Response Of Indica/Japonica Hybrid Rice Yongyou2640 To Nitrogen Application Rates And Its Physiological Basis

Indica-japonica hybrid rice(IJHR)has great yield potential,utilization of heterosis of IJHR has been considered as an effective way to further increase rice yield.Nitrogen is a key factor in rice production.Rational application of nitrogen fertilizer plays a very important role in using heterosis of IJHR and increasing rice yield.However,Little information is available about the response of IJHR to nitrogen application and its physiological basis.They are lack of systematic and in-depth researches.Elucidating the response of IJHR to nitrogen application and its physiological basis would have great significance to understand the mechanism underlying the yield formation of IJHR and its response to nitrogen,increase rice yield potential,realize green development of high yield,high quality,high efficiency,and to ensure national food security.In this study,Yongyou 2640(YY2640),an IJHR variety widely used in the middle and lower reaches of the Yangtze River,and Lianjing 7(LJ-7),a conventional high-yielding japonica rice variety(Yangdao 6,YD-6,an indica rice variety in some experiments),were grown in field with five nitrogen(N)rates(0,100,200,300,400 kg hm-2)or six N rates(0,100,200,300,400,500 kg hm-2).Grain yield,N use efficiency,population quality,leaf photosynthetic characteristics,grain filling processes,root morphology and physiology,rice quality of Yongyou 2640 and the apparent balance of soil nutrients and nitrogen were investigated.The effect of irrigation regimes on grain yield and nitrogen use efficiency of YY2640 was also observed.The main results are as follows.1.Grain yield and nitrogen use efficiencyWith the increase in N application rates,the grain yield of each variety was increased first and then was decreased,and the highest grain yield was at the N rate of 400 kg hm-2 for YY2640 and at the N rate of 300 kg hm-2 for LJ-7 and YD-6.At the same N rate,grain yield of YY2640 was higher than that of LJ-7 or YD-6.A higher grain yield of YY2640 was due mainly to the higher total amount of spikelets.The internal nitrogen efficiency(IEN),partial factor productivity(PFPN),recovery nitrogen efficiency(REN)and agronomic nitrogen efficiency(AEN)of three rice cultivars were decreased with the increase of N application rates.At the same N rate,YY2640 had higher IEN,PFPN,REN and AEN than the control varieties(LJ-7 and YD-6).The physiological N use efficiency(PEN)of YY2640 was lower than that of LJ-7 and higher than that of YD-6 under low N rates(100,200 kg hm-2),and higher than that of LJ-7 and YD-6 under high N rates(300,400,500 kg hm-2).At the same N rate,YY2640 had higher N,P,K harvest index,internal K efficiency and internal P efficiency than the control varieties.The results indicate that YY2640 not only has higher yield,but also has higher nutrient use efficiency.2.Population quality and dry matter production and translocationWith the increase in N rates,plant height,length,width and leaf basic angles of top-three leaves of YY2640 and LJ-7 were increased.When the N application rate was less than 400 kg hm-2 for of YY2640 and less than 300 kg hm-2 for LJ-7,the high efficient leaf area index(LAI),efficient LAI,the crop growth rate and net assimilation rate,the dry weight at heading,the shoot biomass,the dry weight accumulation from heading to maturity were increased with the increase in N rates.Proper increase in N application rates enhanced the content of non-structural carbohydrates(NSC)in the stem and sheath at the heading time and the transport of NSC in stem sheath from heading to maturity.Under the same N application rate,the grain leaf ratio,high efficient LAI,efficient LAI,crop growth rate,net assimilation rate and other population quality index values of YY2640 were higher than those of LJ-7.Th grain yield was significantly and positively correlated with dry matter weight at heading,dry matter weight at maturity,dry matter accumulation from heading to maturity,NSC at heading and NSC translocation from heading to maturity,suggesting that YY2640 has better population quality,which contributes to higher yield and nitrogen use efficiency.3.Leaf physiological traitsWhen the N application rate was less than 400 kg hm-2 for YY2640 and less than 300 kg hm-2 for LJ-7 and YD-6,the leaf SPAD value,photosynthetic rate(Pr),the content of zeatin(Z)+zeatin riboside(ZR)and the activities of nitrate reductase(NR),glutamine synthase(GS)and glutamate synthase(GOGAT)in leaves of the three varieties were increased with the increase in N application rates.When the N application rate of YY2640 exceeded 400 kg hm-2,LJ-7 and YD-6 exceeded 300 kg hm-2,the above indexes of YY2640 did not decrease significantly,while the Pr,the contents of Z+ZR in the flag leaves and the activities