Mesophyll conductance: the leaf corridors for photosynthesis

Besides stomata, the photosynthetic CO₂ pathway also involves the transport of CO₂ from the sub-stomatal air spaces inside to the carboxylation sites in the chloroplast stroma, where Rubisco is located. This pathway is far to be a simple and direct way, formed by series of consecutive barriers that the CO₂ should cross to be finally assimilated in photosynthesis, known as the mesophyll conductance (g_m). Therefore, the g_m reflects the pathway through different air, water and biophysical barriers within the leaf tissues and cell structures. Currently, it is known that g_m can impose the same level of limitation (or even higher depending of the conditions) to photosynthesis than the wider known stomata or biochemistry. In this mini-review, we are focused on each of the g_m determinants to summarize the current knowledge on the mechanisms driving g_m from anatomical to metabolic and biochemical perspectives. Special attention deserve the latest studies demonstrating the importance of the molecular mechanisms driving anatomical traits as cell wall and the chloroplast surface exposed to the mesophyll airspaces (S_c/S) that significantly constrain g_m. However, even considering these recent discoveries, still is poorly understood the mechanisms about signaling pathways linking the environment a/biotic stressors with g_m responses. Thus, considering the main role of g_m as a major driver of the CO₂ availability at the carboxylation sites, future studies into these aspects will help us to understand photosynthesis responses in a global change framework.