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 mu M) and Cd (0, 0.5, 1 mu M) 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 mu M) 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 tM 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 mu M 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.