Effects Of High Temperature During The Reproductive Stages On Rice Yield Formation And Its Phytohormonal Basis

Rice plants are highly susceptible to high temperature stress,especially during reproductive stage,during which high temperature may severely reduce grain yield.High temperature during the early reproductive stage(panicle initiation to booting)poses adverse effects on panicle growth and panicle traits,and induced reduction in yield attributes.Phytohormones respond to high temperature and regulate heat tolerance in plants.However,high temperature effects on rice production and the mechanisms of high temperature tolerance during the early reproductive stage are still unknown.High temperature during the middle to late reproductive stage affects anther dehiscence,pollen shedding and pollen fertility,and leads to spikelet sterility.However,previous studies of high temperature effects during flowering were mainly focused on heat-response of male organs,only few studies paid attention to the exsertion of stigma and its possible roles on pollination.In this study,two hybrids(Shanyou 63,Liangyoupeijiu)and three inbreds(Nagina22,Huanghuazhan,IR64)were cultivated and subjected to high temperature treatments during panicle initiation(young panicle is nearly 1-2mm)and flowering.The objective of this study is to study the response in phytohormones to high temperature treatments and its role on yield formation.We explored the physiological mechanisms underlie high temperature tolerance.We also evaluated the effects of stigma exsertion on pollination and spikelet fertility.In addition,a measure for auxiliary pollination was developed to mitigate the adverse effects on spikelet fertility on rice under high temperature treatments during flowering.(1)High temperature treatments during panicle initiation reduced grain yield.High temperature treatments(high nighttime temperature,high daytime temperature,and high temperature during the whole day)reduced rice yield of Nagina22,Huanghuazhan,Liangyoupeijiu,and Shanyou63 by 61%,57%,59% and 6%.Yield reductions in Nagina22,Huanghuazhan,and Liangyoupeijiu were attributed to reductions in spikelet fertility,spikelets per panicle,and grain weight.High temperature treatments significantly reduced active cytokinins,ghibelline A1(GA1),and indole-3-acetic acid(IAA),but increased abscisic acid(ABA)and bound cytokinins in young panicles.Heat-tolerant Shanyou 63 displayed minor changes in phytohormones,panicle formation,and grain yield under high temperature compared with those of the other three varieties.Exogenous BA application increase grain yield,grain weight,spikelet fertility,and spikelets per panicle of Liangyoupeijiu by 220%,7.8%,116%,10.4% compared with that without exogenous BA under high temperature during the whole day.Correlation analyses revealed that high temperature-induced cytokinin changes may regulate yield components by modulating the differentiation and degradation of branches and spikelets,panicle exsertion,pollen vigor,anther dehiscence,and grain size.These results suggest that phytohormone changes are closely associated with yield formation,and a small reduction or stability in phytohormone content is required to avoid large yield losses under heat stress.(2)High temperature treatments(high nighttime temperature,high daytime temperature,and high temperature during the whole day)reduced spikelets per panicle.The spikelets per panicle of heat susceptible Nagina22,Huanghuazhan,and Liangyoupeijiu were reduced by 7.4%(0-26%),27.3%(19.9-32.3%),24.0%(15.6-32.0%)across three high temperature treatments.The treatments significantly reduced panicle CTK abundance in heat-susceptible LYPJ,HHZ,and N22 varieties,which showed fewer spikelets per panicle in comparison with control plants.Exogenous 6-Benzylaminopurine application mitigated the effect of heat injury on the number of spikelets per panicle.The treatments significantly decreased the xylem sap flow rate and CTK transportation rate,but enhanced cytokinin oxidase/dehydrogenase(CKX)activity in heat-susceptible varieties.In comparison with the heat-susceptible varieties,heat-tolerant variety SY63 showed less reduction in panicle CTK abundance,an enhanced xylem sap flow rate,an improved CTK transport rate,and stable CKX activity under the high temperature treatments.Enzymes involved in CTK synthesis(isopentenyltransferase,LONELY GUY,and cytochrome P450 monooxygenase)were inhibited by the treatments.Heat-induced changes in acropetal CTK transportation through xylem sap flow and panicle CTK degradation via CKX were closely associated with the effects of heat on panicle CTK abundance and panicle size.Heat-tolerant variety SY63 showed stable panicle size under the high temperature treatments because of enhanced transport of root-derived CTKs and stable panicle CKX activity.Our results provide insight into rice heat tolerance that will facilitate the development of rice varieties with tolerance to high temperature.(3)Under high daytime temperature,spikelet fertility and stigma exsertion and their response to high temperature treatment showed different,depending on cultivars.The stigmas were completely hid inside the spikelet after flowering in heat-tolerant Nagina22,in which spikelets per panicle were not affected by high temperature treatments.Contrarily,the percentage of stigma exsertion was the highest in heat-susceptible Liangyoupeijiu,in which spikelets per panicle were reduced significantly by high temperature treatments.Summarily,high temperature tolerant varities were not coupled with high percentage of stigma exsertion,and vise versa.Cultivar variations showed difference in stigma exsertion responed to exogenous GA3 application.The percentages of stigma exsertion in Liangyoupeijiu and Huanghuazhan were increased significantly by application of exogenous GA3,but not in Nagina22,IR64,and Shanyou63,in which stigma exsertion were promoted slightly by exogenous GA3.Under control temperature,exogenous GA3 treatment increased percentage of stigma exsertion in Liangyoupeijiu and Huanghuazhan by 9.3% and 16.4%,respectively,and spikelet fertility with exserted stigma in Liangyoupejiu and Huanghuazhan were increased by 5.8% and 2.3%.Under high temperature,percentage of stigma exsertion in Liangyoupeijiu and Huanghuazhan were increased by 10.7% and 22.0% after application of exogenous GA3,however,the spikelet fertility with exserted stigma of Liangyoupejiu and Huanghuazhan were reduced by 4.4% and 3.7%,respectively.It suggests that stigma exsertion is not considered as a criterion for high pollination and high spikelet fertility under high temperature in rice.High temperature treatment delayed anther dehiscence and led fewer pollen grains deposited on stigmas.In addition,the vigor of the exserted stigmas was decreased due to exposure to high temperature.The two observations may explain that the reason for the non-relation between stigma exsertion and spikelet fertility under high temperature.(4)High temperature posed adverse effects on pollination,induced spikelets sterility,and further resulted in huge yield loss.Measures of auxiliary pollination without or with chemical application increased spikelet fertility.Under control temperature(27.6?),auxiliary pollination combined with chemical application increased spikelet fertility in Liangyoupeijiu and Shanyou63 by 4.9% and 3.7%,respectively.Under high daytime temperature treatment,auxiliary pollination increased spikelet fertility more than under control temperature,e.g.,auxiliary pollination without chemical application increased spikelet fertility in Liangyoupeijiu and Shanyou63 by 5.4%,16.6%,61.1% and 2.4%,7.1%,12.0% when suffering high temperature of 28.6?,29.2?,31.5?,respectively,and auxiliary pollination with chemical application increased spikelet fertility in Liangyoupeijiu and Shanyou63 by 15.8%,33.8%,133.0% and 6.7%,10.7%,28.6% when suffering high temperature of 28.6?,29.2?,31.5?,respectively.Auxiliary pollination measure may relief the adverse effects of high temperature on spikelet fertility,and has the advantage for its real-time and easy for operating.