Substantial role for carbonic anhydrase in latitudinal variation in mesophyll conductance of Populus trichocarpa Torr. & Gray

In Populus trichocarpa (black cottonwood), net photosynthesis (A(n)) varies with latitude and, in northern genotypes, is supported by higher stomatal conductance (g(s)). We report here a parallel cline in mesophyll conductance (g(m)) and link this variation to carbonic anhydrase (CA) activity. Using concurrent carbon isotope discrimination and chlorophyll fluorescence methods, we examined the effects of acetazolamide, an inhibitor of CA, on g(m) in six representative genotypes (three from either end of the north-south cline). Acetazolamide reduced CA activity, g(m), g(s), chloroplast CO2 concentration (C-c) and A(n) at normal CO2 (400molmol(-1)), the latter being reversible at saturating CO2. Absolute reductions in A(n), g(m) and CA activity were greater in northern genotypes than in southern genotypes (P<0.025) but percent reductions were similar. In contrast, northern genotypes showed lower percent reduction in C-c compared to southern genotypes (P<0.025). The northern genotypes had greater CA activity relative to both leaf area (two-fold) and mass (1.8-fold) (P<0.016). The relationship between CA activity and g(m) was similar whether the variation was inherent or inhibitor induced. We suggest that greater CA activity contributes to higher g(m) in northern P. trichocarpa genotypes, but other diffusion pathway components may also be involved.