Rice(Oryza Sativa L.) Yield Formation And Its Nitrogen Regulation Under High Temperature During Panicle Initiation Stage

High temperature（HT）is one of abiotic stresses affecting rice yield under global warming.The decreased seed setting rate and spikelet sterility are the main factors for the decrease of rice yield under HT treatment.However,little information is available on the effects of HT during PI on anther anatomical structure,assimilates partitioning and their relationships with spikelet fertility.It is not yet clear whether HT injury during PI could be alleviated by regulating nitrogen application rate and time.Information is not yet available regarding the effects of different nitrogen rates on nitrogen,phosphorus uptake and partitioning for cultivars with different heat resistance under HT stress during PI.Understanding the mechanism of HT injury is of great significance,it is worthy to develop measures for alleviating HT injury.Therefore,the purpose of this study is to explore the effects of different nitrogen application rates on yield formation under high temperature during PI and its mechanism in rice,focusing on pollen development and fertitlity.The main contents include three parts:（1）effect of HT on rice spikelet fertility and its relationship with anther structure characters and carbohydrate distribution,（2）effect of increasing nitrogen fertilizer rate at different stages（basal,tillering stage,and panicle initiation）on rice yield and underlying mechanism,（3）effects of different nitrogen application rates on rice nitrogen（N）and phosphorus（P）uptake and partitioning under HT stress during PI.Therefore,using two rice varieties（Liangyoupeijiu,LYPJ,HT sensitive）and（Shanyou 63,SY63,HT tolerant）,the potted experiments were conducted with two temperature treatments（CK,average daytime/nighttime temperature 30/27℃;HT treatment,average daytime/nighttime temperature 37/27℃）,and different nitrogen treatments were the high and low application rates for basal and tillering and for panicle initiation).The main results are as follows:HT treatment during PI significantly decreased the grain yield by 84%of heat sensitive variety LYPJ,which could be ascribed to the decreased spikelets per panicle（decreased by35%）,seed setting percentage（69%）and 1000-grain weight（17%）.The reduction of the seed setting percentage of LYPJ was mainly attributed to lower spikelet fertility（69%）,which was caused by the decreased pollen viability（46%）,the reduced anther dehiscence rate（5%）and the poor pollen shedding of the anthers（11%）.HT had slightly adverse effect on the grain yield of heat-tolerant variety SY63,which was attributed to stable pollen viability,anther dehiscence,and pollen shedding of the anthers.At stage 10 of anther development in LYPJ plants under HT,the number of vacuolated microspores was decreased,tapetum degenerated slowely,Ubisch bodies were unevenly arranged,the outline of pollen exine were obscure.At stage 11 in LYPJ under HT,the anther wall layers were disordered and broken,microspores were degraded and irregularly shaped,the tapetum became less condensed,and the lacuna between the septum and stomium became smaller.At stage 13 in LYPJ under HT,the anthers were smaller,the anther wall was shrunken and irregularly shaped,the tapetal cells were visible,and fewer fertile pollen grains per anther locule were observed,sporopollnins were unevenly desposited on the pollen surface.However,HT did not substantially change the development of the anther wall and microspores at the late stage of pollen formation in SY63.Therefore,our results suggest that disturbed anther walls and pollen development are responsible for the reduced spikelet fertility and grain yield of the tested heat susceptible variety,and noninvasive anthers and pollen formation in response to HT are associated with heat tolerance.HT significantly decreased pollen viability and spikelet fertility by 41%and 83%in LYPJ plants,respectively,whereas HT had slight effects on pollen viability and spikelet fertility in SY63 plants.Under HT condition,concentration of non-structural carbohydrates（NSC）in LYPJ anthers decreased by 44%;however,anther NSC concentration in SY63 did not change significantly.In LYPJ plants,HT decreased ¹³C abundance of anther and leaf by10%and 24%,respectively;but increased