Photosynthetic Electron Transport Activity Of Three Representative Plants

To clarify the function of PSⅡ and PSⅠ in Hedera nepalensis,Zostera japonica,and Sargassum thunbergii,chlorophyll fluorescence of were examined in response to varying environmental stresses.The main results showed as following:1.Photosynthetic electron transport activity of Hedera nepalensisTo examine the coordination between photosystem Ⅱ（PSⅡ）and photosystem I（PSⅠ）in response to varying environmental conditions,both diurnal fluctuations and seasonal variability of photosynthetic electron transport activity in ivy（Hedera nepalensis,Araliaceae）were investigated: by measuring prompt fluorescence,delayed fluorescence（DF）and modulated reflection of 820 nm light（MR）.During diurnal fluctuations,the PSⅡ electron donor side was damaged,as evidenced by decreases of the fast amplitude of DF decay kinetics at I1,although there was no significant change in variable fluorescence at K-step relative to amplitude of FJ-Fo.Decreases in the maximum photochemical efficiency（i.e.,PSⅡ photoinactivation）were accompanied by an increased maximum decrease in the slope of MR/MRo（i.e.,PSⅠ photoactivation）.Subsequently,PSⅡ recovery and PSⅠ relaxation occurred in the afternoon.Throughout the season,alternations between PSⅡ and PSⅠ were also suggested by the down-regulation of PSⅡ and the up-regulation of PSⅠ from summer to winter.Significant negative linear correlations between the activity of PSⅡ and PSⅠ across both diurnal fluctuations and seasonal variability were verified by correlation analyses.In addition,PSⅠ was active throughout the year,suggesting PSⅠ is independent from high temperatures.High PSⅠ activity may maintain the functional integrity of the photosynthetic apparatus in overwintering ivy.The alternation between PSⅡ and PSⅠ activity may regulate the distribution of excitation energy between the two photosystems and balance the redox state of the electron transport change,thereby enabling ivy to respond to varying environmental conditions.2.Photosynthetic electron transport activity of Zostera japonicaSeawater warming is emerging as a result of increasing global temperature.In this study,Z.japonica（Zostera japonica Ascherson & Graebner）collected from the intertidal zones of Changdao（37°N,120°E）were used to investigate the responses of photosynthetic electron transport to thermal exposure during April 2016.As seawater temperature rose from 20 to 32°C,the increase of the relative variable fluorescence at the K-step to the amplitude of FJ-Fo（Wk）and the maximum photochemical efficiency（F_v/F_m）indicated an inhibition in the oxygen-evolving complex（OEC）and photosystem Ⅱ（PSⅡ）reaction center,respectively.When exposed to 32°C for 4 hr,both OEC and PSⅡ reaction center suffered an irreversible damage,confirming that 32°C is the upper critical seawater temperature for Z.japonica photosynthetic electron transport.PⅠ_ABS,as a thermal inhibition indicator,exhibited a time-dependent linear decrease,an irreversible damage of PSⅡ reaction center occurred once the value of PⅠ_ABS was less than 10.60.Based on results of the photosynthetic performance,thermal response strategies were summarized as:（1）an enhancement in the efficiency of the active PSⅡ reaction centers;（2）an increase in the activity of the PSⅡ electron acceptor side;（3）enhancement in the activities of both PSⅠ and the cyclic electron transport around PSⅠ;and（4）the alternation between PSⅡ and PSⅠ.Such adaptive strategies may balance the redox state of electron transport and regulate the distribution of excitation energy between the two photosystems,and thereby protecting Z.japonica from ocean warming.3.Photosynthetic electron transport activity of Sargassum thunbergiiThe present study evaluates the seasonal photosynthetic performances of Sargassum thunbergii via chlorophyll fluorescence technique.During summer and early winter,no significant change was observed in photosynthetic capacity(rETR_max),maximum photochemical efficiency（F_v/F_m）,and performance index(PⅠ_ABS),indicating sound performance of photosystem Ⅱ（PSⅡ）.During late winter and early spring,the minimum saturating irradiance（Ek）,rETR_max,F_v/F_m,and PⅠ_ABS decreased significantly,implying that S.thunbergii was vulnerable to light stress and that its PSⅡ suffered severe photoinhibition.Subsequently,PSⅡ gradually recovered during late spring and summer,as evidenced by an increase of both parameters.The down-regulation of PSⅡ was always consistent with an inhibition of the PSⅡ acceptor side,which was similar to the so-called“energy crisis” induced by low temperatures as well as light stress in terrestrial evergreen plants.Throughout the year,both the maximum decrease in the slope of MR/MR0 and the initial rate of P700+ re-reduction maintained low values.This indicates that photosystem I（PSⅠ）and cyclic electron transport（CET）were inactive;nevertheless,a seasonal down-regulation during late winter and early spring could be detected.The weak performance of PSⅠ and CET can potentially limit the flexibility in response to winter stress and result in a delayed recovery of PSⅡ.In conclusion,the seasonal variability of S.thunbergii photosynthetic activity was characterized by three periods: active state,down-regulation and restoration.The rapid growth during early spring was accompanied by weak photosynthetic performance,indicating that the carbohydrates consumed duringthis period were derived from previously stored starch.