The Responses Of Different Rice Varieties In Agronomic Performance To Seedling Ages With Varied Seeding Or Transplanting Dates

Transplanting is the main crop establishment method in rice production due to the potential of achieving high-yielding and increasing multiple-cropping index.In transplanted rice,seedling age(SA),the rice growth duration in the seedling nursery,has a profound effect on rice growth and yield formation.Different SA can be achieved by varying seeding dates with the same transplanting date(SAs)or varying transplanting dates with the same seeding date(SAt).Generally,studies have been conducted to investigate optimum SA for different rice varieties with one of the two methods for achieving different SA.There was limited information about the difference in responses of rice agronomic performance to SA between the two methods for varying SA.Therefore,four field experiments were conducted to determine the response of different rice varieties in agronomic performance to SA using the two methods for varying SA in Wuxue County,Hubei Province,China in the middle-season rice of 2016-2018.The experiments were arranged in a split-plot design with SA as main plots and varieties as subplots and with four replications.The SA treatments were achieved by SAs in 2016 and 2017 and by SAt in 2017 and 2018.Two SA(20-and 40-day-old seedling,SA20 and SA40)were used in all experiments.Three widely grown rice varieties,Huanghuazhan(HHZ),Yiangliangyou6(YLY6),and Y-liangyou900(YLY900)were used as experimental materials.Rice crop phenology,dry matter accumulation and translocation,grain yield and yield components,and climatic conditions were measured.The main results were as follows:(1)Rice crop phenology Compared with SA20,the crop phenology of each variety in SA40 was advanced in SAs but delayed(except for panicle initiation of HHZ)in SAt.The duration of vegetative growth in the main field of SA40 was shortened by 4-10 d and9-25 d compared with SA20 in SAs and SAt,respectively.SA had a small effect on reproductive growth duration except for HHZ in SAs,for which old seedlings increased reproductive growth duration by 11-15 d.Grain filling durations were similar between SA in SAs,but it was increased by 3-10 d for old seedlings in SAt.Compared with SA20,crop duration in the main field(from transplanting to maturity)of SA40 were shortened by 4-8 d and 1-6 d in SAs and SAt,respectively.However,old seedlings increased total crop growth duration(from sowing to maturity)by 12-16 d and 14-19 d in SAs and SAt,respectively.(2)Rice grain yield In SAs,HHZ in SA40 had 8.4%higher and 11.7%lower grain yield than that in SA20 in 2016 and 2017,respectively.The difference in grain yield between SA for YLY6 was not significant in both 2016 and 2017.The grain yield of YLY900 in SA40 decreased by 15.3%and 8.8%in 2016 and 2017,respectively compared with that in SA20.The average grain yield across varieties for SA20 and SA40 were 9.16and 8.87 t ha^-1 in 2016 and 8.94 and 8.27 t ha^-1 in 2017 in SAs,respectively.The difference in grain yield between SA was only significant in 2016 but not in 2017.In SAt,old seedlings significantly increased grain yield,with the increase of 20.6%and 8.5%for HHZ and YLY900 in 2017,respectively.However,the grain yield of YLY6 in SA40 was significantly lower than that in SA20 in 2017.There was no significant difference in grain yield between SA for both HHZ and YLY6,whereas the grain yield of YLY900 in SA40decreased significantly,with a reduction of 6.7%,compared with that in SA20 in 2018.The average grain yield for SA20 and SA40 was 8.34 and 8.82 t ha^-1 in 2017 and 9.68 and9.47 t ha^-1 in 2018 in SAt,respectively.The difference in grain yield between SA was not significant in both years.In general,the grain yield of SA40 was not necessarily lower than that of SA20 in the two methods for varying SA.(3)Yield components The significant increase in spikelets m^-2 and the decrease in grain filling percentage were responsible for yield increase of HHZ in 2016 and yield reduction of HHZ in 2017 and YLY900 in both years for old seedlings in SAs,respectively.In SAt,yield improvements of HHZ and YLY900 in old seedlings in 2017were attributed to increased grain filling percentage,while yield reduction of YLY6 in2017 and YLY900 in 2018 in old seedlings were due to the decrease of spikelets m^-2.In general,SA had an insignificant effect on panicles m^-2,regardless of the methods used for varying SA.The responses of spikelets m^-2 and grain filling percentage to SA were opposite between SAs and SAt.Averaged across varieties and years,the spikelets m^-2 in SA40 was increased by 7.8%,but the grain filling percentage was decreased by 6.3%,compared with that in SA20 in SAs.However,the spikelets m^-2 and grain filling percentage in SA40 were decreased and increased by 11.7%and 13.5%,compared with SA20 in SAt,respectively.The different response of yield components to SA was related to the variation of crop phenology and associated meterological condition.The crop phenology in the main field for SA40 was advanced compared with SA20 in SAs,which led to higher thermal and solar radiation during reproductive growth stage and lower heat risk due to high temperature around panicle initiation but higher heat risk around heading.While the crop phenology was delayed in old seedlings in SAt,which resulted in lower thermal and solar radiation during reproductive growth stage and higher heat risk around panicle initiation but less risk around heading.Therefore,the response of yield components to SA was opposite between the two methods.(4)Dry weight accumulation and translocation Seedling dry weight of old seedlings was significantly increased for both methods.In SAs,SA had small effect on dry weight accumulation at mid-tillering,heading,and maturity.In SAt,SA40 increased the dry weight accumulation at early stage,but decreased the dry weight accumulation at heading and maturity.The correlations between relative grain yield and relative harvest index of old seedlings over young seedlings were significantly positive for both methods.The results suggested that harvest index was responsible for the response of grain yield to SA.In addition,the increase of pre-heading dry matter translocation contributed to stabilizing grain yield in SA40 as compared to SA20.In conclusion,the responses of crop growth phenology and yield components to SA were opposite between SAs and SAt.Compared with SA20,the crop phenology in the main field for SA40 was advanced in SAs,which resulted in less heat stress around panicle initiation but more heat stress around heading compared with SA20.Therefore,spikelets m^-2 and grain filling percentage of SA40 were increased and decreased relative to those in SA20,respectively.In contrast,the crop phenology was delayed in SA40 in SAt,which resulted in more heat stress around panicle initiation but less heat stress around heading,compared with SA20.Therefore,spikelets m^-2 and grain filling percentage of SA40 were decreased and increased relative to those in SA20,respectively.The grain yield of SA40 was not necessarily lower than that of SA20 under both methods used for varying SA because of the compensation among yield components.In addition,increasing harvest index and pre-anthesis dry matter translocation were responsible for yield stabilization of old seedlings compared with young seedlings.Overall,the results from this study indicated that achieving high yielding with old rice seedlings is critical to promoting the adoption of multi-cropping system in the context of resource and labor shortage and contributing to the sustainable development of simplified and large-scale agricultural production.