| Xinjiang,located in the hinterland of Asia-European continent,becomes an important arid and semi arid area producing top quality cotton.In recent years,along with the changes in the system of planting and irrigating and powerful development of new technology in agriculture,the yield of cotton has increased significantly.Because of unique geographic positions and climatic environment,however,the growth and development of cotton has been disrupted due to long or short-term chilling stress,and ultimately the yield and fiber quality of cotton has also been affected.This study mainly focused on the effects of low temperature on the mechanism of photosynthesis,including energy distribution in PSⅡ,the redox state of PSⅠ,photosynthetic electron and proton transport and photodamage defence mechanism,especially cyclic electron flow around PS Ⅰ,in cotton during the yield-forming stage(i.e.mid-July to late-August in Xinjiang).The C hl fluorescence and P700+ absorption signal were simultaneously measured by PAM-100 to clarify the relation of physiological change in cotton leaves under low temperature during boll-forming stage.There are three parts in this paper: Part one,the effects of low temperat ure on PSⅠand PSⅡphotoinhibition in cotton leaves at boll stage;Part two,The effects of light and low temperature stress on the electron and proton transport of photosynthetical appartus in cotton leaves at boll stage;Part three,the effects of high root zone te mperature on photosynthesis and energy distribution of photosynthetic apparatus in cotton under low night temperature at boll stage.Part one: The results showed that the light-adapted maximum quantum yield of PSⅡ(Fv′/Fm′),photochemical quenching cofficient(qP),effective quantum yield of PSⅡ [Y(Ⅱ)] decreased significantly under low temperature stress.Low temperature significantly increased non-photochemical quantum yield of PSⅠ caused by donor side limitation [Y(ND)],the yield of regulated energy dissipation [Y(NPQ)] and non-regulated energy dissipation of PSⅡ [Y(NO)] and ultimately led to reversible photoinhibition in cotton leaves.Compared with control,low temperature stress significantly decreased the acceptor s ide limitation of PSⅠ [Y(NA)] and increased donor side limitation of PSⅠ[Y(ND)],but the value of effective PSⅠ complex contents(Pm)was not significantly decreased under low temperature,suggesting that PSⅠ in cotton leaves is insensitive to low temperature compared with PSⅡ.The quantum yield of cyclic electron flow [Y(CEF)] and the ratio of Y(C EF)to the effective quantum yield of PSⅡ[Y(C EF)/Y(Ⅱ)] were stimulated under low temperature stress in cotton.These results suggested that stimulation of cyclic electron flow plays an important role in protecting PSⅠand PS Ⅱ from photoinhibition caused by chilling stress in cotton.Furthermore,the non-photochemical quenching(NPQ)and regulated heat dissipation [Y(NPQ)] had significant positive correlation with the quantum yield of cyclic electron flow [Y(CEF)],indicating that the strong excess excitation energy due to the overclosure of PSⅡreaction centers led to reversible photoinhibition of PSⅡunder low-temperature stress.In conclusion,we found that the strong stimulation of cyclic electron flow and regulated heat dissipation powerfully prevented PS Ⅱ and PS Ⅰ of cotton from photoinhibition and photodamage induced bylow temperature stress,and which may be the main mechanism of the non-susceptibility of PSⅠin cotton to the low temperature stress.Part two: The physiological response of cotton exposed to different low temperature with weak and moderate light stress,including PSⅡand PSⅠactivity of photosynthetic apparatus,electron and proton transport characteristics,was studied in phytotron.The results showed that the value of acceptor side limitation of PSⅠ[Y(NA)] induced by treatment of low temperature with low light was much higher than moderate at the same low temperature level.Interestingly,compare with control,the photoinhibition of PSⅠwasn’t induced by low temperature treatments with moderate and low light stress.Low temperature stress with low and moderate light stress significantly increased the quantum yield of cyclic ele ctron flow of cotton at boll stage,and at the same low temperature level,the quantum yield of cyclic electron flow [Y(CEF)] was higher in treatment of low temperature with moderate light than low light,suggesting that cyclic electron flow plays an impor tant role in preventing PSⅡand PSⅠfrom photoinhibition caused by low temperature with light in cotton at boll stage.In addition,low temperature with light stress significantly increased the transmembrane proton gradient((35)PH)and inhibited the generation of zeaxanthin(Z)but significantly decreased transmembrane potential((35)Y)and ultimately favor to maintain stable proton motive force(pmf)which efficiently guarantee the supply of energy consumed by carbon assimilation.Part three: High root zone temperature significantly increased the photosynthetic rate by alleviating stomatal closure induced by low night temperature.The inhibition of PSⅡactivity and linear electron flow and accumulation of excess excitation energy due to low night temperature were alleviated by increasing root zone temperature.PSⅠwas insensitive to low night temperature stress,and high root zone temperature increased t he acceptor side limitation of PSⅠ[Y(NA)] mainly related to the low donor side limitation of PSⅠ[Y(ND)].Non-photochemical energy dissipation played an important role in dissipating excess excitation energy caused by night chilling,by contrast,high root zone temperature signifiantly decreased the accumulation of excess excitation energy and the fraction of non-photochemical energy dissipation.Cyclic electron flow played an significant role in preventing the photodamage of PSⅡand PSⅠdue to the night chilling.However,high root zone temperature significantly decreased the quantum yield of cyclic electron flow,this is because the high activity of PS Ⅱ and PS Ⅰ were maintained by increasing root zone temperature under night chil ing stress. |
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