Distribution and uptake dynamics of mercury in leaves of common deciduous tree species in Minnesota, U.S.A.

A sequential extraction technique for compartmentalizing mercury (Hg) in leaves was developed based on a water extraction of Hg from the leaf surface followed by a solvent extraction of the cuticle. The bulk of leaf Hg was found in the tissue compartment (90-96%) with lesser amounts in the surface and cuticle compartments. Total leaf concentrations of Hg varied among species and was most closely correlated with the number of stomates per sample, supporting the hypothesis that stomatal uptake of atmospheric Hg (most likely Hg(0)) is a potential uptake pathway. Mercury concentrations in leaves were monitored from emergence to senescence and showed a strong positive correlation with leaf age. Leaves accumulated Hg throughout the growing season; the highest uptake rates coincided with periods of high photosynthetic activity. Concentrations of Hg in leaf tissue increased steadily throughout the season, but no such trends were observed for surficial or cuticular accumulation. Factors affecting the variability of Hg in leaves were analyzed to improve protocols for the potential use of leaves as passive monitors of atmospheric Hg. Results show that total leaf Hg concentrations are affected by leaf age and leaf placement in the crown.