Studies On Physiological Mechanisms Of Optimized Nitrogen Managments Increasing Nutrient Use Efficiency In Rice

Nitrogen(N),which is the major restriction for rice plant growth and development,plays an important role in improving rice yield.However,the current rice production in China is faced the problems of slow increase in rice yield,unreasonable application of N fertilizer and low nitrogen use efficiency.Therefore,exploring scientificly optimized N management becomes an important measure to synergistically increase rice yield and nitrogen use efficiency(NUE),and is the urgent need of the sustainable development of agriculture.Both field experiments and laboratory experiments were conducted in this study.The field experiments with four different N managements methods were conducted in long-term location experiment station of,Liyang City,Jiangsu Province at 2011-2012.Treatments included:no N fertilizer(NO),farmers' fertilizer practices(FFP),N application of high efficiency(HE),N application of high yield and high efficiency(HYHE),in which HE and HYHE were the optimiztd N managments(OPTs).The effects of different N management methods on rice yield,nitrogen use efficiency(NUE),the accumulation of dry matter and N were studied in field experiments.Laboratory experiments with three different N supply patterns were conducted according to the results of field experiments through nutrient solution culture method,simulating FFP,HE,and HEHY,respectively.The effects of different N management methods on the growth of rice root and aboveground parts,distribution of dry matter,N accumulation,distribution,and assimilation,root morphology and physiological characteristics,and yield formation process were further analysed,in order to provide a theoretical basis in OPTs both increasing rice yield and NUE.The major results are list as follows:1.Field experiments showed that the total N rate of HE and HYHE reduced by 40%and 20%respectively,however,rice yield of HE and HYHE increased by 1.9%and 4.3%respectively when compared with FFP.The higher grains per panicle,seed setting rate and 103-grain weight contributed to higher rice yield in OPTs.Compared with FFP,nitrogen recovery efficiency(REN),nitrogen agronomic efficiency(AEN),and nitrogen partial factor productivity(PFPN)were significantly increased,in which,the REN increased by 71.7%and 63.0%respectively,the AEN increased by 64.9%and 50.9%,and the PFPN increased by 65.9%and 30.1%respectively.Simulated hydroponic experiments also showed that the rice yield of HE and HYHE increased by 14.6%and 18.4%respectively when compared with FFP,and grains per panicle,seed setting rate and103-grain weight also significantly increased.2.In field experiments,the leaf area index(LAI)and SPAD value of HE were significantly lower than that of HYHE and FFP at the early growth stage of rice,however,no difference of LAI and SPAD value were observed in all treatments at anthesis and maturity stage,and leaf SPAD value order was HYHE>HE>FFP.The leaf SPAD value in FFP was significantly lower than in OPTs especially under the simulated hydroponic experiments at anthesis and grain-filling stage.The leaf photosynthetic rate(Pn)in HE and HYHE increased by 13.4%and 20.1%,39.5%and 40.7%respectively at anthesis and grain-filling stage respectively when compared with FFP.Results in simulated hydroponic experiments showed that HE and HYHE improved the aboveground growth rate at the middle and late growth stage of rice,furthermore,post-anthesis dry matter accumulation(post-anthesis DMA)and the transportation of post-anthesis assimilates produced by photosynthesis to grain(APA)increased by 12.7%and 11.3%?16.3%and 21.2%respectively when compared with FFP.The leaf Pn at grain-filling stage was significantly positively correlated with post-anthesis DMA and APA.Significantly positive correlation between the post-anthesis DMA or APA and rice yield with the correlation coefficient of 0.88 or 0.84 respectively.3.Results in simulated hydroponic experiments showed that:compared with FFP,(1)OPTs could maintain higher plant N level at the late growth stage and increased the aboveground N accumulation rate at middle and late growth stage of rice.Moreover,post-anthesis N accumulation(post-anthesis NA)in HE and HYHE increased by 12.7%and 11.3%respectively,and ultimately increased the total N accumulation by 7.3%and 16.8%respectively,which is the reason of higher REN in OPTS;(3)the transportation of post-anthesis absorbed N to grain in FFP,HE and HYHE were 30.9 mg plant-1,57.4 mg plant-1 and 66.3mg plant-1 respectively.Rice yield showed significanltly positive correlation with post-anthesis N accumulation and the transportation of post-anthesis absorbed N with both the correlation coefficient of 0.88.4.Results in simulated hydroponic experiments showed that:(1)compared with FFP,rice plants in HE and HYHE showed higher root dry matter,root/shoot ratio,root length,total root surface urea and root volume throughout the growth season,morever,the xylem flow sap rate increased by 13.2%and 19.9%,19.8%and 29.5%respectively at anthesis and grain-filling stage;(2)there was significantly positive correlation between xylem flow sap rate and leaf Pn;(3)significantly positive correlation between root growth rate and N accumulation was observed,moreover,the root dry matter,morphological parameters and root activity at anthesis were positively correlated with N accumulation during each growth stage,and with grains per panicle,seed setting rate a 103-grain weight.Consequently,good root morphology and higher root activity.5.Simulated hydroponic experiments showed that:(1)compared with FFP,OPTs could maintain higher glutamine synthetase(GS)activity in roots and leaves at late growth stage of rice.As a result,total free amino acids content in xylem sap and leaf soluble protein content increased accordingly;(2)compared with FFP,leaf nitrate reductase(NR)activity in HE and HYHE increased by 19.0%and 14.1%?76.4%and 80.6%at anthesis and grain filling stage respectively;(3)significantly positive correlation between N uptake post-anthesis and leaf GS activity or leaf NR activity at anthesis were observed.6.(1)OPTs decreased the degradation of secondary rachis branch,the primary flort and the secondary flort by improving N accumulation and aboveground growth rate during booting to anthesis,consequently resulting in higher total number of flort,which laid the good foundation for the formation of large panicle;(2)Different N management methods showed no effects on grain-filling characteristics of superior spikelets.Compared with FFP,OPTs increased the soluble sugar content in the flag leaf and increased sucrose content in flag leaf sheath phloem sap during grain-filling stage,which provided more raw materials for starch synthesis,and thus promoted the synthesis of starch in the inferior spikelets,accordingly promoting the grain-filling of inferior spikelets.HE and HYHE shortened the active grain filling period of inferior spikelets by 2.4 and 3.0 days respectively,however,increased grain filling rate of inferior spikelets by 18.9%and 17.0%respectively than those of FFP,and thus the final grain weight was increased by 12.1%and 11.6%in HE than in FFP..In summary,OPTs reduced the total N rate and N rate at early growth stage of rice,however,increased N application at the late growth stage of rice.And thus OPTs could regulate the dynamics of rice root growth and development,the accumulation and allocation of abroveground dry matter and N,N physiological process and grain-filling process,and thus achieving the synergistic improvement in rice yield and NUE.