Physiological Acclimation Of PS? To Environmental Fluctuations For Artemisia Ordosica And Leymus Secalinus In Mu Us Sandland

The chlorophyll fluorescence(ChlF)parameters of photosystem ?(PS?)are often regarded as an important method of exploring plant acclimation to environmental stress,because of its high sensitivity to environmental fluctuations.Plants in arid and semi-arid area often suffer from high light intensity,high temperature,drought and other stressful environments,but the mechanisms of plant photosynthetic acclimation to fluctuated environment remain incomplete.In this study,we continuously examined the ChlF parameters in two different sand-fixing speices Artemisia ordosica(shrub)and Levmus secalinus(herb)and environments factors in the south edge of Mu Us desert during 2015-2016.The research aim to explore the physiological acclimation mechanisms and differences between two species to the stressful environment The main findings are as follows:(1)ChlF parameters of A.ordosica and L.secalinus have obvious daily and seasonal variations.The diurnal variation of the actual photochemical quantum yield(?PS?)is approximately 'U' type,and reach the daily minimum at noon.In opposite,non-photochemical quenching(NPQ)peaked the daily maximum at noon.The thermal dissipation at night was observed for both two species.The vaule of NPQ ranged from 0 to 0.5.Maximum photochemical quantum yield(Fv/Fm)?PS? and NPQ were high in spring and autumn and low in summer.Some Fv/Fm below the healthy line(0.73)in spring and summer.(2)First,mechanisms of short-term acclimation in photo active radiation(PAR)for plants were same.But the sensitivity of mechanism was different,With increasing PAR,A.ordosica closed PS?reaction center rapidly with higher slope of diurnal ?PS?-PAR regression,and L.secalinus had a larger thermal dissipation rate with higher y-intercepts of diurnal NPQ-PAR regression.Second,there was no differences in mechanisms of long-term acclimation in PAR for plants.When PAR was increasing,two plants avoided photoinhibition by increasing thermal dissipation.While A.ordosica could maintain higher PSII potential acitivity and light energy transformation efficiency(higher Fv/Fm and ?PS?)simultaneously.L.secalinus suffered photoinhibiton and photodamage under higher PAR,with decreasing Fv/Fm and NPQ.(3)First,mechanisms of short-term acclimation in air temperature(Ta)for plants were same.The two plants developed time regulation mechanism maintaining a higher utilization of light energy under higher Ta,in order to protect PSII from photoinhibition or photodamage.The result showed that maximum and minumum value of ?PS? and NPQ had a time lag 2-3h at short-term scale.When Ta reached its maximum,the ?PS? values of two species were higher.Second,there was significantly differences acclimation mechanisms in two plants response to Ta at long-term scale.With increasing Ta,A.ordosica avoided photoinhibition by higher thermal dissipation and light energy conversion efficiency,and then supplied stable photosynthetic energy to dark reaction.While Ta>21 ?,L.secalinus protected PSII by reduced thermal dissipation and light energy conversion efficiency.The Fv/Fm and ?PS? of L.chinensis decreased rapidly with increasing Ta,indicating bad thermal stability.Fv/Fm values of A.ordosica are relatively stable.The A.ordosica had better resistance to higher Ta than L.secalinus.(4)First,mechanisms of short-term acclimation in soil water content(VWC)for plants were same.,but the tolerant of PSII in L.chinensis was bad under drought.They avoided photoinhibition by increasing thermal dissipation in all stage,but the non-regulated energy dissipation(?NO)was higher for L.chinensis.In rainfall and recovery stage,the ?NO were significantly lower than that of the drought stage for both two species.In drought stage,Levmus chinensis suffered drought stress(Fv/Fm was lower than 0.73,and ?NO increased in the early morning and decreased at noon).Passive thermal dissipation was the main mechanism dissipating excess light energy in the dry stage.Second,there was significantly differences acclimation mechanisms in two plants response to VWC at long-term scale.A.ordosica avoided photoinhibition by higher thermal dissipation and light energy conversion efficiency.While L.secalinus responsed to changed VWC by reduced thermal dissipation and light energy conversion efficiency,resulted by closing PS? reaction.With variation of VWC in summer,L.chinensis had obvious water stress range.When VWC<0.08 or>0.14 m3·m-3,photosystem ? of L.secalinus was experiencing water stress,which showed that Fv/Fm and NPQ decreased(Fv/Fm below 0.73).In summary,A.ordosica and L.secalinus had different acclimation to light intensity,temperature and soil water content at short-and long-term scales.And Artemisia ordosica was adjusted by internal physiological time and variable trade-off between light acclimation and light protection mechanism when dealing with fluctuating environment.The thermal dissipation mechanism of L.secalinus was sensitive to changes in temperature and soil water content,and photosystem ? was more susceptible to photoinhibition or even photodamage.From the perspective of physiological acclimation mechanism,we inferred that L.secalinus is more susceptible to environmental stress under climate change in future.