rachis and husk ¹³C abundance by 26%and 23%,respectively;leaf,anther,rachis and husk ¹³C abundance of SY63 significantly increased under HT by 89%,40%,237%and 204%,respectively.HT had no significant effect on photosynthetic rate,vascular bundle characters of peduncle,area of anther sieve elements for both cultivars.Lower anther sucrose concentration of LYPJ was possibly related to obstacle in sucrose phloem unloading in both symplastic pathway（reflected by the decline in plasmodesmata density between companion cell and its adjacent parenchyma cells,and between parenchyma cells）and apoplastic pathway（reflected by the decreased expression of Os CIN1,Os CIN2,Os SUT1,Os SUT5）in LYPJ plants compared with CK.However,HT had no substantial effects on anther phloem unloading in SY63 plants.Compared with LYPJ,SY63 had larger area of anther vascular bundle,plasmodesmata density between companion cell and its adjacent parenchyma cells,and between parenchyma cells under HT.Overall,our results demonstrate that high assimilate unloading of anther contributes to heat tolerance of SY63 under HT during PI.HT markedly decreased yield of LYPJ by 84%at low panicle nitrogen application rate for panicle initiation（LPN）,while less yield decline（48%）was observed at high panicle nitrogen application rate for panicle initiation（HPN）under HT;the yield of SY63 was decreased by 31%at LPN and 36%at HPN under HT,the results suggest that increasing N application at PI can improve rice yield of heat susceptible cultivar LYPJ,mainly via improving pollen viability and pollen shedding percentage of the anthers.The following three reasons might explain higher pollen viability and spikelet fertility of LYPJ at HPN under HT during PI.Firstly,higher evapotranspiration and lower panicle temperature were observed at HPN than those at LPN in the study.Secondly,the negative effects of HT on the expression of anther Os INV2 and Os INV3 in LYPJ were alleviated,and the inhibition of high temperature on assimilate transport was reduced,resulting in the increased sucrose concentration of anther.Thirdly,the activity of POD enzyme increased,resulting in lower lipid peroxidation of anther and spikelet.HT during PI reduced the yield in LYPJ by 82%（low nitrogen application rate for the basal and tillering,LEN）and 39%（high nitrogen application rate for the basal and tillering,HEN）,and in SY63 by 40%（LEN）and 60%（HEN）,respectively,indicating that the increased nitrogen fertilizer rate could improve the yield of LYPJ,but aggravate the yield decline of SY63.The spikelet fertility of LYPJ was 11%at LEN and 62%at HEN under HT during PI.Increasing basal and tillering fertilizer rates could improve spikelet fertility of LYPJ under HT during PI,mainly via reducing panicle temperature,increasing anther NSC concentration and delaying panicle development.HT during PI reduced dry matter allocation to panicles and increased stem dry weight at heading and maturity stages of LYPJ.Increasing both basal and tillering fertilizer rate and panicle nitrogen fertilizer rate promoted panicle dry matter accumulation at heading stage and maturity stage under HT during PI.HT during PI reduced the number of differeniated spikelets on secondary branches,increased the number of degenerated spikelets on secondary branches,and declined the size of spikelets of LYPJ.HT during PI had no significant effect on the number of differeniated spikelets on secondary branches,but increased in the number of degenerated spikelets on secondary branches,and declined the size of spikelets of SY63.Increasing both basal and tillering fertilizer rate and panicle nitrogen fertilizer rate could not allievate the decrease of spikelets per panicle and spikelet size of LYPJ under HT during PI.HT during PI significantly decreased the distribution of N and P to grains and increased N and P distribution in leaves and stems.Increasing both basal and tillering fertilizer rate and panicle nitrogen fertilizer rate promoted the distribution of N and P to grains in LYPJ,however,decreased the distribution in SY63 when compared with those under medium nitrogen application rate for the basal and tillering.Therefore,the results indicate that varieties with different tolerance to HT show different responses of N and P absoroption and distribution among various parts of plants.