Characterization of the stimulating effect of low-dose stressors in maize and bean seedlings.

The effect of some more or less harmful compounds like Cd, Pb, Ni, Ti salts and DCMU at low concentrations on the development of chloroplasts in maize and bean seedlings was investigated. Chlorophyll content, chlorophyll a/b ratio, photosynthetic activity (14CO2 fixation), chlorophyll-protein composition of thylakoid membranes, fluorescence spectra of chloroplasts, fluorescence induction parameters of leaves and electron microscopic structure of maize and bean chloroplasts as well as growth parameters were studied. Stimulation of chlorophyll synthesis and photosynthetic activity was observed at different intervals during all of the treatments, while chlorophyll a/b ratios and fluorescence properties of leaves or chloroplasts did not change considerably except in DCMU treated plants. Heavy metal treatments increased the amount of photosystem I and light-harvesting complex II, while decreased amount of photosystem I and higher amount of light-harvesting complex II was found in DCMU treated thylakoids. Electron microscopy showed only sligth differences in the morphology of chloroplast lamellar system (mostly in DCMU treated plants), while the status of the plasmalemma and tonoplast seemed to be altered as a result of certain metal treatments. Results showed the expression of a cytokinin-like effect on the development of chloroplasts. It is assumed, that these low-dose stressors generate non-specific alarm reactions in plants, which may involve changes of the hormonal balance.