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

In Populus trichocarpa (black cottonwood), net photosynthesis (A ) varies with latitude and, in northern genotypes, is supported by higher stomatal conductance (g ). We report here a parallel cline in mesophyll conductance (g ) 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 in six representative genotypes (three from either end of the north-south cline). Acetazolamide reduced CA activity, g , g , chloroplast CO concentration (C ) and A at normal CO (400 μmol mol ), the latter being reversible at saturating CO . Absolute reductions in A , g 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 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 was similar whether the variation was inherent or inhibitor induced. We suggest that greater CA activity contributes to higher g in northern P. trichocarpa genotypes, but other diffusion pathway components may also be involved.