Abstract
Mesophyll conductance is thought to be an important photosynthetic limitation in gymnosperms, but they currently constitute the most understudied plant group in regard to the extent to which photosynthesis and intrinsic water use efficiency are limited by mesophyll conductance. A comprehensive analysis of leaf gas exchange, photosynthetic limitations, mesophyll conductance (calculated by three methods previously used for across-species comparisons), and the underlying ultra-anatomical, morphological and chemical traits in 11 gymnosperm species varying in evolutionary history was performed to gain insight into the evolution of structural and physiological controls on photosynthesis at the lower return end of the leaf economics spectrum. Two primitive herbaceous species were included in order to provide greater evolutionary context. Low mesophyll conductance was the main limiting factor of photosynthesis in the majority of species. The strongest sources of limitation were extremely thick mesophyll cell walls, high chloroplast thickness and variation in chloroplast shape and size, and the low exposed surface area of chloroplasts per unit leaf area. In gymnosperms, the negative relationship between net assimilation per mass and leaf mass per area reflected an increased mesophyll cell wall thickness, whereas the easy-to-measure integrative trait of leaf mass per area failed to predict the underlying ultrastructural traits limiting mesophyll conductance.
Highlights
A comprehensive analysis of leaf gas exchange, photosynthetic limitations, mesophyll conductance, and the underlying ultra-anatomical, morphological and chemical traits in 11 gymnosperm species varying in evolutionary history was performed to gain insight into the evolution of structural and physiological controls on photosynthesis at the lower return end of the leaf economics spectrum
A large variability was observed in their macroscopic anatomy (Fig. 1A), and structural and ultrastructural parameters (Figs 1B–D and 2), e.g. a P. sylvestris cross-section exhibited lobed cells, increasing mesophyll surface area exposed to the intercellular airspace (Fig. 1C)
The variation in leaf mass per area (LMA) was attributed to variation in both of its components, but there was a stronger positive relationship with leaf density (Dleaf) than leaf thickness (Tleaf) (Fig. 3A, B)
Summary
The 36-fold variation of net photosynthetic rate across C3 species (Wright et al, 2004) cannot be explained only by stomatal restrictions and biochemical potentials, because the CO2 diffusion efficiency from substomatal cavities to chloroplasts (mesophyll diffusion conductance; gm) plays an important role in shaping photosynthetic capacities and leaf resourceuse efficiency across Earth’s ecosystems (Warren et al, 2003; Niinemets et al, 2009b, 2011; Terashima et al, 2011; Tosens et al, 2012b, 2016). Mesophyll cell wall thickness (Tcwm) and chloroplast surface area exposed to intercellular airspace (Sc/S), have been highlighted as the strongest limiting factors of gm, but these anatomical traits are highly variable among species (Evans et al, 2009; Terashima et al, 2011; Tosens et al, 2012b). Gm and its underlying traits cannot be extended across plant groups, and additional factors such as evolutionary age can play an important role in its determination
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