Growth, metal partitioning and antioxidant enzyme activities of mung beans as influenced by zinc oxide nanoparticles under cadmium stress

Context: Cadmium (Cd) toxicity and zinc (Zn) deficiency are of major concerns for crop growth and quality. Moreover, their interactive effects exert some controversial reports. Aims: The effects of zinc oxide nanoparticles (ZnO NPs) and Cd on growth, physiology, and metal distribution in mung beans (Vigna radiata L.) was investigated. Methods: Seven-day-old seedlings were treated with Zn (0, 1, 2, 4, 8, 16 and 32 μM) and Cd (0, 0.5, 1 μM) for 14 days. Key results: Photosynthetic pigments, antioxidant enzyme activities, dry matter yield and metal concentration in tissues were significantly influenced by ZnO NPs and Cd. Considered on its own as a main effect, Zn application (16 μM) enhanced its accumulation in roots, stem and leaf by about 33-fold (314 mg kg−1), 10-fold (60.6 mg kg−1) and 17-fold (110.8 mg kg−1), respectively, compared to control. However, accumulation was slower for interactions with Cd. While leaf Zn increased approximately 27 times (180 mg kg−1) at 32 μM Zn, its interactions with lower and higher Cd increased only 6-fold (41.2 mg kg−1) and 3-fold (21.4 mg kg−1), respectively. Added ZnO NPs up to 4 μM under Cd contamination elevated the leaf Cd, which was restricted by higher supply. However, Cd accumulation in stem and root consistently rose, indicating a synergistic effect. ZnO NPs induced an upregulation of antioxidant enzymes to avert oxidative stress and maintain growth performance. Implications: These findings may be suitable for formulating nanomaterials of desired particle sizes and testing on other crop to remediate Cd.