• Elevated atmospheric CO₂ (eC_a) may benefit plants during drought by reducing stomatal conductance (g_s) but any ‘water savings effect’ could be neutralized by concurrent stimulation of leaf area. We investigated whether eC_a enhanced water savings, thereby ameliorating the impact of drought on carbon and water relations in trees.
• We report leaf‐level gas exchange and whole‐plant and soil water relations during a short‐term dry‐down in two Eucalyptus species with contrasting drought tolerance. Plants had previously been established for 9 to 11 months in steady‐state conditions of ambient atmospheric CO₂ (aC_a) and eC_a, with half of each treatment group exposed to sustained drought for 5 to 7 months.
• The lower stomatal conductance under eC_a did not lead to soil moisture savings during the dry‐down due to the counteractive effect of increased whole‐plant leaf area. Nonetheless, eC_a‐grown plants maintained higher photosynthetic rates and leaf water potentials, making them less stressed during the dry‐down, despite being larger. These effects were more pronounced in the xeric species than the mesic species, and in previously water‐stressed plants.
• Our findings indicate that eC_a may enhance plant performance during drought despite a lack of soil water savings, especially in species with more conservative growth and water‐use strategies.

中文翻译：

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树木中二氧化碳相互作用引起的干旱：节水机制的测试

• 在干旱期间，升高的大气CO ₂（eC _a）可能会通过降低气孔导度（g _s）使植物受益，但任何“节水效果”都可以通过同时刺激叶面积而抵消。我们调查了eC是否_可以提高节水效果，从而减轻干旱对树木碳与水关系的影响。
• 我们报道了两种桉树在短期干旱期间叶片水平的气体交换以及整个植物与土壤水之间的关系，并具有相对的耐旱性。先前已在环境大气CO ₂（aC _a）和eC _a的稳态条件下建立了9至11个月的植物，每个处理组中有一半暴露于持续干旱5至7个月。
• 在eC _a下较低的气孔电导率并没有导致土壤水分的节省，这是由于增加了整株植物叶面积的反作用。尽管如此，尽管eC _a生长的植物较大，但其光合速率和叶片水势仍保持较高，使它们在干燥过程中的压力较小。在干性物种中，这些效应比中性物种以及以前的水分胁迫植物更为明显。
• 我们的发现表明，尽管缺乏土壤节水，但eC _a可能会增强干旱期间的植物性能，尤其是在具有较保守的生长和节水策略的物种中。