Ensuring global food security is a worldwide major concern considering the predicted climate change scenarios for the main agricultural regions of the world. Stomatal conductance (g_s) and mesophyll conductance (g_m) are major drivers limiting photosynthesis (A). Both conductances frequently impose about two-thirds of the total photosynthetic limitation under optimum conditions. However, under abiotic stress, like drought or salinity, the diffusional limitations can reach more than 85 % of the total. Thus, knowledge about both conductances is essential to improve water use efficiency (WUE) through targeted crop breeding programs and to promote sustainable, resource-efficient, and environmental-friendly agriculture strategies.

Intriguingly, knowledge obtained from both conductances from decades of research differs importantly by their focus. Whilst the role of both mechanics and metabolism of guard cells on the regulation of stomatal movements remains much less understood, the signaling pathways that regulates stomatal movements are well-documented. The opposite is true for gm, in which the biochemical regulation and signaling pathways remain mostly still unexplored. Even more surprising is the lack of information about the putative molecular mechanisms that should drive the known coupled behavior of both conductances in response to the environment. Here, we discussed the main mechanisms driving the responses of each of the conductances, but highlighting a special focus into the possible common determinants that could link their coupled behavior. Further integrative multidisciplinary studies joining molecular biology and ecophysiology are required in order to improve our understanding of both conductances, the major actors limiting photosynthesis and WUE in a changing environment.