Stomatal Response to Humidity: Blurring the Boundary between Active and Passive Movement

Land plants often experience conflicting demands for carbon assimilation and water conservation. The epidermal surface of plant leaves includes pores, or stomata, that allow atmospheric CO2 to enter the air space within leaf for photosynthetic carbon fixation by the mesophyll cells. Stomata also facilitate transpiration, that is, the diffusion of water vapor from the leaf air spaces to the outer atmosphere. The water vapor content within the leaf is usually close to saturation. So, a dry atmosphere gives rise to a large gradient in water vapor across the stomatal pore, that is, a leaf-to-air vapor pressure deficit (VPD), which increases as the relative humidity outside falls. Increasing VPD leads to a proportional increase in transpiration rate through the stomatal pore, driven by diffusion. Stomata therefore generally close at high VPD to prevent excessive water loss. Pairs of guard cells surround each stoma to regulate the stomatal aperture and, hence, the stomatal conductance (g s, defined as the rate of transpiration divided by the VPD, the driving force). Guard cells open and close the pore, driven by solute uptake and loss—notably of K+ and Cl−—and by the synthesis and metabolism of organic solutes, especially malate. Solute flux and metabolism generate osmotic gradients for water flux across the guard cell plasma membrane, which in turn leads to changes in guard cell volume and turgor. Increasing volume and turgor promotes stomatal opening as the guard cells press apart from one another, and decreasing volume and turgor reverses this effect. Much research has focused on the water stress hormone abscisic acid (ABA) and its connection to stomatal closing. These studies have highlighted roles for Ca2+-dependent signaling, cytosolic pH, reactive oxygen and nitrogen species, and protein phosphorylation in regulating a number of critical ion transport processes of the guard cell needed to generate the osmotic gradients for stomatal movement (Jezek and Blatt, 2017).