PREMISE The impact of elevated CO2 concentration ([CO2 ]) and climate warming on plant productivity in dryland ecosystems is influenced strongly by soil moisture availability. We predicted that the influence of warming on the stimulation of photosynthesis by elevated [CO2 ] in prairie plants would operate primarily through direct and indirect effects on soil water. METHODS We measured light-saturated photosynthesis (Anet ), stomatal conductance (gs ), maximum Rubisco carboxylation rate (Vcmax ), maximum electron transport capacity (Jmax ) and related variables in four C3 plant species in the Prairie Heating and CO2 Enrichment (PHACE) experiment in southeastern Wyoming. Measurements were conducted over two growing seasons that differed in the amount of precipitation and soil moisture content. RESULTS Anet in the C3 subshrub Artemisia frigida and the C3 forb Sphaeralcea coccinea was stimulated by elevated [CO2 ] under ambient and warmed temperature treatments. Warming by itself reduced Anet in all species during the dry year, but stimulated photosynthesis in S. coccinea in the wet year. In contrast, Anet in the C3 grass Pascopyrum smithii was not stimulated by elevated [CO2 ] or warming under wet or dry conditions. Photosynthetic downregulation under elevated [CO2 ] in this species countered the potential stimulatory effect under improved water relations. Warming also reduced the magnitude of CO2 -induced down-regulation in this grass, possibly by sustaining high levels of carbon utilization. CONCLUSIONS Direct and indirect effects of elevated [CO2 ] and warming on soil water was an overriding factor influencing patterns of Anet in this semi-arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change.

中文翻译：

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气候变暖改变了草原植物对二氧化碳升高的光合反应

前提 CO2 浓度升高 ([CO2 ]) 和气候变暖对旱地生态系统植物生产力的影响受到土壤水分可用性的强烈影响。我们预测，变暖对草原植物中升高的 [CO2] 光合作用刺激的影响将主要通过对土壤水的直接和间接影响起作用。方法我们测量了草原采暖和二氧化碳富集 (PHACE) 中四种 C3 植物物种的光饱和光合作用 (Anet)、气孔导度 (gs)、最大 Rubisco 羧化速率 (Vcmax)、最大电子传输能力 (Jmax) 和相关变量在怀俄明州东南部进行实验。测量是在降水量和土壤水分含量不同的两个生长季节进行的。结果 C3亚灌木Artemisia frigida和C3 forb Sphaeralcea coccinea中的Anet在环境和温暖温度处理下受到升高的[CO2]刺激。在干旱年，变暖本身降低了所有物种的 Anet，但在湿润年刺激了 S. coccinea 的光合作用。相比之下，C3 草 Pascopyrum smithii 中的 Anet 不受 [CO2 ] 升高或潮湿或干燥条件下变暖的刺激。该物种在升高 [CO2] 下的光合作用下调抵消了改善水关系下的潜在刺激作用。变暖还降低了这种草中 CO2 引起的下调幅度，这可能是通过维持高水平的碳利用来实现的。