Chromium Toxicity in Crop Plants: Mechanisms, Physiological Responses, and Remediation Strategies: A Review

Abstract

Chromium (Cr) contamination has emerged as a serious environmental concern driven by rapid industrialization and diverse anthropogenic activities. Chromium enters agricultural soils and water systems primarily through effluents from leather tanning, electroplating, metal processing, pigment manufacturing, and mining operations. Among its various oxidation states, hexavalent chromium [Cr(VI)] is considerably more toxic, mobile, and bioavailable than its trivalent counterpart [Cr(III)], with Cr(VI) being absorbed more readily by plants and forming free radicals during intracellular reduction. Elevated chromium concentrations adversely affect seed germination, plant growth, photosynthesis, mineral nutrition, water relations, and cellular metabolism across diverse crop species including wheat, rice, maize, sunflower, and legumes. A key mechanism underlying this phytotoxicity is the induction of oxidative stress via the generation of reactive oxygen species (ROS), including superoxide radicals (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (OH·), leading to membrane damage through lipid peroxidation, protein degradation, and DNA alterations. In response, crop plants deploy a range of physiological and biochemical defenses, most notably the upregulation of antioxidant enzyme systems including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX). Certain plant species capable of accumulating and tolerating chromium, such as Brassica juncea, Helianthus annuus, Sorghum bicolor, and Vetiveria zizanioides, hold significant promise for phytoremediation applications. Recent advances have identified novel amelioration strategies including the application of nanoparticles, plant growth regulators, chelating agents, and beneficial microorganisms that enhance chromium tolerance and remediation efficiency. This review synthesizes current knowledge on the sources, environmental distribution, uptake mechanisms, physiological and biochemical impacts, and remediation strategies associated with chromium toxicity in crop plants, with emphasis on molecular mechanisms and practical applications for sustainable agriculture.