Seasonal Sensitivity In Light Response Of Diurnal Photochemical Efficiency For Tow Desert Shrub Species

Plants in arid and semi-arid areas are sensitive to global climate changes. Desert plants are exposed to high radiation, extreme temperature, water deficit, and other climatic anomalies. Chlorophyll fluorescence has been widely accepted as an indicator of plant stresses due to its highly sensitivity to environmental stresses. The light energy utilization of plant mainly controlled by photosynthetic active radiation. Analysis of seasonal sensitivity in light response of diurnal photochemical efficiency for desert shrub species can provide important information related to physiological condition or photoacclimation status. The program aims to continuously monitor Photosystem ? (PS?) chlorophyll fluorescence of Artemisia ordosica and Hedysarum mongolicum in situ. We studied the variation of photochemical efficiency of PS?, characteristics of light response of diurnal photochemical efficiency and the relationships with environmental factors.(1)The two desert shrub species have strong acclimation abilities to environmental fluctuations. Environmental fluctuations caused the change of the activity of PSII reaction center, but did not result in damage of PS?. The maximal quantum yield of PS? photochemistry (Fv/Fm) of Artemisia ordosica ranged from 0.68 to 0.83, with monthly means of Fv/Fm being all above 0.75 from June to September; Fv/Fm of Hedysarum mongolicum ranged from 0.60 to 0.83, with monthly means of Fv/Fm being all 0.77 from August to September.(2)The photosynthetic active radiation (PAR) affected the diurnal changes in effective quantum yield of PSII photochemistry (?PS?) for two desert shrub species. OPS? decreased with increasing PAR. The proportion of light energy absorption of the photochemical reaction and light energy utilization were reduced with the increase of photosynthetic active radiation for Artemisia ordosica and Hedysarum mongolicum.(3)The key factors affecting Fv/Fm,?PS?, the slopes and y-intercepts of diurnal Opsii-PAR regression for two shrub species varied under different soil water conditions. During water stress period, Fv/Fm, ?PS?, the slopes and y-intercepts of diurnal Opsii-PAR regression showed strong correlation with PAR and soil water content. Under higher soil water condition, PAR, air temperature, relative humidity (RH) and vapor pressure deficit all affecting the slopes of diurnal Opsii-PAR regression. PAR is the only vital factor affecting Fv/Fm and the y-intercepts of diurnal ?PS?-PAR regression for Artemisia ordosica. PAR and RH are two vital factors affecting Fv/Fm and the y-intercepts of diurnal ?PS?-PAR regression for Hedysarum mongolicum.(4)The variation trend of the y-intercepts of diurnal ?PS?-PAR regression were similar to Fv/Fm. The y-intercepts of diurnal ?PS?-PAR regression were linearly related to Fv/Fm, increasing with increasing Fv/Fm.We conclude that:Artemisia ordosica and Hedysarum mongolicum avoid the photoinhibition and adapt environmental fluctuations by adjusting the use of light energy absorption, which may explain their dominance in Mu Us. The characteristics of light response of diurnal photochemical efficiency and the dominant environmental factors exist difference under different water conditions. So, when choosing sand-fixation plants should be considering the plant physiological characteristics and habitat conditions. The y-intercept of diurnal effective quantum yield of PSII photochemistry versus photosynthetically active radiation can be a proxy for maximal quantum yield of PSII photochemistry as a sensitive indicator of plant photosynthetic performance. It may solve the measuring difficulty of maximal quantum yield of PSII photochemistry in long-term monitoring. The discovery provides an important basis for plant growth condition in early diagnosis by monitoring chlorophyll fluorescence in situ.