CO2 diffusion inside photosynthetic organs

In the present chapter, we review the current state-of-the-art of knowledge on mesophyll (internal) CO2 diffusion conductance of photosynthetic tissues (for simplification, g(m)). We show that, despite concerns regarding the methodological approaches currently used for its estimation, a large and consistent body of evidence has accumulated showing that g(m) is finite and significantly limiting for-photosynthesis, as well as being highly variable among photosynthetic organisms and in response to environmental changes. Part of this variation results from different anatomies of the photosynthetic tissues, with a particularly strong influence of chloroplast distribution and cell wall thickness. Besides these, it appears that a biochemical modulation of g(m) also occurs, likely involving aquaporins and, possibly, carbonic anhydrases and other metabolic components. Further efforts are needed in the near future to improve CO2 diffusion models, both for the estimation of g(m) and for the precise physiological understanding of the CO2 assimilation process in different plants, as well as to increase our knowledge of the mechanistic base for gm and its regulation.