Zinc–Cadmium Interaction Affects Growth, Metal Accumulation, Photosynthesis, and Antioxidant Activity in Two Mung Bean Varieties

Zinc (Zn) is an essential element, and zinc sulfate (ZnSO4) has been extensively applied to crops to combat its deficiency. Moreover, adding Zn brings benefits by reducing cadmium (Cd) accumulation by crops, although its impact on Cd uptake is inconsistent in the literature. Two promising mung bean (Vigna radiata) varieties, released as Jade-AU and Celera II-AU by the Australian Mung Bean Association, were grown hydroponically to examine the effects of ZnSO4 application on Cd accumulation in tissues. This was done by measuring photosynthetic pigments, plant biomass production, Zn and Cd accumulation, and antioxidative enzyme systems. Seven-day-old seedlings were exposed to different levels of Zn (0, 1, 2, 4, and 8 µM) and Cd (0, 0.5, and 1 µM) for 14 days. While the addition of Zn significantly (p < 0.01) enhanced photosynthetic pigments, plant biomass, and Zn and Cd concentrations in tissues, the prevalence of Cd revealed damaging outcomes. Zn accumulation in shoots increased as much as 6-fold (95 mg kg⁻¹ in Jade-AU) with the application of Zn alone, while adding Cd restricted the trend. In contrast, Zn accumulation in roots rose 28-fold (146 mg kg⁻¹ in Celera II-AU) when both Zn and Cd were present. Cd concentration in the shoot rose by as much as 22-fold (1.8 mg kg⁻¹ in Celera II-AU) with the application of Cd alone and virtually tripled (5.42 mg kg⁻¹ and 6.99 mg kg⁻¹, in Jade-AU and Celera II-AU, respectively), when Zn and Cd interacted synergistically. Zn addition restricted Cd accumulation in roots under low-Cd level (0.5 M), while it was unable to limit the translocation of Cd to shoots. Zn application significantly improved antioxidant enzyme activities, with increases observed up to 13% for APX, 41% for CAT, 40% for POD, and 35% for SOD relative to the control. This refers to both varieties of mung beans, indicating the benefit of Zn application in nullifying oxidative stress and maintaining plant growth. Of the two varieties, Jade-AU displayed less Cd translocation from the roots to shoots and produced higher biomass, demonstrating its Cd-tolerant advantages.