Response Of Photosynthesis To Extreme High Temperature In Rice Genotypes With Contrasting Heat Tolerance

In recent years,with the development of global warming,the heat damage caused by high temperature has become a major threat to rice production.Photosynthesis is the primary determinant of rice yield,and the efficiency by which rice captures light and converts it into biomass over the growing season is a key determinant of final yield.Photosynthesis is one of the important physiological processes that are most sensitive to heat stress.Globally,many studies are attempting to improve the photosynthetic adaptation to heat stress through various bioengineering techniques,for achieving high and stable rice yield under climate changes.In this study,the objectives are to identify rice varieties with heat tolerance from the existing rice varieties,and illustrate the heattolerant mechanisms from photosynthesis and metabolism.These would have important implications for the utilization of heat-resistant planting resources,efficient selection of evaluation indicators related to high temperature adaptability,and mining of heat-resistant candidate genes and heat-resistant breeding in rice production.The current study was conducted in an artificial climate chamber equipped with natural supplementary light at Agricultural College,Yangtze University in 2021.Threegradient temperature treatments were set as 38°C,40°C,42°C respectively.Gas exchange characteristics(Pn,Gs,Ci,Tr),chlorophyll fluorescence characteristics(Fv/Fm,Y(II),ETR,q P,NPQ),key photosynthetic enzymes(Rubisco,RCA)characteristics and photosynthetic pigment content(Chl,Car)content(Chl,Car)were measured to illustrate physiological characteristics of typical resistance/sensitivity varieties.In addition to these,dry matter production of heat-resistance/sensitivity varieties under long-term(30d)high temperature was also measured.To verify whether the difference in the reaction of photosynthesis to high temperature of the cultivar will eventually be reflected in the results of its substance production.The main results are as follows.1.Under extreme high temperatures,photosynthesis decreased,and the resistant varieties adapt to high temperatures longer and the stress response rate is more sluggish than that of the sensitivity varieties.After 42°C treatment for one day,the net photosynthetic rate and stomatal conductance of N22 and SDWG005 were significantly higher(p<0.05)than Mianhui101 11.1%,17.9%,40.7% and 33.8%.The intercellular carbon dioxide concentration(Ci)and transpiration rate(Tr)were significantly higher than those of Mianhui 101 9.1%,5.1%,10.7% and 12.0%,respectively.2.Under extreme high temperature,the damage degree of photosystem II(PSII)reaction center of resistant varieties is less than that of sensitivity varieties.After 42°C treatment for 1 day,the maximum photochemical quantum yield(Fv/Fm),actual photochemical quantum yield(YII),and electron transfer rate(ETR)of PSII of the resistant varieties N22 and SDWG005 were significantly higher than the sensitivity varieties Mianhui 101.3.Under extreme high temperatures,the activities of Rubisco and RCA in rice leaves were significantly reduced.After 42 °C treatment for 5 days,the Rubisco enzyme activity of the resistant cultivar N22 and SDWG005 was 48.9% and 32.3% higher than that of the sensitivity variety Mianhui 101,and the RCA enzyme activity was 28.0% and 34.5%higher than that of Mianhui 101,respectively,and the difference reached a significant level(p<0.05).4.Under extreme high temperatures,the photosynthetic pigment content is reduced,and the photosynthetic pigment content of the resistant varieties is higher than that of the sensitivity varieties.After 5 days of treatment at 42 °C,the total chlorophyll content of the resistant cultivar N22 and SDWG005 was significantly higher than that of the sensitivity varieties Mianhui 101 16.0% and 21.2%,and the carotenoid content was significantly higher than that of the sensitivity varieties 101 8.2% and 15.2%.5.After 30 days of high temperature treatment,the dry matter accumulation of rice was significantly reduced at 38 °C compared with the control(31 °C)(p<0.05).Resistant varieties have a higher accumulation of dry matter than sensitivity varieties.Under the high temperature treatment of 42°C,the dry matter accumulation of N22 and SDWG005 was significantly higher than that of the i sensitivity varieties Mianhui 101 53.5% and 27%(p<0.05),respectively.In summary,the resistant varieties N22 and SDWG005 can maintain a stable photosynthetic rate and performance than that of sensitive varieties at extreme high temperatures,and thus obtain higher material accumulation outcome.From the perspective of photosynthesis,it can be considered that the physiological reason for the heat tolerance of rice is that at high temperatures,the resistant varieties have higher stomatal conductivity(Gs),photosynthetic activity(q P)and heat dissipation capacity(NPQ)as well as the activity and chlorophyll/carotenoid content of photosynthetic key enzymes(Rubisco,RCA enzyme),which can effectively alleviate the photoinhibition generated by high temperature and reduce the thermal damage of the photosynthetic system.