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 O-2 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 O-2 evolution calculated per chlorophyll basis, DeltaO(2)/(Deltat chlorophyll), increased. High concentrations of heavy metals significantly suppressed plant development and inhibited photosynthetic O-2 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 DeltaO(2)/(Deltat 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 DeltaO(2)/(Deltat chlorophyll) (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.