In this review, I first address the basics of gas exchange, water-use efficiency and carbon isotope discrimination in C3 plant canopies. I then present a case study of water-use efficiency in northern Australian tree species. In general, C3 plants face a trade-off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO2 assimilation rate, but a lower photosynthetic water-use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water-use efficiency. This may explain why community-level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water-use efficiency that take place during leaf expansion in C3 plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water-use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO2 can increase whole-plant water-use efficiency. Finally, I discuss the role of water-use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO2 . In coming decades, increases in plant water-use efficiency caused by rising CO2 are likely to partially mitigate impacts on plants of drought stress caused by global warming.

中文翻译：

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植物冠层的气体交换和水利用效率。

在这篇综述中，我首先介绍了C3植物冠层中气体交换，水利用效率和碳同位素歧视的基础知识。然后，我介绍了澳大利亚北部树种的水利用效率的案例研究。通常，C3植物需要权衡取舍，从而在给定条件下增加气孔导度将导致更高的CO2同化率，但降低光合用水效率。一项常见的花园实验表明，能够通过高气孔导度在澳大利亚北部较干燥地区建立和生长的树木能够快速使用水，但这样做的代价是用水量低效率。这可以解释为什么在北澳大利亚热带样带降水减少时，社区水平的碳同位素歧视没有像在其他一些降水梯度上观察到的那样急剧下降。接下来，我讨论了C3植物叶片扩叶过程中水分利用效率的变化。最近，人们认识到叶片物候是常绿森林冠层冠层气体交换的重要驱动力，叶片扩展涉及光合能力和水分利用效率的变化。在此之后，我将讨论木质组织呼吸作用在冠层气体交换中的作用，以及呼吸的CO2的光合作用固定如何增加整株植物的用水效率。最后，我讨论了水分利用效率在推动陆地植物对全球变化的响应中的作用，特别是大气中CO2浓度的上升。在未来的几十年中，二氧化碳上升引起的植物水分利用效率的提高可能会部分缓解全球变暖对干旱胁迫对植物的影响。