Abstract:

Mercury pollution is a major environmental problem. Unlike most heavy metals, plants uptake mercury primarily through leaves rather than the root system. Most of the mercury existed as complexes of Hg2+, which could cause the inhibition of photosynthesis as well as plant growth in higher plants. Since photosynthesis is very sensitive to heavy metals in higher plants, green algae and cyanobacteria, the inhibition of photosynthesis by Hg²⁺ had been examined in CO₂ fixation and photosystems activities. However, in most of the researches, Hg²⁺ solutions were applied in vitro or not directly on plant leaves, the mechanism of inhibition on photosynthesis by Hg²⁺ is still in controversy. In order to discover its effect on leaf photosynthesis, HgCl₂ solutions were applied to intact broad bean (Vicia faba L.) leaves. In the experiment, Hg²⁺ contents in intact broad bean leaves were determined after 30 min treatment with different concentrations of HgCl₂ solution. And leaf gas exchange and chlorophyll fluorescence were measured under a definite light intensity and CO₂ concentration. The results showed that the Hg content in leaves increased with the increase of treated Hg concentrations at the range from 0.5 mg L^-1 to 2 000 mg L^-1. When the applied HgCl2 concentration was 0.5 mg L^-1, the increase of leaf mercury content compared with that of control was less than 20%. If the applied HgCl₂ increased to 500 mg L^-1, Hg content in leaves was about 800% of control. Meanwhile, leaf net photosynthetic rate (P_n) was unaffected at the treated HgCl₂ concentrations from 0.5 mg L^-1 to 5.0 mg L^-1, and gradually decreased with the increase of treated Hg concentrations from 10 mg L^-1 to 2 000 mg L^-1. The inhibition of leaf’s P_n was about 36% after 2000 mg L^-1 HgCl₂ treatment. The results suggested that broad bean could absorb mercury solution through the leaves and thus lead to the inhibition of leaf photosynthetic activities.
Further studies showed that both leaf’s apparent quantum yield (AQY) and electron transport rate (ETR) were decreased significantly, with the increase of HgCl2 concentrations. Moreover, the quantum yield of PSⅡ(DF/F_m′) were inhibited whereas the non-photochemical quenching (NPQ) was increased by HgCl₂ concentrations from 20 mg L^-1 to 2 000 mg L^-1, indicating that the decrease of Pn in broad bean leaves by low concentrations of HgCl₂ solutions short time treatment might be mainly due to the inhibition of photosynthetic electron transport caused by the decrease of light energy transferring to active center and the increase of the light energy dissipation in PSⅡ.

Key words: Mercuric chloride (HgCl₂), Net photosynthetic rate (P_n), Apparent quantum yield (AQY), Chlorophyll fluorescence