High relative air humidity (RH ≥ 85%) is frequent in controlled environments, and not uncommon in nature. In this review, we examine the high RH effects on plants with a special focus on stomatal characters. All aspects of stomatal physiology are attenuated by elevated RH during leaf expansion (long-term) in C₃ species. These include impaired opening and closing response, as well as weak diel oscillations. Consequently, the high RH-grown plants are not only vulnerable to biotic and abiotic stress, but also undergo a deregulation between CO₂ uptake and water loss. Stomatal behavior of a single leaf is determined by the local microclimate during expansion, and may be different than the remaining leaves of the same plant. No effect of high RH is apparent in C₄ and CAM species, while the same is expected for species with hydropassive stomatal closure. Formation of bigger stomata with larger pores is a universal response to high RH during leaf expansion, whereas the effect on stomatal density appears to be species- and leaf side-specific. Compelling evidence suggests that ABA mediates the high RH-induced stomatal malfunction, as well as the stomatal size increase. Although high RH stimulates leaf ethylene evolution, it remains elusive whether or not this contributes to stomatal malfunction. Most species lose stomatal function following mid-term (4–7 d) exposure to high RH following leaf expansion. Consequently, the regulatory role of ambient humidity on stomatal functionality is not limited to the period of leaf expansion, but holds throughout the leaf life span.

在受控环境中，经常会出现较高的相对空气湿度（RH≥85％），这在自然界中并不罕见。在这篇综述中，我们研究了对植物的高RH效应，特别关注气孔特征。在C ₃物种的叶片扩展（长期）过程中，相对湿度的升高会削弱气孔生理的所有方面。这些包括打开和关闭响应减弱以及diel振荡减弱。因此，高RH生长的植物不仅易受生物和非生物胁迫，而且在吸收CO ₂和水分流失之间也受到放松的调节。单个叶片的气孔行为由膨胀过程中的局部微气候决定，并且可能不同于同一株植物的其余叶片。C _4中没有明显的高RH效应和CAM种类，而具有水被动气孔关闭的种类预计也是如此。具有较大孔的较大气孔的形成是叶片扩张过程中对高RH的普遍反应，而对气孔密度的影响似乎是物种和叶片侧特异性的。有力的证据表明，ABA介导了由RH引起的高气孔功能失调以及气孔大小的增加。尽管高RH会刺激叶片乙烯的释放，但是否导致气孔功能障碍仍然难以捉摸。大多数物种在叶片扩张后的中期（4-7 d）暴露于高湿度下会失去气孔功能。因此，环境湿度对气孔功能的调节作用不仅限于叶片膨胀的时期，而且在整个叶片寿命中都有效。