Hexavalent chromium leads to differential hormetic or damaging effects in alfalfa (Medicago sativa L.) plants in a concentration-dependent manner by regulating nitro-oxidative and proline metabolism

Chromium has been proven to be extremely phytotoxic. This study explored the impacts of increasing Cr(VI) exposure (up to 10 mg L^-1 K₂Cr₂O₇) on the growth and development of alfalfa plants and adaptation responses employed, in an environmentally relevant context. The threshold concentration of K₂Cr₂O₇ in irrigation water beyond which stress responses are initiated is 1 mg L^-1. Lower Cr(VI) exposure (0.5 mg L^-1 K₂Cr₂O₇) induced hormesis, evident through increased biomass and larger leaves, likely mediated by increased NO content (supported by elevated NR enzymatic activity and overexpression of NR and ndh genes). Elevated Cr(VI) exposure (5 and 10 mg L^-1 K₂Cr₂O₇) resulted in reduced biomass and smaller leaves, and lower levels of photosynthetic pigment (10 mg L^-1 K₂Cr₂O₇). Higher levels of lipid peroxidation, H₂O₂ and NO contents in these plants suggested nitro-oxidative stress. Stress responses included increased SOD and CAT enzymatic activities, further supported to some extent by MnSOD, FeSOD, Cu/ZnSOD and CAT transcripts levels. GST7 and GST17 gene expression patterns, as well as proline content, P5CS enzymatic activity and corresponding P5CS and P5CR gene expression levels emphasized the role of proline and GSTs in the adaptation responses. Results highlight the importance of managing Cr(VI) levels in irrigation water.

Keywords: Alfalfa; Antioxidants; Chromium; Nitric oxide; Phytotoxicity; Proline.