of NR,GS and GOGAT in leaves of LJ-7 and YD-6 were decreased significantly.Under the same N application rate,leaf SPAD value,Pr,leaf Z+ZR content and N metabolism enzyme activities of YY2640 were all higher than those of the control cultivars,indicating that IJHR has better leaf physiological traits than conventional high-yielding varieties under the same or higher N application rate.4.Grain filling of riceThe maximum grain filling rate(Gmax)and mean grain filling rate(Gmean)of the superior spikelets(S)and inferior spikelets(I)for YY2640 and LJ-7 were increased first and then decreased with the increase of N application rates.Compared with LJ-7,the asynchronous grain filling of S and I of YY2640 was more obvious with the increase of N application rates.At the same N rate,the Gmean and Gmax of S of YY2640 were greater than those of LJ-7,and the time to reach Gmax and the active grain-filling period(D)were significantly less than those of LJ-7.The variation trend of I was opposite to that of S.The final brown rice weight of the S and I of the two varieties had the same trend,i.e.,increased first and then decreased with the increase of N application rates.The contents of Z+ZR,indole-3-ecetic acid(IAA)and abscisic acid(ABA)in grains showed a single peak curve during grain filling period.The contents of Z+ZR,IAA and ABA in I were lower than those in S during the early grain filling period.With the increase of N application rate,the peak values of Z+ZR,IAA and ABA in S and I were increased first and then decreased.When the N application rate was at 300 kg hm-2,YY2640 and LJ-7 had the peak values of hormones.The contents of Z+ZR,IAA and ABA in grains were significantly and positively correlated with grain filling rate(G),indicating that the contents of Z+ZR,IAA and ABA in grains were closely related to grain filling rates.The Lower G and grain weight(GW)for I are related to the lower hormone contents during the early grain filling period.The contents of Z+ZR,IAA and ABA in spikelets could be increased by properly increasing N application rates,and consequently,G and GW could be increased.5.Root morphology and physiology of riceWhen the N application rate of YY2640 was less than 400 kg hm-2 and LJ-7 was less than 300 kg hm-2,the root dry weight,root length and root diameter of YY2640 were increased at main growth stages with the increase of N application rates,while the root to shoot ratio was decreased with the increase of N application rates.Under the same N rate,the root dry weight,root length and root diameter of YY2640 were all higher than those of the control cultivars,but the root to shoot ratio had no significant difference between varieties.The root oxidation activity,total root absorbing surface area,active root absorbing surface area,activities of root nitrogen metabolizing enzymes(nitrate reductase,glutamate synthase)and root hormone content(Z+ZR,IAA)were increased with the increase of N application rates at each main growth stage of rice.Under the same N application rate,the root oxidation activity,total root absorption surface area,root active root absorption surface area,root N metabolism enzyme activities and root hormone content of YY2640 were all higher than those of the control cultivars.Correlation analysis showed that root dry weight,root length,root diameter,root shoot ratio,root oxidation activity,root total absorption surface area,root active absorption surface area,root enzyme activity and root hormone content were linearly and positively correlated with grain yield.When the N application rate was less than 400 kg hm-2,the application of nitrogen fertilizer could improve the root morphological and physiological characteristics and increase grain yield of IJHR YY2640.When the N application rate was more than 300 kg hm-2,the grain yield was decreased for LJ-7 and YD-6 with the increase in N rates due mainly to the decreases in root dry weight,root length and root diameter,the root oxidation activity,total root absorption surface area,and root active root absorption surface area.6.Grain qualityWhen the N rate was less than 400 kg hm-2,with the increase of N application rates,the brown rice rate,milled rice rate,head milled rice rate,peak viscosity,hot viscosity,break down and final viscosity of YY2640 and LJ-7 were increased first and then decreased,while the chalky grain rate,chalky area,chalkiness,total protein content,albumin,globulin and prolamin contents were increased,and the amylose content and gel consistency were decreased,and set back was decreased first and then increased.Under the same N rate,the brown rice rate,milled rice rate,head milled rice rate,gel consistency and glutelin were significantly lower for YY2640 than for than LJ-7,and chalky grain rate,chalky area,chalkiness,amylose content,total protein content,albumin,globulin and prolamin,peak