We developed a model that coupled leaves net photosynthetic rate, transpiration rate and instantaneous water use efficiency (WUEI) and tested against observational data from an apple orchard. The sensitivity of WUEI to microclimatic factors and leaf water potential ( ψl ) was systemically assessed in this model. The leaves photosynthetic and transpiration rates were simulated by the C3 photosynthesis model and Penman-Monteith equation. For convenience,WUEI was expressed as the ratio of net photosynthetic rate to transpiration rate. It was shown that WUEI increased rapidly with increasing CO2 concentration and relative humidity (RH); but decreased with increasing temperature ( Ta ) and ψl. WUEI consistently increased with the increase of PAR when PAR was below light saturation point. And it mainrained a constant when PAR excess light saturation point. The results showed that the influences of these factors were different and there were strong interactive effects of various microclimatic factors on WUEI. On clear days, the peak of WUEI was reached early after sunrise; it then decreased to a stable level maintained through most of the day until falling to zero at sunset.%通过构建叶片净光合速率和蒸腾速率耦合模型可模拟出不同小气候条件和叶片水势下苹果叶片瞬时水分利用效率(WUEI)动态变化.该模型利用C3植物叶片光合生化模型和Penman-Monteith公式计算叶片净光合和蒸腾速率,而删是根据净光合速率和蒸腾速率比值求出.利用在富士苹果园观测数据拟合了相关模型参数.模拟结果表明WUEI随CO2浓度和湿度的增加而迅速增加,随温度和叶片水势的增大而减少;当有效光合辐射(PAR)低于光饱和点时WUEI随PAR的增加而增加,超过光饱和点后,随PAR的增加而略有减少.数值模拟还表明各种小气候因子对WUEI的影响不同,并且各因子之间还存在强烈的交互作用.在晴天WUEI在太阳升起不久就达到了最大值,然后迅速下降到一个比较稳定的水平,并几乎全天都保持此水平.直到太阳落山时降到0.
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