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Abstract: The effects of high concentrations of Hg2+(10‐2M and 10‐3M) were investigated on the ultrastructure and on the light‐induced transformation of isolated prolamellar bodies (PLBs) of dark‐grown wheat leaves. Our earlier work on wheat leaf homogenates (Solymosi et al. [2004], Plant Biology 6, 358 ‐ 368) showed that, depending on the concentration, Hg2+reacts with protochlorophyllide, NADPH and the NADPH: protochlorophyllide oxidoreductase (POR, EC 1.3.1.33) enzyme and induces disaggregation of the macrodomain structure of this latter. Spectroscopic analyses confirmed that 15 min incubation with 10‐2M Hg2+at 4°C completely inhibited the activity of POR also in isolated PLBs. Ultrastructural investigations revealed the loosening of the PLB structure in the Hg2+‐treated sample, i.e., intensive vesicle formation on the surface of the PLB membranes. The hexagonal geometry of the inner lattice was not disturbed, however, the unit cell size significantly increased. The disruption of the PLB membranes upon irradiation was studied after 40 min incubation with 10‐3M Hg2+at 4°C and a subsequent irradiation for 40 min at 20°C. Equimolar concentrations (10‐3M) of NADPH and Hg2+were added to the samples 10 min prior or after the addition of Hg2+. Our results suggest that Hg2+accelerates the disruption of the PLB membranes and that NADPH can only partially prevent this process. These membrane transformations were similar to those observed in the initial steps of the Shibata shift of control samples.

Abstract: The molecular background of Hg2+‐induced inhibition of protochlorophyllide (Pchlide) photoreduction was investigated in homogenates of dark‐grown wheat leaves. Our earlier work showed that 15 min incubation with 10‐2M Hg2+completely inhibits the activity of NADPH: Pchlide oxidoreductase. Detailed analysis of spectra recorded at 10 K indicated the appearance of emission bands at 638 and 650 nm, which are characteristic for NADP+‐Pchlide complexes. Fluorescence emission spectra recorded with different excitation wavelengths, fluorescence lifetime measurements and the analysis of acetone extractions revealed that Hg2+can also react directly with Pchlide, resulting in protopheophorbide formation. At 10‐3M Hg2+, the phototransformation was complete but the blue shift of the chlorophyllide emission band speeded up remarkably. This indicates oxidation of the NADPH molecules that have a structural role in keeping together the etioplast inner membrane components. We suggest a complex model for the Hg2+effect: depending on concentration it can react with any components of the NADPH: Pchlide oxidoreductase macrodomains.