Chromium-induced phytotoxicity and its impact on plant metabolism

Chromium (Cr) is a familiar heavy element and is mainly available in nature with a couple of stable oxidation states such as Cr⁺³ and Cr⁺⁶. Its soil–plant transfer is accomplished mostly by sulphate or iron ions which serve as carriers without a transport system. The entry of Cr⁺⁶ into the plant cells induces biochemical, molecular, cytotoxic and genotoxic alterations leading to the inhibition of seed germination, seedling growth and subsequent development. The harmful effects of Cr⁺⁶/Cr⁺³ on plants comprise growth inhibition of roots, shoots and leaves which affect plant biomass and productivity. Cr⁺⁶/Cr⁺³ also causes detrimental consequences on many plant metabolic processes, viz. absorption of water, mineral uptake, hormonal distribution, flowering, senescence, photosynthesis, respiration, carbohydrate, nitrogen, lipid and stress metabolism. Chromium induces phytotoxicity by the generation of free oxygen radicals, while antioxidant enzyme may alleviate the stress. Basically, plant stress resistance may occur through either an avoidance or tolerance mechanisms. The studies in this review highlight the behaviour of Cr in the soil plant system, Cr-speciation, Cr-phytoavailability and Cr-phytotoxicity with fragmentary information on Cr-induced toxicity effects on plant metabolism. Thus, this comprehensive review addresses available information published during the last 3–4 decades. We focussed on the inhibition of growth and development by ions of chromium, starting from seed germination, and with an emphasis on metabolic processes in plant organs. The issue of plant stress resistance mechanisms was also included. We suggest the need to reduce pollution with chromium compounds for a cleaner environment and ensuring the security of sustainable developments.