Abstract: [Objective] [Objective] This research was carried out to investigate the photosynthesis and chlorophyll fluorescence characteristics and explore the response mechanism in terms of photosynthetic physiology of Nitraria tangutorum under different rain addition treatments. [Method] Field plots were set and a typical desert plant, Nitraria tangutorum, was used as the research object at the eastern margin of the Ulanbuh Desert. The species were subjected to five water regimes by adding extra rainfall (increased 0, 25%, 50%, 75% and 100% of the local mean annual precipitation, respectively). A Li-6400xt portable photosynthesis measurement system was used to determine the dynamic changes of net photosynthetic rate, the light and CO₂ response curve, and the chlorophyll fluorescence parameters. Total daily photosynthesis and photosynthetic parameter were calculated according to the net photosynthetic rate daily dynamic curve, the light response curve and the CO₂ response curve under five rain addition treatments. [Result] 1) Average and total daily photosynthesis increased significantly under 75% and 100% rain addition treatments. The average daily photosynthesis of N. tangutorum treated with 75% and 100% rain addition was higher than those in control by 32.74% and 37.64%, respectively. The total daily photosynthesis of N. tangutorum with 75% and 100% rain addition was higher than those in control by 32.01% and 38.43%, respectively. The results indicated that photosynthetic capacity and daily photosynthesis accumulation were enhanced after rain addition. 2) With the increase of precipitation, apparent quantum yields (AQY) and light saturation points (LSP) increased gradually. AQY under 25%, 50%, 75% and 100% rain additions were 17.24%, 31.03%, 37.93% and 24.14% higher than control, respectively, and the corresponding LSP increased by 14.6%, 6.0%, 3.0% and 26.1%. These results suggested that rain additions promoted the capacity of utilizing optical energy at low light levels, the utilization of bright light, and conversion efficiency of light energy of N. tangutorum, which would improve the efficiency of photosynthesis. 3) With the increasing of precipitation, carboxylation efficiency (CE) increased gradually. The CE under 100% rain addition was significantly higher than control and the increment rate was 5.73%. The CO₂ saturation points (CSP) were greater in watered plots than in control plots. The CSPs in 50%, 75% and 100% rain additions were approximately 30.31%, 26.56%, and 50.94% higher than control, respectively. These results suggested that rain addition enhanced the activities of photosynthetic carbon cycle enzymes and increased utilization amplitude of high carbon dioxide concentration for the desert plant. 4) Rain addition led to the increase of primary photochemical quantum efficiency (F_v/F_m), PSⅡquantum yield (Ф_PSⅡ), electron transport rate (ETR) and photochemical quenching (q_P) of N. tangutorum. The results indicated that rain addition was conducive to increase the activity and opened proportion of PSⅡ reaction center, which should be beneficial to transform the captured light energy into biochemical energy for N. tangutorum leaves and make more energy used to promote photosynthetic electron transport.[Conclusion] Thus, under global change scenario, N. tangutorum would mediate the function of photosynthetic apparatus, change the physiological characteristics and enhance the ability of utilization environmental resources to adapt to precipitation pattern variation in the future.

Key words: Nitraria tangutorum, simulated rain addition, photosynthesis, chlorophyll fluorescence, response mechanism