Study On The Regulation Of Photosynthetic Characteristics In Cotton Leaves Under Fluctuating Light

Light is the raw material for photosynthesis.In the natural environment,leaves of plant are always experience more fluctuating light（FL）levels due to the gradual changes of the sun-altitude angle and clouds during a day.Although fluctuating light usually lasts only a few seconds to a few minutes,but it can contribute 20-80%of the total solar energy received by the leaves,which can be effectively used by the leaves for growth.In order to adapt to this light condition,plants usually need to adjust their leaf structure and function.Therefore,the changes of plant growth and photosynthetic performance need to be further studied under fluctuating light.The development of cotton is inseparable from light,but there is no research on cotton photosynthesis under fluctuating light.Aiming at the changes of photosynthetic physiological mechanism of cotton under fluctuating light,this paper studied the changes of leaf structure and physiological characteristics of cotton under fluctuating light,and revealed the adaptive mechanism of photosynthetic characteristics of cotton under fluctuating light.On this basis,the photochemical activity and photoprotection mechanism of the photosynthetic mechanism of cotton under fluctuating light were studied,and the ability of the influence of fluctuating light on the light absorption,photochemical activity and heat dissipation of cotton was preliminarily clarified,and the reason for the limitation of heat dissipation under fluctuating light was further explained.In addition,the changes of stomatal conductance in response to fluctuating light were found through monitoring and light induction of changes in cotton gas exchange parameters under fluctuating light,and the limiting factors of cotton photosynthetic rate and water use efficiency under fluctuating light were discussed.The results will enrich the light energy utilization theory of cotton and lay a theoretical foundation for further improving the light energy utilization and water use efficiency of cotton.The main research contents and results are as follows:1)The regulation mechanism of cotton photosynthetic capacity under fluctuating light was studied,and the changes of leaf gas exchange parameters,leaf mass per area,leaf thickness,leaf area and biomass were measured.The results showed that compared with the plants growing under constant light,the cotton growing under fluctuating light was heavier than the leaves,and its leaf area and biomass were significantly reduced.Under constant light treatment,the net CO₂ assimilation rate per unit blade area,saturated net CO₂assimilation rate and maximum CO₂ assimilation rate were significantly higher than those under fluctuating light conditions,but there was no significant difference in the net CO₂ assimilation rate per unit blade mass under different light treatment.In addition,the lower maximum electron transfer rate estimated from the CO₂ response curve indicated that plants growing under fluctuating light were more restricted by RuBP regeneration,while plants under constant light showed higher intercellular CO₂ concentration and higher apparent mesophyll conductance.This study showed that fluctuating light inhibited the photosynthetic capacity of cotton,leading to its photosynthetic material production capacity decreased,and its leaf ratio was reduced to control its photosynthetic capacity under fluctuating light.2)The photochemical activity and photoprotection mechanism of cotton photosynthetic apparatus under fluctuating light were studied.The results showed that the content of chlorophyll in cotton leaves increased under constant light,while the content of chlorophyll a and chlorophyll b decreased under fluctuating light,and the chlorophyll a/b ratio increased.The actual photochemical efficiency and photosynthetic electron transfer rate of photosynthetic apparatus of plants growing in fluctuating light are lower,and the proportion of photosynthetic apparatus photosystem II reaction center opening is also lower.With the increase of fluctuating light level,the content of carotenoids in cotton decreased gradually and the content of zeaxanthin increased gradually,indicating that the cycle of lutein was not restricted,but the circular electron flow did not promote the formation of transmembrane proton gradient.Therefore,the present study showed that fluctuating light reduced the light absorption,photochemical activity of photosynthetic apparatus and the ability of light protection of cotton.3)The regulation mechanism of unsteady fluctuation of light on cotton water use efficiency was studied.By monitoring the change of cotton gas exchange parameters under fluctuating light,the study showed that the photosynthesis of leaves under fluctuating light could respond to the change of light intensity instantaneously,which was consistent with the trend of light intensity change,but the response of stomatal conductance to the change of light intensity was always delayed,and stomatal conductance presented a highly fluctuating state under high light.In this study,it was found that during the high-light induction process,there was almost no difference between the WUE of cotton under fluctuating light treatment and that under constant light treatment,indicating that the stomatal conductance and transpiration rate of cotton under fluctuating light were reduced,thus limiting the net CO₂ assimilation rate and making the internal WUE of the plant unchanged.Stomatal conductors control plant transpiration,which may further indicate that cotton maintains normal growth mainly by changing stomatal characteristics under fluctuating light.