Concomitant effects of mercuric chloride on mesophyll conductance and carbonic anhydrase activity in Populus trichocarpa Torr. & Gray

Abstract

Mercuric chloride, an inhibitor of CO2 transport via aquaporins, reduces mesophyll conductance in black cottonwood, and more so in northern than in southern genotypes, and it also reduces carbonic anhydrase activity. To investigate the potential involvement of aquaporins (AQPs) in latitude-dependent variation in mesophyll conductance (g m) of Populus trichocarpa Torr. & Gray (black cottonwood), photosynthetic susceptibility to mercuric chloride (HgCl2; a chemical inhibitor for AQPs) was tested in three northern and three southern representative cottonwood genotypes. The chlorophyll fluorescence method combined with a gas exchange system, was used to estimate g m in leaves under distilled water (control) and 1.5 mM aqueous HgCl2 fed through the petiole. Compared to the control, in both northern and southern genotypes photosynthetic rate (A n), g m, stomatal conductance (g s) and chloroplast CO2 concentration (C c) decreased significantly under HgCl2 (P < 0.008). The effect of HgCl2 on A n was reversible at saturating CO2. In the northern genotypes, A n, g m and g s (which are intrinsically higher in northern genotypes than in southern genotypes), were more susceptible to HgCl2 in both absolute and percentage terms (P < 0.05). In contrast, northern and southern genotypes had similar C c under control conditions, and showed statistically similar reductions under HgCl2. These results are consistent with a greater proportional involvement of AQPs in determining g m in northern genotypes. However, treatment with HgCl2 resulted in a concurrent reduction (P < 0.05) in carbonic anhydrase (CA) activity, which has previously been linked to variation in g m in black cottonwood. Mercuric chloride may thus reduce g m by acting on either AQPs or CA, or the two in combination.