When it comes to water, plants are wasteful organisms. Although water is one of the most limiting factors for plant growth, most water taken up by the roots is directly released by the leaves into the atmosphere. This is because plants have to make a compromise. For every molecule of CO2 that is absorbed through the stomata, hundreds of water molecules are lost in the form of water vapor. To restrict water loss, plants can close their stomata, but the reduced CO2 uptake will inhibit photosynthesis and growth. Hence, if water is plentiful, stomata will remain open and water is lost at a high rate. However, there are more aspects to the water economy of a leaf than opening and closing of the stomata. After water is absorbed by the roots, it is transported upwards through the xylem vessels and into the leaf veins. To reach the inner leaf tissue, water has to cross a tissue layer that tightly enwraps the leaf veins: the bundle sheath cells. The bundle sheath cells act as a selective barrier that can block the transport of certain solutes, such as sodium and boron. Several years ago, researchers found that bundle sheath cells can also act as a barrier to the passage of water (Shatil-Cohen et al., 2011). In this way, the bundle sheath cells control the water transport capacity of the leaf, or leaf hydraulic conductance, which in turn controls the rate at which water is lost. How the barrier behavior of the bundle sheath cells was modulated remained an open question. A mechanism that involves the pH of the xylem sap was postulated, but proof was lacking (Geilfus, 2017). Recently, researchers from The Hebrew University of Jerusalem found that the gene encoding Arabidopsis H+-ATPase 2 (AHA2) is highly expressed in the bundle sheath cells (Wigoda et al., 2017). Plasma membrane proton pumps transport H+ ions from the cytosol to the extracellular space (in case of the bundle sheath cells: the xylem). If the barrier mechanism of bundle sheath cells is regulated by the pH of xylem sap, they thought that AHA2 would be an interesting candidate gene to characterize. Therefore, they investigated the effects of AHA2 on the pH of xylem sap and on leaf hydraulic conductance, and the results of this study are reported in this issue of TPJ (Grunwald et al., 2021). The authors compared wild-type plants and aha2 mutants. Using a gas exchange system, they measured the amount of water vapor released by excised leaves. Immediately after these transpiration measurements, they determined the leaf water potential. Individual leaves were placed in a pressure chamber and the pressure was increased until water appeared at the excision cut. With these measurements, the authors calculated the leaf hydraulic conductance for single leaves. Hydraulic conductance in leaves from aha2 mutant plants was significantly lower than in leaves from wild-type plants, suggesting that AHA2 has a function in regulating leaf hydraulic conductance. Subsequently, the authors examined the effects of AHA2 on the pH of xylem sap. They found that when AHA2 function was disrupted, either by treating leaves with a proton pump inhibitor or in aha2 mutant plants, the xylem sap became more alkaline. Conversely, treatment with a proton pump stimulator caused acidification. To test if the pH of xylem sap modulates leaf hydraulic conductance, the authors treated wild-type leaves with solutions that were buffered at different pHs. Leaves that were treated with a solution at pH 6.0 – which is in the range of normal growth conditions – showed similar hydraulic conductance as leaves that were treated with a non-buffered control solution. However, leaves treated with a slightly alkaline solution (pH 7.5) showed a strong reduction in hydraulic conductance, which indicates that the pH of xylem sap has a direct effect on the hydraulic conductance of the leaves. To understand how pH affects hydraulic conductance, the authors tested whether the permeability of bundle sheath cells is affected by pH. To do this, they isolated protoplasts from bundle sheath cells and flooded them with a hypotonic solution that causes water to enter the protoplasts. By measuring the rate at which the protoplasts were swelling, the authors could calculate the water permeability of the membrane. At a higher pH, the protoplast swelled more slowly. Hence, the lower hydraulic conductance at a higher pH is most likely caused by a decreased permeability of the bundle sheath cell membranes. These results suggest a mechanism in which bundle sheath cells regulate leaf hydraulic conductance by modulating their permeability through control of xylem sap pH (Figure 1). Understanding how plants prevent water loss is vital to understand and predict the effects of climate change, as well as for the development of climate-resilient crops. However, why membrane permeability changes under different pH values remains an enigma. The authors think that aquaporins might be involved. Aquaporins are membrane channels that allow water to cross membranes and are important in the hydraulic regulation in response to environmental stimuli, such as drought and flooding. Moreover, aquaporin activity can be regulated by pH changes (Kapilan et al., 2018). Further research will tell us if this is indeed part of the mechanism by which plants control their water economy.