Study On Physiological Mechanism Underlying Improvement In Nitrogen Utilization Efficiency Of Late-sown Winter Wheat

The field experiment was carried out in Dongwu Village,Dawenkou Town,Tai'an City,Shandong Province,during the two consecutive reproductive seasons of 2016-2017 and2017-2018.Tainong 18?T18?,a widely planted winter wheat variety,was used as the test material.We set up three sowing date treatments,including early sowing?24 September?,normal sowing?8 October?,and late sowing?22 October?.The effects of sowing date on grain yield formation and nitrogen uptake and utilization efficiency of winter wheat were studied.The findings are as follows:1 Effect of sowing date on grain yield formation of winter wheatSowing date significantly affected the growth of winter wheat during the whole growth period.The number of tillers per plant,dry weight per plant,population size and dry matter accumulation per unit area were significantly different in the early stage of growth?before wintering and jointing?during three sowing date treatments.Although the difference of population size still existed in the later stage of growth,the dry weight per plant increased significantly,which resulted in the same dry matter accumulation per unit area.As sowing date delayed,the grain weight of winter wheat was not affected,but the spike number per unit area decreased significantly,and the grain number per spike increased significantly.A trade-off between spike number per unit area and grain number per spike resulted in consistent grain number per unit area between three sowing date treatments,which led to the same grain yield level,so sowing date did not effect the grain yield of winter wheat.At the same time,the harvest index remained the same during the three sowing date treatments,indicating that the sowing date did not affect the allocation ability of dry matter to the grain.2 Effect of sowing date on nitrogen nutrition indexSowing date significantly affected pre-anthesis nitrogen nutrition index of winter wheat.At wintering and jointing,the nitrogen nutrition index of the three sowing date treatments was less than 1,that is,the plants were in the deficit state,which suggests that nitrogen fertilizer should be properly used to supplement at jointing stage.At booting and anthesis,the nitrogen nutrition index of early sowing and normal sowing was obviously higher than 1,that is to say,the plants were in surplus state.While that of late sowing was close to 1,that is,nitrogen was in balance between supply and demand.3 Effect of sowing date on nitrogen distribution of winter wheat at anthesisSowing date significantly affected nitrogen distribution in winter wheat.At anthesis,the aboveground nitrogen uptake decreased significantly as sowing date delayed,but the storage nitrogen content in reserve nitrogen increased significantly,while the accumulation nitrogen content decreased significantly.In addition,the nitrogen distribution on a single plant was optimized under late sowing,compared to that under normal sowing,mainly as follows:Firstly,the nitrogen allocation to leaves was increased.Secondly,the nitrogen allocation to upper leaves?flag leaf and second leaf?was increased,and the nitrogen allocation to lower layer leaf?third leaf and fourth leaf?was reduced.Third,the dry weight and nitrogen content per unit area of upper leaves increased,while the dry weight and nitrogen content per unit area of lower leaves remained unchanged or decreased.Fourthly,the nitrogen allocation to Rubisco enzyme in upper leaves was increased,while the nitrogen allocation to Rubisco enzyme in lower layer was maintained or decreased.Fifthly,the nitrogen allocation to activated Rubisco enzyme was increased,while the nitrogen allocation to inactivated Rubisco enzyme was decreased.Sixth,the nitrogen allocation to the cell wall of each leaf layer was decreased.As a result of the optimal nitrogen allocation,the maximum carboxylation efficiency of the upper leaves increased significantly,while the maximum carboxylation efficiency of the lower leaves remained unchanged or decreased.On the other hand,late sowing optimized the diffusion pathway of carbon dioxide from atmosphere to carboxylation site in leaves,which mainly increased the stomatal conductance and mesophyll conductance of leaves.Combined with the maximum carboxylation efficiency and the improved carbon dioxide diffusion ability,the light-saturated net photosynthetic rate of the upper leaves increased significantly,and the increase of light-saturated net photosynthetic rate of lower leaves mainly attributed to the enhancement of carbon dioxide supply capacity.Outperformance by light-saturated net photosynthetic rate over specific green leaf area nitrogen led to improved photosynthetic nitrogen use efficiency in upper leaves.Enhanced light-saturated net photosynthetic rate coupled with unchanged or decreased specific green leaf area nitrogen resulted in improved photosynthetic nitrogen use efficiency in lower leaves.Therefore,under the absorbing less nitrogen of late sowing,the grain yield could be maintained the same level with early and normal sowing.Ultimately,the nitrogen utilization efficiency was improved significantly under late sowing,compared to that under normal sowing.4 Effect of sowing date on nitrogen productivitySowing date significantly affected nitrogen productivity of winter wheat.With the delay of sowing date,the nitrogen accumulation and dry matter accumulation of shoot from emergence to flowering stage were significantly different,but the difference of nitrogen productivity was not significant or slightly different,indicating that the delayed sowing date did not affect the nitrogen productivity at this growth stage.However,nitrogen accumulation and dry matter accumulation in shoot decreased significantly.From flowering stage to mature stage,there were significant differences in nitrogen accumulation,dry matter accumulation and nitrogen productivity in the shoot,indicating that the delayed sowing date significantly reduced the above-ground nitrogen accumulation,but significantly increased the dry matter accumulation and nitrogen productivity.5 Effect of sowing date on nitrogen use efficiencyIn our study,the sowing date did not affect the nitrogen use efficiency of winter wheat,and there was no correlation between nitrogen use efficiency and nitrogen nutrition index,mainly because the sowing date did not affect the grain yield of winter wheat at the maturity stage.The same grain yield level was mainly due to the trade-off between the number of panicles per unit area and the number of grains per panicle,as well as the result of the invariance of grain weight.Sowing date significantly affected the nitrogen uptake efficiency,that is to say,the nitrogen uptake efficiency decreased significantly with the delay of sowing date,and there was a significant negative correlation between nitrogen uptake efficiency and nitrogen nutrition index,which was mainly due to the consistency of nitrogen supply in the three sowing date treatments.However,the total nitrogen uptake in shoot decreased significantly with the delay of sowing date.Sowing date significantly affected the nitrogen use efficiency,that is to say,the nitrogen use efficiency increased significantly with the delay of sowing date,and there was a significant positive correlation between nitrogen use efficiency and nitrogen nutrition index,which was mainly due to the consistency of grain yield in the ripening stage of winter wheat.However,the total amount of nitrogen uptake in shoot decreased significantly with the delay of sowing date.Finally,the decrease of nitrogen uptake efficiency was basically consistent with the increase of nitrogen use efficiency,so the nitrogen use efficiency remained the same.6 Effect of sowing date on soil nitrogen balanceSowing date significantly affected soil nitrogen balance at maturity,including soil mineral nitrogen content and soil apparent nitrogen loss.In our study,nitrogen uptake in early sowing treatment was significantly higher than that in normal sowing stage treatment,which reduced soil mineral nitrogen content and soil apparent nitrogen loss in mature stage.This will reduce groundwater pollution caused by nitrogen leaching and Greenhouse Effect caused by the discharge of N₂O.The nitrogen uptake in late sowing treatment was significantly higher than that in normal sowing stage,which increased the soil mineral nitrogen content and soil apparent nitrogen loss in mature stage.This will increase groundwater pollution caused by nitrogen leaching and Greenhouse Effect caused by the discharge of N₂O.It can be seen that late-sowing winter wheat will lose a lot of nitrogen and aggravate environmental pollution.Therefore,the nitrogen supply of late sowing treatment should be reduced,which will improve the nitrogen uptake efficiency,while the nitrogen use efficiency and grain yield will not be affected,and ultimately improve the nitrogen use efficiency.