Studies On The Stability Of Photosynthetic Apparatus And Its Photoprotection Mechanisms Of Cotton (Gossypium Spp.) Under Water Deficit

The processes of plant growth and development are offen accompanied by biotic and abiotic stresses,and high irradiance,drought,salinity,high temperature are the most limiting environmental factors in yield formation.Ⅰn view of the fast change in global climate,drought is likely to become more serious in the future.The strategies of tolerance and adaption will be of major importance for plants growing under drought conditions.Ⅰn this paper we regarded the photosynthetic electron transport chain as the core,dissipating excess excitation energy as the main line,combining the upstream leaf movement which regulates the abosorption of solar radiation and the downstream antioxidant system which controls the levels of reative oxygen species to research the effect of soil water deficit on the activity of photosystem Ⅱ,the distribution of photosynthetic electron flow and as well as carbon assimilation.The results determined the relationship between the aborption and distribution of light energy.Further,the physiological mechanisms of the stability of the photosynthetic apparatus and its photoprotection were explored.Moreover,the relationships among different photoprotective mechanisms were also discussed.This research may help to（i）increase understanding about drought resistance of cotton,（ii）provide new ideas for water-saving irrigation of cotton,and（iii）assist with the breeding of drought resistant cotton varieties.The main results of this research are listed below.1.Effect of water deficit on the distribution of photosynthetic electron flow and the corresponding physiological responses were studied.With increasing water deficit,net photosynthetic rate significantly decreased.The total electron flux through photosystem Ⅱ gradually decreased and the fraction of electron flux required to sustain CO₂ assimilation markedly declined.Simultaneously,the ratio of quantum efficiency of photosystem Ⅱ to the quantum efficiency of CO₂ fixation increased,accompanied by an increase in the alternative electron flux,indicating the electrons had been transported to O2 in the Mehler-peroxide reaction and/or used for nitrate reduction.Ⅰn addition,mild water deficit increased the proportion of electron flux for the photorespiratory carbon oxidation.Water deficit significantly increased surperoxide radical production rate and hydrogen peroxide content,and the activities of superoxide dismutase,ascorbate peroxidase,peroxidase,catalase and nitrate reductase and glutamine synthetase in cotton leaves also increased under water deficit.Therefore,the Mehler-peroxidation reaction,photorespiration and nitrate reduction helped to dissipated excess light energy,being important photoprotective mechanisms for adapting the photosynthetic apparatus to water deficit in cotton.2.The diurnal time course of dissipating the excess energy of photosynthetic apparatus in water deficit cotton leasves was studied.The results showed that leaf diaheliotropic movement was pronounced in all treatments,water deficit significantly increased leaf temperature.Compared with the control,moderate water deficit obviously decreased the photochemical efficiency of PSⅡ,but the maximum photochemical efficiency of PSⅡ showed no differences among three treatments.It means that moderate drought stress induced down-regulation of photosynthetic apparatus but not caused chronic photoinhibition of PSⅡ.The diurnal time course of quantum yield of △p H-and xanthophyll-regulated thermal energy dissipation showed a maximum between 14:00 and 16:00 hours,and the value was greatest in the moderate drought stressed plants.The diurnal time courses of the distribution of photosynthetic electron flow showed that the drought-induced decrease in the proportion of electron flux for photosynthetic carbon reduction was mostly compensated by the electron flux for the photorespiratory carbon oxidation and the alternative electron flux,indicating that cotton uses electron transport flux under mild drought whereas electron transport flux and regulated non-photochemical energy dissipation under moderate drought for excess light energy dissipation Therefore,we concluded that active leaf diaheliotropic movement works well under drought can optimize the incident light available for photosynthetic apparatus and photosynthetic electron transport flux is insensitive to drought because of stronger alternative electron sinks in cotton.3.The adaptation mechanisms of upland cotton and pima cotton to water deficit were compared.Water deficit decreased photosynthesis in both cotton species,but did not decrease chlorophyll content,nor did it induce any chronic photoinhibition in either cotton species.Water deficit increased ETR/4-AG.The increase in ETR/4-AG,which represents an increase in photorespiration and alternative electron fluxes,was particularly pronounced in Xinluzao 45.In Xinluzao 45,water deficit increased the activities of antioxidative enzymes,as well as the contents of reactive oxygen species（ROS）.By contrast,moderate water deficit particularly increased non-photochemical quenching（NPQ）in Xinhai 21.Our results suggest that Xinluzao 45 appeared to rely on enhanced electron transport such as photorespiration and the Mehler reaction to dissipate excess light energy under mild and moderate water deficit.Xinhai 21 used enhanced photorespiration for light energy utilization under mild water deficit but,when subjected to moderate water deficit,possessed a high capacity for dissipating excess light energy via heat dissipation.4.The role of cyclic electron transport in the photoprotective mechanism of cotton leaves under water deficit was studied.Compared with the control,the quantum efficiency of PSⅡ decreased,the fraction of energy dissipated in from of heat via the regulated non-photochemical quenching mechanism obviously increased and the fraction of energy that is passively dissipated in form of heat and fluorescence slightly increased.The quantum efficiency of PSⅠ slightly increased under high light,the fraction of over P700 that is oxidized in a given state of photosystem Ⅰ maintained at a high leval.There was no difference in the energy distribution of PSⅡ and the redox state of P700 in cotton leaves between control and mild water deficit.In addition,the ratio of electron flow through PSⅡ and PSⅠ was significantly higher in mild water deficit leaves than that in control,indicating that cyclic electron transport was excited.With increasing water deficit,the activity of ATPase and the formation of zeaxanthin decreased gradually,but water deficit induced the formation of proton gradient across the thylakoid mechanisms（△pH）.The results suggest that the stimulation of cyclic electron transport（CEF）is essential for protecting PSⅠ against photodamage through alleviating the over-reduction of PSⅠ accepter side under moderate water deficit.Furthermore,CEF is essential for protecting PSⅡ against excess excitation pressure due to CEF ependent build-up of a △pH helps the activation of NPQ.Therefore,the activation of CEF is important for protecting the photosystems of cotton leaves from photoinhibition under moderate water deficit.5.The effects of water deficit and rewatering on the recovery of photosynthesis of cotton leaves were studied.The results show that water deficit led to a reversible reduction in net CO₂ assimilation rate,stomatal conductance,intercellular CO₂ concentration.The activities of photosystem Ⅱ（PSⅡ）and photosystem Ⅰ（PSⅠ）were relatively stable during water deficit and recovery.Water deficit caused an enhanced production of reactive oxygen species（ROS）and increased lipid peroxidation.Proline accumulation and the antioxidative enzymes along with the antioxidant ascorbate（AsA）increased during water deficit.On re-watering,the ROS generation rate,antioxidative enzymes activities and the extent of the lipid peroxidation returned to near control values.Overall,rapid recovery of photosynthetic rate is related to the stability of the photosystems,and the stable of the photosystems depend on strong regulation of ROS metabolism and osmotic adjustment.