In situ detection of heavy metal substituted chlorophylls in water plants

The in vivo substitution of magnesium, the central atom of chlorophyll, by heavy metals (mercury, copper, cadmium, nickel, zinc, lead) leads to a brea kdown in photosynthesis and is an important damage mechanism in heavy metal -stressed plants. Tn this study, a number of methods are presented for the efficient in situ detection of this subsitution (i.e. in whole plants or in chloroplasts). While macroscopic observations point to the formation of he avy metal chlorophylls at higher concentrations, fluorescence microscopy en ables the detection of this reaction at very low substitution rates. Theref ore, the course of the reaction can be followed by continuously measuring t he fluorescence of whole plants. Furthermore absorbance spectroscopy of who le cells or isolated chloroplasts also enables the ill situ detection of he avy metal chlorophylls. These methods provide practicable approaches in det ecting the formation of these compounds in situ, avoiding artefacts that mi ght occur using extraction methods based on polar solvents. In addition to the new methods for in situ detection, an extreme heterogeneity in the reac tion of cells in the Same tissue upon heavy metal stress was observed: whil e some cells are already disintegrating, others still show normal fluoresce nce and photosynthetic activity. Measurements of fluorescence kinetics gave a further hint that in high light intensity a substitution of Mg by heavy metals might take place specifically in PS II reaction centres.