Differences In Nitrogen Absorption And Utilization Between Early And Late Seasons And Underlying Physiological Basis In Rice

Nitrogen absorption and utilization play important roles in the yield formation of rice.With the global climate change,more and more attentions are being drawn on the effects of climatic factors on growth and yield and on the nitrogen absorption and utilization;however,most of the previous researches were focused on the nitrogen absorption and utilization of rice under differently ecological regions.The effects of temperature and solar radiation conditions on the nitrogen absorption and utilization of rice has rarely been studied in the different growth seasons in the same location.Therefore,the objectives of present study were to examine the effects of temperature and light conditions on the nitrogen absorption and utilization of rice under double season rice systems in central China,and to explore the physiological changes in nitrogen absorption and utilization in response to temperature and solar radiation.In 2016 and 2017,two varieties,Ezao 18?inbred rice?and Liangyou 287?hybrid rice?,were used,which are widely cultivated in the middle and lower reaches of the Yangtze River.In the study,we investigated the effects of different nitrogen application rates on the grain yield formation and nitrogen absorption and utilization of rice in early and late seasons in the same experimental site.The main results are as follows:1.In the same season?early and late seasons?,increased nitrogen application rates significantly increased the grain yield,and the yield increase was higher in the early season than in the late season.For the same variety,the yield in late season was higher than that in early season;however increase of nitrogen application rate decreased the yield difference between the early and late rice.In the same season,the differences in grain yield were mainly attributed to the increase in spikelets per unit area?11%-84%?and aboveground biomass?11%-81%?.When compared between early and late seasons,the differences in yield were mainly attributed to the increase in spikelets numbers?61%?and aboveground biomass?33%?,as well as decrease of grain filling rate?8%?and grain weight?12%?,suggesting that higher spikelets per unit area and bimass resulted in the high grain yield in late rice.2.In the same season,the differences in intercepted radiation were significantly and positively correlated with differences in spikelets and yield among different nitrogen application rates.The differences in intercepted radiation were consistent with differences of yield,spikelets,grain filling rate,grain weight and biomass between early and late seasons.These results indicated that nitrogen application rates and intercepted radiation showed interactive effects on investigated agronomic traits,and increased nitrogen application rate enhanced intercepted radiation,and further facilitated the yield formation in rice.3.In the same season,total nitrogen acculumation?TNA?gradually increased with the increase in nitrogen application rates,while both NUE_b and NUE_g decreased.Total nitrogen acculumation in late season was higher than that in early season,while NUE_b,NUE_g,and NAE were lower than that in early season.Intercepted radiation was significantly and positively correlated with total nitrogen accumulation,and negatively correlated with nitrogen use efficiency.4.In the same season,leaf nitrogen accumulation and re-translocation increased gradually with the increase in nitrogen application rates,however,the increase in leaf nitrogen accumulation and re-translocation by the increase of nitrogen application rates were lower in the late season than that in early season.Additionally,leaf nitrogen accumulation and re-translocation in late season were higher than that in early season.Intercepted radiation was significantly and positively correlated with leaf nitrogen accumulation and re-translocation,respectively,indicating that the differences in intercepted radiations among different nitrogen rates were closely associated with the differences in leaf nitrogen accumulation and re-translocation.5.In the same season,the GS and GOGAT activities in flag leaves increased with the increase of nitrogen application rates in panicle initation and heading stages,while the increase in GS and GOGAT activities of flag leaves in late season were lower than that in early season.The GS and GOGAT activities of the flag leaves in late season were higher than those in early season,and the differences in GS and GOGAT activities between both seasons gradually decreased with the increase of the nitrogen application rates.The daily mean temperature and mean daily radiation in early season(24.5?,15.2 MJ m^-2 d^-1)were lower than that the late season(28.8?,16.4 MJ m^-2 d^-1).These results indicated that the better temperature and radiation conditions in late season may have positive effects on GS and GOGAT activities,and increased nitrogen application rates attenuated the differences in GS and GOGAT activity that were caused by differences in temperature and intercepted radiation.Therefore,our results suggest that there is an interactive effection between nitrogen application rate and temperature and radiation conditions,which together affect the activities of GS and GOGAT.6.Our results showed that GS and GOGAT activities showed significantly positive correlation with leaf nitrogen accumulation and re-translocation during panicle initation and heading duration,respectively,indicating that higher GS and GOGAT enhanced leaf nitrogen accumulation and re-translocation.In summary,there was an interaction between nitrogen application rate and planting season.The favorable temperature and light conditions in late season were useful for plant growth and yield formation of rice,via enhancing leaf nitrogen accumulation and re-translocation and leaf GS and GOGAT activities.There were differences in temperature and radiation between early and late seasons,and increase in nitrogen application rate may relief to some degress the effects of temperature and radiation on grain yield differences between early and late seasons.