viscosity,hot viscosity,break down,final viscosity and set back of YY2640 were higher than those of LJ-7.Generally,,when N application rate was<300 kg hm-2,increasing N application could be beneficial to improve rice quality of YY2640,while when the N application rate was more than 300 kg hm-2,rice quality could be decreased.7.Distribution of fertilizer-N in plants and the loss of fertilizer-NThe effects of N application rates(0,100,200,300,400,500 kg hm-2)on the distribution of fertilizer-N in plants and the loss of fertilizer-N into environment were observed by using 15N tracer micro-plot experiment.With the increase of N rates,the total N content,nitrate-N content,ammonium-N content,and activities of invertase soil urease in the soil showed an increasing trend,and acid phosphatase activity was increased first and then decreased,and they reached the maximum at the N rate of 300 kg hm-2.When the N application rate was less than 400 kg hm-2 for YY2640 and less than 300 kg hm-2 for LJ-7,application of N increased the fertilizer-N accumulation in panicles of YY2640,LJ-7 and YD-6 at maturity,but when the N application rate exceeded the rates mentioned above,the fertilizer-N accumulation did not increase significantly for YY2640,and decreased significantly for LJ-7 and YD-6,but significantly increased in the stems and leaves.15N tracer results showed that the accumulation of fertilizer-N in plants(PNdff)and residual-N in the soil(NRS)were increased with the increase in N application rates.However,when the N application rate was more than 300 kg hm-2,the increase of NRS became smaller,and the loss of fertilizer-N into the environment(N loss)was increased sharply.With the increase in N application rates,the contribution rate of fertilizer-N to plant-N accumulation(PNdff%)aboveground and the loss rate of fertilizer-N into environment(loss%)were increased,the contribution rate of soil-N to the plant-N accumulation(PNdfs%),fertilizer-N absorption and utilization rate(Re-15N)and fertilizer-N residue rate(NRS%)were decreased.Under the same N rate,the amount of fertilizer-N accumulation,Re-15N,PNdff,PNdff%and PNdfs%of YY2640 were higher than those of the control varieties,while N loss,NRS%and loss%were lower than those of the control varieties.Overall,21.0%-35.7%,6.7%-23.7%and 42.3%-72.6%of fertilizer-N were absorbed by plants,remained in soil and lost to ecosystem,respectively.Data normalization showed that the major contribution of increasing nitrogen application rates was to increase the loss of fertilizer-N into the environment,the intermediate was to enhance N-uptake by rice shoots,and the least was to increase grain yield.In overall,the recommended N application rate for YY2640 was 300 kg hm-2.8.Effect of irrigation regimes on grain yield,water and N use efficienciesIn order to explore an suitable irrigation regime and the synergistic interaction effect between water and N on the grain yield and N use efficiency(NUE)of YY2640,two experiments of irrigation regimes were conducted.In the experiment 1,three irrigarion regimes,alternate wetting and moderate drying(AWMD),alternate wetting and severe drying(AWSD),conventional irrigation(CI)were designed.In the experiment 2,three irrigation regimes(CI,AWMD and AWSD)with three N rates of the low N rate(LN,100 kg hm-2),medium N rate(MN,200 kg hm-2)and high N rate(HN,300 kg hm-2)were conducted.Compared with CI,AWMD significantly increased grain yield by 8.41%and 7.05%,respectively,for YY2640 and LJ-7,whereas AWSD decreased grain yield by 8.88%and 23.64%,respectively,for YY2640 and LJ-7,implying that YY2640 is more suitable for AWMD and has stronger drought resistance than LJ-7.The AWMD significantly increased WUE and NUE of both varieties,with more increase for YY2640 than for LJ-7.AWMD significantly increased the percentage of productive tillers,grain-leaf ratio,photosynthetic rate of the flag leaf after heading,crop growth rate,root oxidation activity,whereas AWSD decreased the above-mentioned population quality indexes.Under the same irrigation regime,population quality of YY2640 was higher than that of LJ-7.Under all the N application rates(LN,MN,HN),the grain yield,WUE and NUE of YY2640 were higher under AWMD than under CI or than under AWSD,with highest under AWMD+HN.Under the same irrigation regime,the grain yield and WUE were increased with the increase in N application rate,whereas PFPN and IEN were decreased with the increase of N application rates.Higher percentage of productive tillers,higher cytokinin(Z+ZR)contents in roots,higher dry weight accumulation after heading and higher harvest index accounted for higher grain yield,NUE and WUE under AWMD+HN treatment combination.The results indicate that the grain yield,NUE and WUE of YY2640 can be further improved by using AWMD technique at the N application rate of 300 kg hm-2.