Regulation Of Non-photochemical Quenching In Photosynthetica Acclimation Of Two Xerophytic Shrubs To Fluctuating Environments

Desert ecosystem is one of the most sensitive terrestrial ecosystems to climate change. Desert vegetation are exposed to high radiation, extreme temperature, water deficit, and other climatic anomalies. However, the responses and acclimation strategies of desert vegetation to environmental stresses remain unclear. The study aims to continuously monitor PSII Chlorophyll fluorescence (ChlF) of Salix psammophila and Artemisia ordosica in situ and environmental factors simultaneously, combining periodically manual measurements of ChlF. The objectives are:(1) to demonstrate changes of ChlF parameters diurnally, seasonally, and interannually, (2) to understand the controlling factors of ChlF parameters and their thresholds, (3) to examine the mechanism and strategies of responses and acclimation of two shrub species to environmental stresses. The results show as follows:(1) ChlF parameters varies in different time scales. The basic ChlF parameters Fm’ and ΦPSⅡ has a negative correlation with PAR diurnally, being higher during night and lower at noon, while ETR, NPQ and ΦNPQ have a positive correlation with PAR, being close to 0 during night. At seasonal scale, ΦPSⅡ and Fv/Fm of Artemisia ordosica have high values in summer days and low values in spring; those values of Salix psammophila rise to the highest in spring, followed by summer and autumn.(2) Two species have different adaptability to PAR fluctuations. Artemisia ordosica has a smaller sensitivity to high PAR than Salix psammophila, indicating that Artemisia ordosica has greater photoprotective ability to copy with high light intensity. When PAR is greater than 1200 μmol m-2 s-1, the photo-inhibition or photo-damage occurs in Salix psammophila as showed by increased ΦNO and downregulated ETR. Fv/Fm and Fv/F0 of Salix psammophila have a higher response sensitivity to the daily maximum PAR, PSII reaction centres are damaged seriously when PAR>1300μmol m-2 s-1 indicated by lowest Fv/Fm less than 0.4. Artemisia ordosica has an attenuate photo-protective ability under low light condition to environmental stresses. When PAR<300 μmol m-2 s-1, Artemisia ordosica is vulnerable to photo-damage, with a higher ΦNO and lower NPQ than Salix psammophila. Seasonal difference are obvious between the two species. ΦSPII of Artemisia ordosica is the lowest in autumn with highest NPQ and ΦNPQ, while Salix psammophila has a lowest ΦSPII in summer with high NPQ and ΦNPQ. It is concluded that Artemisia ordosica are more adapted to high radiation.(3) There is difference between two species in adaptation and tolerance to high and low temperature. When temperature is greater than 27℃, PSII reaction centres of Salix psammophila are damaged being showed by increased ΦNO and downregulated ΦNPQ and Fv/Fm. By contrast, Artemisia ordosica has a self-protection mechanism through upgrade xanthophylls cycle conversion rate to increase regulated energy dissipation in the same situation. When temperature is less than 7.5℃, photo-inhibition occurred with decrease in ΦSPII for two species. It is concluded that Artemisia ordosica is more adapted to high temperature.(4) There is a different sensitivity between two species to VWC.①Uprush in 70cm VWC (over 14%) caused damage in PSII of Salix psammophila reflected by decreased ΦSPII and upregulated ΦNO and ΦNPQ, which need recovery phase of 2 days, the recovery phase added to 5days when VWC(70cm)>16%. ② The improve of 30cm VWC is a promotion for photosynthetic physiological activity of Salix psammophila indicated by increased ETR and ΦSPII, decrease ΦNO and ΦNPQ. When VWC(30cm)>16%, photo-inhibition occurred in Artemisia ordosica PSII reaction centres indicated by decreased ΦSPII and improved ΦNO and ONPQ.③The optimum VWC of two species is VWC(30cm)=10% in leaf-expanded period, with a highest ΦSPII and NPQ.④rtemisia ordosica has a drought-sensitive period in August, Salix psammophila has a drought-sensitive period in June; both species are vulnerable to PAR with upregulated ΦNO.Overall, as the excellent sand-fixing vegetation, both of the two species have a strong resistance to the desert environment. Artemisia has a better adaptability and tolerance to high temperature and intensity, with low sensitivity to pulses of drought and rainfall; while Salix psammophila is more adaptive to low temperature and low light stress. Thus we recommend greater use of Artemisia ordosica as sand-fixing and vegetation recovery species in dry and hot area.