Cadmium Toxicity in Plants: Recent Progress on Morpho-physiological Effects and Remediation Strategies

Cadmium (Cd) is one of the non-essential, highly toxic environmental pollutants worldwide causing serious environmental and agricultural problems. Elevated Cd doses are carcinogenic to humans. It is ranked seventh in the list of top 20 toxic metals and classified as a group 1 carcinogen. The median range of Cd dietary intake (66.5–116 µg Cd kg−1 body weight per month) is much higher than maximum limit (25 µg Cd kg−1 body weight per month) reported by FAO/WHO. Toxicity of Cd causes a range of damages to plants from germination to yield; however, the extent of damage is concentration and time-dependent. Reduction in seed germination and plant growth is primarily due to Cd interference with enzymatic and photosynthetic activities and membrane damage. Cadmium exposure at higher rates disturbs the nutritional and water relations of plants and causes oxidative damage. Moreover, Cd-induced structural changes in the photosynthetic apparatus disturb the yield of plants. In this review, adverse effects of Cd on seed germination, stand establishment, plant growth, uptake and assimilation of nutrients, enzymatic activities, ultra-structural and oxidative damages, changes in antioxidant defense system and stress proteins, carbon metabolism, and yield formation are reported. Moreover, Cd dynamics in soil rhizosphere and factors affecting Cd dynamics in soil have also been discussed. Furthermore, remediation strategies (physical, chemical, biological, and amendments) to decontaminate Cd-polluted soils have also been described in this review. Through phytoremediation, Cd can be extracted and stabilized in the soil while through microbes Cd can be sequestrated into their bodies. Increased Cd uptake in hyperaccumulator plants to remediate and convert the toxic form of Cd into nontoxic forms. While in chemical remediation, Cd can be washed out, immobilized and stabilized in the soil through chemical amendments. Bioremediation of polluted sites is considered effective and reliable due to its eco-friendly features. Moreover, Cd uptake and toxicity in rice can be decreased by proper application of essential nutrients such as nitrogen, zinc, iron, and selenium in Cd contaminated soils. The organic amendments may help through an increase in soil pH, adsorption in its functional groups, the formation of complexations, and the conversion of exchangeable to residual forms. Adoption of some agricultural practices are also found to be effective in reducing the Cd uptake and accumulation in plants and harvesting quality food from Cd contaminated soils.