The interplay of short-term mesophyll and stomatal conductance responses under variable environmental conditions

Understanding the short-term responses of mesophyll conductance (g_m) and stomatal conductance (g_sc) to environmental changes remains a challenging yet central aspect of plant physiology. This review synthesises our current knowledge of these short-term responses, which underpin CO₂ diffusion within leaves. Recent methodological advances in measuring g_m using online isotopic discrimination and chlorophyll fluorescence have improved our confidence in detecting short-term g_m responses, but results need to be carefully evaluated. Environmental factors like vapour pressure deficit and CO₂ concentration indirectly impact g_m through g_sc changes, highlighting some of the complex interactions between the two parameters. Evidence suggests that short-term responses of g_m are not, or at least not fully, mechanistically linked to changes in g_sc, cautioning against using g_sc as a reliable proxy for g_m. The overarching challenge lies in unravelling the mechanistic basis of short-term g_m responses, which will contribute to the development of accurate models bridging laboratory insights with broader ecological implications. Addressing these gaps in understanding is crucial for refining predictions of g_m behaviour under changing environmental conditions.