| IPCC predicted that the frequency of extreme precipitation events will increase at middle and high latitude area and precipitation will increase in the arid areas of northwestern, China.As the main driving element of desert ecosystem structure and function, precipitation is an important limiting factor for plant physiological ecology process in arid areas. In recent years,there are some studies on the effects of simulated rain addition on desert plants, while the studies on the responses of desert plants in physiological and ecological characteristics and its adaptive strategies to arid environment are still relatively scarce. In this study, Natraria tangutorum, a desert plant distributed in the eastern edge of the Ulan Buh Desert, was selected as the research object. An artificially simulated rain addition experiment was conducted on N.tangutorum nebkhas continuously from 2008 to 2014, with four rain addition treatments(increased 25%, 50%, 75% and 100% of local mean annual precipitation, respectively). The responses of photosynthetic capacity, physiological characteristics, leaf traits, plant growth and biomass allocation features of N. tangutorum to rain addition were measured, and the photosynthesis physiological mechanism and adaptive strategies to environment were explored.The main results and conclusions are as follows:(1) After artificially simulated rain addition, the photosynthetic capacity of N. tangutorum showed a positive response at different time scales. Rain addition strengthened the ability of N.tangutorum in physiological metabolism and photosynthetic product accumulation. There was a threshold of rain addition for accelerating the photosynthesis of N. tangutorum. The rain addition of 75% or more of local mean annual precipitation made the photosynthetic production capacity of N. tangutorum significantly increased. In order to maintain high level photosynthetic production capacity, N. tangutorum sustains high level transpiration rate to provide more raw material for photosynthesis, even at the expense of bringing down its water use efficiency.(2) After artificial simulated precipitation, photosynthetic physiological characteristics were affected significantly. Rain addition enhanced the abilities to use optical energy at low light levels and activities of photosynthetic carbon cycle enzyme, changed the light utilization amplitude to high intensity and CO2 utilization amplitude to high concentration, and increased the optical energy conversion efficiency of N. tangutorum. After rain addition, activity and opening ratio of PSⅡ reaction center increased after rain addition which suggested more captured light energy were transform into biochemical energy. Non photochemical quenching coefficient(qN) increased with the increase of rain addition, which illustrated the increase stability of optical system response center.(3) After artificially simulated rain addition, N. tangutorum leaves’ water status were improved obviously. With the increase of rain addition ammout, specific leaf area and leaf nitrogen content increased gradually. Rain addition facilitated leaf physiological metabolism and water circulation, increased the leaf area of unit quality, improved the activity and number of photosynthesis enzyme, and finally resulted in high CO2 assimilation rate and photosynthetic capacity. The sequence of main leaf traits that affecting photosynthetic capacity was leaf relative water content > succulent index > leaf nitrogen content > specific leaf area.(4) After artificially simulated rain addition, N. tangutorum foliar δ13C decreased with the increase of rain addition and the water use efficiency of N. tangutorum decreased. The water use efficiency of N. tangutorum had obvious seasonal variation. It was high in the early stage of growing season, then declined, and achieved its minimum value in the late stage of growing season.. δ15N were enhanced in watered plots obviously. Rain addition accelerated the speed of import, conversion and export of the nitrogen in plant body of N. tangutorum, which were benefitial to the photosynthesis and photosynthetic product accumulation.(5) After artificially simulated rain addition, the growth of vegetative and reproductive branches of N. tangutorum increased obviously, aboveground biomass accumulation and the proportion of photosynthetic product allocation to leaves were enhanced. Rain addition increased vegetation cover of N. tangutorum shrubs and then improved the photosynthetic effective area. These results illustrated N. tangutorum shrubs in watered plots would capturemore optical energy and absorb more carbon dioxide compared with unwaterd plots. The facts that rain addition leading to the increase of aboveground biomass and L/B suggested that N.tangutorum shrubs would allocate more photosynthetic product to the aboveground and simultaneously allocate more photosynthetic product to leaves compared with unwaterd plots.Thedesert species N. tangutorum had ability to adapt to rain addition.(6) Under global climate change scenario, N. tangutorum would mediate the morphology and physiological function in order to utilize environmental resources effectively and accommodate new water conditions gradually. The patterns include: 1) N. tangutorum increased light energy absorption, transformation, and utilization efficiency by increasing activity of PS Ⅱ reaction center and the electron transfer rate; 2) N. tangutorum increased assimilation rate, promoted matter accumulation and transformation by increasing the number and activity of RuBP carboxylase; 3) In order to capture more optical energy and absorb more carbon dioxide, N. tangutorum increased the photosynthetic effective area on community scale by increasing the growth of branches and vegetation cover; 4) N. tangutorum accelerated material and energy turnaround speed by increasing physiological metabolism to adapt to new water condition. |
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