Stomatal aperture, VPD and transpiration - The chicken or the egg?

Stomata regulate gas exchange and water loss by closing in response to high vapor pressure deficit (VPD). Yet, it remains unclear whether stomata directly sense external conditions (e.g., relative humidity [RH] or temperature) or instead respond to internal changes associated with VPD (e.g., transpiration rate [E] or water status). Moreover, the stomatal response to VPD integrates both passive hydraulic and active metabolic mechanisms and involves inherent coupling between E and RH, which complicates our understanding of stomatal signal perception and response. In addition, recent findings challenging the long-held assumption that leaf airspaces are saturated, raise new questions about the VPD that stomata actually experience and how internal regulation of RH interacts with stomatal control. In this review, we examine the relationships between stomatal aperture, transpiration, and conductance, alongside the underlying mechanisms governing the stomatal response to VPD. We also revisit and refine a previously proposed mechanism by which guard cells may sense changes in VPD by perceiving local changes in RH. A deeper understanding of these mechanisms is critical for improving models of plant water use and for breeding drought-resilient crops that optimize CO₂ uptake while minimizing water loss in a changing climate.