It is widely acknowledged that many plant species can keep stomata open during night. We examined how nocturnal stomatal conductance differs among potted saplings of nine temperate tree species from diverse native habitats in wet and dry soil conditions, and how it affects plant predawn water status. Nocturnal stomatal conductance in dry soil conditions was low in all the species (with a maximum value of 14.6 mmol m^–2 s^–1); in wet conditions, it was the highest in Populus tremula L., a fast-growing and anisohydric pioneer species, and the lowest in Quercus robur L., a late-successional and isohydric species. Relatively high nocturnal stomatal conductance in wet conditions in P. tremula compared with the other species resulted in the highest difference in water potential values between the leaves and soil at predawn. As drought progressed, different species tended to keep stomata almost closed at night, and the observed differences between anisohydric and isohydric species disappeared. At an ample soil water supply, nocturnal stomatal behaviour was species dependent and varied according to both the water-use and the life strategies of the species. Keeping that in mind, one should therefore be careful when using predawn leaf water potential as a proxy for soil water potential, sampling different species.

众所周知，许多植物可以在夜间保持气孔开放。我们研究了湿润和干燥土壤条件下来自不同原生生境的九种温带树种的盆栽树苗的夜间气孔导度如何不同，以及它如何影响植物的黎明前水分状况。在所有土壤种类中，干燥土壤条件下的夜间气孔导度均较低（最大值为14.6 mmol m ^–2 s ^–1）。在湿润条件下，它在快速增长的和等水先驱种杨木（Populus tremula L.）中是最高的，而在后来的又是等水的种栎木（Quercus robur L.）中则最低。银耳潮湿条件下夜间气孔导度相对较高与其他物种相比，黎明前的叶子和土壤之间的水势值差异最大。随着干旱的进行，不同物种趋向于在夜间保持气孔几乎关闭，并且观察到的等渗和等渗物种之间的差异消失了。在充足的土壤水供应下，夜间气孔行为取决于物种，并根据物种的用水和生命策略而变化。请牢记这一点，因此，在使用黎明前的叶子水势作为土壤水势的代表时，应该对不同物种进行采样时要小心。