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

Momayyezi, Mina, Guy, RobertD.
Trees 2018 v.32 no.1 pp. 301-309
Populus trichocarpa, aquaporins, carbon dioxide, carbonate dehydratase, chlorophyll, chloroplasts, enzyme activity, gas exchange, genotype, mercuric chloride, mesophyll, petioles, photosynthesis, stomatal conductance
KEY MESSAGE: 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.