Abstract: Based on the monitoring data of groundwater level at the typical sections in lower reaches of Tarim River, three survey plots nearby the ecological monitoring wells with groundwater depths > 6 m were selected to investigate the chlorophyll fluorescence characteristics of Tamarix hispida and its photosynthetic activity of PSII under effects of different groundwater depths. With increasing groundwater depth, the chlorophyll fluorescence parameters such as actual photochemical efficiency of PSⅡ in the light (Φ_PSII), electron transport rate (ETR), and photochemistry quenching (q_P) of T. hispida decreased, while the non-photochemistry quenching (q_N, NPQ) and the yield for dissipation by down-regulation (Y_NPQ) increased remarkably, and the maximal photochemical efficiency of PSII (F_v/F_m) maintained an optimum value. All the results suggested that the PSII photosynthetic activity of T. hispida under drought stress declined with increasing groundwater depth, and the greater excess energy could result in more risk of photo-inhibition. However, the good adaptability and drought tolerance of T. hispida could make its PSII not seriously damaged, though the drought stress actually existed.

Key words: chlorophyll fluorescence, groundwater level, fluorescence quenching, photochemical efficiency, Tarim River, scale effect, response time, rainfall intensity, karst.