021）and44.6%（2021）for FLYX1,respectively,and for LY6326,the aboveground biomass at maturity averagely increased by 11.4%（2020）and 45.5%（2021）,and sink capacity averagely increased by 8.9%（2020）and 54.0%（2021）under the two bud-promoting fertilizer applications.Therefore,bud-promoting fertilizer applications could increased dry matter accumulation and sink capacity of ratoon rice.（3）Under the three bud-promoting fertilizer treatments,the high panicle fertilizer application rate had no effect on pre-full heading dry matter translocation（DMT）and post-full-heading dry matter accumulation（DMA）for both varieties in ratoon season.Under the two panicle fertilizer treatments,the two applications of bud-promoting fertilizer increased DMA but decreased DMT and harvest index of main season for both varieties when compared with no application of bud-promoting fertilizer.Under the two panicle fertilizer treatments,compared with no application of bud-promoting fertilizer,the two bud-promoting fertilizer applications averagely increased DMA by 35.5%（2021）across the two varieties,but had no effect on DMT for both varieties in ratoon season.Therefore,bud-promoting fertilizer applications increased ratoon rice yield mainly via enhancing DMA of ratoon rice.（4）Under the three bud-promoting fertilizer treatments,the high panicle fertilizer application rate had no effect on the regenerated bud length,survival number of buds,and bud growth rate in 2020 and 2021.In 2021,under the two panicle fertilizer treatments,the bud length of the top 2^nd,3^rd,and 4^th averagely increased by 18.7%,76.4%and 1863.2%for FLYX1and averagely increased by 10.0%,51.4%and 473.4%for LY6326,respectively,and the survival number of buds and bud growth rate per stem averagely increased by 40.3%and 88.8%for FLYX1 and averagely increased by 27.0%and 55.0%for LY6326,respectively,under the two bud-promoting fertilizer applications when compared with no application of bud-promoting fertilizer.Therefore,bud-promoting fertilizer applications could prompting bud growth,especially for the axillary bud from the lower node.（5）Under the three bud-promoting fertilizer treatments,the high panicle fertilizer application rate averagely decreased stubble soluble sugar concentration,NSC concentration and NSC content by 60.2%,47.6%and 47.0%for FLYX1,respectively,and averagely decreased stubble NSC by 18.8%for LY6326 in 2021.Under the two panicle fertilizer treatments,compared with no application treatment of bud-promoting fertilizer,the two bud-promoting fertilizer treatments averagely decreased stubble soluble sugar by 44.7%（2020）and47.9%（2021）,and averagely decreased starch content by 34.4%（2020）,and averagely decreased stubble NSC content by 43.4%（2020）and 32.0%（2021）across varieties.（6）There were significant differences in grain weight and grain filling rate in the upper,middle,and lower parts of the panicle at different nodes in ratoon season.Grain weightand grain filling rate in the upper part of the panicle were significantly higher than those in the lower part of the panicle.The difference in grain filling rate and grain weight in different part of panicle may limit the high yield potential of ratoon rice.The high panicle fertilizer application rate had no effect on grain weight at different parts of the panicle,but increased the grain filling rate at different parts of the panicle of the ratoon rice for FLYX1,and had no significant effect on grain filling rate at different parts of the panicle for LY6326.Compared with no application treatment of bud-promoting fertilizer,the two bud-promoting fertilizer applications significantly decreased grain weight in the middle and lower parts of the panicle of different nodes,and had no significant effect on grain weight in the upper part of the panicle of different nodes for both varieties at maturity stage.The bud-promoting fertilizer treatments had no significant effect on grain filling rate at different parts of the panicle on the top 2^nd node,but decreased the grain filling rate in the middle part of the panicle on the top 3^rd node for both varieties.Therefore,more application rate of bud-promoting nitrogen fertilizer was not benificial to grain-filling and grain weight formation of ratoon rice.