Contrasting anatomical and biochemical controls on mesophyll conductance across plant functional types

Mesophyll conductance (gm) limits photosynthesis by restricting CO2 diffusion between the substomatal cavities and chloroplasts. Although it is known that gm is determined by both leaf anatomical and biochemical traits, their relative contribution across plant functional types (PFTs) is still unclear. We compiled a dataset of gm measurements and concomitant leaf traits in unstressed plants comprising 563 studies and 617 species from all major PFTs. We investigated to what extent gm limits photosynthesis across PFTs, how gm relates to structural, anatomical, biochemical, and physiological leaf properties, and whether these relationships differ among PFTs. We found that gm imposes a significant limitation to photosynthesis in all C3 PFTs, ranging from 10–30% in most herbaceous annuals to 25–50% in woody evergreens. Anatomical leaf traits explained a significant proportion of the variation in gm (R2 > 0.3) in all PFTs except annual herbs, in which gm is more strongly related to biochemical factors associated with leaf nitrogen and potassium content. Our results underline the need to elucidate mechanisms underlying the global variability of gm. We emphasise the underestimated potential of gm for improving photosynthesis in crops and identify modifications in leaf biochemistry as the most promising pathway for increasing gm in these species

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, gm). 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 gm 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 gm 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 gm 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