Abstract: Understanding the adaptive strategy of psammophyte plants under drought stress has drawn much research attention. Artemisia ordosica is among the most important sand-fixing species in the Mu Us Sandland. We conducted continuous in situ measurements of chlorophyll fluorescence on A. ordosica under both rainfall exclusion and natural precipitation conditions to examine the response of PSII photochemical efficiency to water stress. Continuous measurements of the chlorophyll fluorescence parameters were made in August, 2013 at the Yanchi Research Station, Ningxia Hui Autonomous Region, northwest China. Chlorophyll fluorescence parameters were measured on Artemisia ordosica under both rainfall exclusion and natural precipitation conditions using a Monitoring-Pam fluorometer. Climatic variables were simultaneously measured with automated sensors. The maximum quantum yield of PSII photochemistry (F_v/F_m), photochemical efficiency (Φ_PSII) and non-photochemical quenching (NPQ) were analyzed in relation to environmental factors at the hour and diel scales. Photosynthetically active radiation was the direct control over Φ_PSII and NPQ at the diel scales. Φ_PSII and NPQ were significantly different in different water cinditions. The sensitivity of Φ_PSII to incident photosynthetically active radiation, air temperature and relative humidity was lower under rainfall exclusion than under natural rainfall, while the sensitivity of NPQ to these factors was increased by rainfall exclusion. The decrease of F_v/F_m was by the reversible inactivation of PSII reaction center. Our results indicated that drought stress strongly affects the dynamics of chlorophyll fluorescence parameters of Artemisia ordosica, which copes with harsh desert environments by adaptively increasing its thermal dissipation to avoid damages to the photosynthetic apparatus.

Key words: chlorophyll fluorescence, rainfall, PSII photochemical efficiency, arid and semiarid area