Chromium uptake and consequences for metabolism and oxidative stress in chamomile plants

Chromium Cr(III) toxicity toward chamomile metabolism and oxidative stress-related parameters after 7 days of exposure was studied. Cr preferentially accumulated in the roots and evoked extensive both dose-dependent and dose-independent increase in fluorescence signals of ROS, NO and thiols. Superoxide increased mainly at the highest Cr dose, whereas H2O2 accumulation revealed a discontinuous trend in relation to external Cr supply, and this could be owing to variation in activities of peroxidases. Glutathione and ascorbate quantification, using LC-MS/MS equipment, revealed strong stimulation despite low shoot Cr amounts. Phenolic enzyme activities, except for PAL, were depressed by Cr presence, whereas phenolic metabolites were stimulated, indicating various time dynamics. Among free amino acids, their sum and even proline decreased in the roots, whereas soluble proteins increased. Mineral nutrients showed negligible responses with only Zn and Cu being depleted in both shoots and roots. Cr staining using Cr(III)-specific (naphthalimide-rhodamine) and metal nonspecific (Phen Green) dyes indicated that the former correlated well with AAS quantification of Cr amount. Use of Phen Green is also discussed. These data indicate that Cr-induced oxidative stress is not simply a function of exposure time and applied concentration. Microscopic observations in terms of oxidative stress and chromium uptake are presented here for the first time.