3) Dr. Yotam Zait

Stomatal are microscopic pores on the leaf surface which regulate H₂O and CO₂ exchange with the environment. Despite the large number of works on stomatal functioning, there is still no mechanism to explain how stomata sense VPD and what is being sensed since VPD, RH, and transpiration are all coupled. To address this, we utilized a system that allows control of VPD and transpiration rate independently by using air (21% oxygen:79% nitrogen) and Helox (21% oxygen:79% helium), where water vapor diffuses 2.33 times faster than in air. This allows control of VPD and transpiration rate independent of one another while maintaining a constant leaf temperature. We investigated the stomatal responses of several plant species, including: Vitis vinifera, Musa acuminata cv. Cavendish , Solanum lycopersicum Commelina communis, and the fern Nephrolepis exaltata, which have diverse stomatal characteristics. In all angiosperms tested, we observed that stomatal closing was triggered exclusively by reductions in RH rather than by transpiration rate increases. When transpiration increased but relative humidity remained constant, we detected an increase in stomatal conductance (g_sw) without a decrease in the experimental time span. Moreover, switching from air to Helox led to a significant increase in g_sw, while stomatal aperture remained unchanged, suggesting an uncoupling between stomatal conductance and aperture.  These results challenge the current understanding of the mechanisms underlying stomatal responses to VPD and the well-recognized relationship between stomatal aperture and conductance.  In addition, our set-up enabled to evaluate the limitation of intercellular airspace conductance (g_ias) on CO₂ assimilation. We developed a novel method to quantify g_ias for CO₂ and found that photosynthetic rate is limited by g_ias more than previously report in many species. These results indicate that optimizing the factors regulating g_ias could improve CO₂ fixation in C3 plants.