Changes in the Photosynthetic Apparatus of Broad Bean Leaves as Dependent on the Content of Heavy Metals in the Growth Medium

The content of chlorophyll, the rate of O₂ evolution, the slow phase of fluorescence induction, and photoinduced changes in the intensity of electron paramagnetic resonance (EPR) signal I from the reaction center of photosystem I (P⁺700) were studied in leaves of Vicia faba L. grown in 10^–7–10^–3 M aqueous solutions of CdCl₂, SnCl₂, CuCl₂, and MgCl₂. At low concentrations of heavy metal (Cd, Sn, and Cu) chlorides, the content of chlorophyll per fresh weight decreased. However, the rate of O₂ evolution calculated per chlorophyll basis, O₂/(t chlorophyll), increased. High concentrations of heavy metals significantly suppressed plant development and inhibited photosynthetic O₂ evolution. In contrast, plant treatment with MgCl₂ (10^–5–10^–3 M) resulted in an increase in the content of chlorophyll and a stimulation of leaf photosynthetic activity. A positive correlation between the F _M/F _T ratio and O₂/(t chlorophyll) (r = 0.89, P > 0.999) was found. We observed a negative correlation between the values of P/P ₀ of EPR signal I (white/far-red light) and O₂/(tchlorophyll) (r = –0.89, P > 0.999). The data obtained indicate nonspecific and nonmonotone changes in the photosynthetic apparatus of plants treated with heavy metals: low concentrations of heavy metals (10^–7–10^–6 M) stimulated photosynthetic activity, whereas high concentrations (10^–4–10^–3 M) suppressed it.