[Effect of Foliar Zinc Application on Bioaccessibility of Cadmium and Zinc in Pakchoi]

Abstract

Two cultivars of pakchoi with different cadmium (Cd) accumulation were grown in nutrient solutions containing low and sufficient zinc (Zn) levels. ZnSO₄ and ZnNa₂ EDTA were applied as foliar fertilizers. The bioaccessibilities of Cd and Zn in pakchoi were assessed by the in vitro digestion method, and the bioaccessible established daily intakes (BEDI) of Cd and Zn from pakchoi were calculated. The effect of foliar zinc application on concentrations, bioaccessibilities, and BEDIs of Cd and Zn in pakchoi was evaluated. Results show that the Cd concentrations in shoots of the tested pakchoi cultivars under sufficient Zn condition were significantly lower than those under low Zn condition, and foliar application of ZnNa₂ EDTA significantly decreased the Cd concentrations of pakchoi. The tested pakchoi cultivars with sufficient Zn had a significantly higher mean shoot Zn concentration than those with low Zn. Foliar Zn treatments significantly increased shoot Zn concentrations of pakchoi, with the highest in the ZnSO₄ treatment. Cd bioaccessibility in the tested pakchoi cultivars with sufficient Zn was significantly lower than that with low Zn. Foliar applied Zn could significantly reduce Cd bioaccessibility in the gastric phase, with a maximal reduction of 35.81% compared to the control. Foliar treatment with ZnSO₄ could significantly decrease Cd bioaccessibility in the small intestinal phase, with a maximal reduction of 59.24% compared to the control. Foliar Zn treatments reduced significantly the Zn bioaccessibility of pakchoi in the gastric and small intestinal phases, with a maximal reduction of 68.90% compared to the control. The reduction of Zn bioaccessibility was higher in the ZnSO₄ treatment than in the ZnNa₂ EDTA treatment. Via the consumption of the Cd-contaminated common cultivar Hanlv, the BEDI values of Cd were higher than that of the provisional tolerable daily intake (PTDI) standard of WHO [0.83 μg·(kg·d)^-1]. However, via the consumption of the low-Cd accumulating cultivar Huajun or pakchoi with foliar application of ZnSO₄, the BEDI values of Cd decreased significantly and were below the PTDI value. Via the consumption of pakchoi in the control or low-level ZnNa₂ EDTA treatments, the BEDI values of Zn were below that of the recommended nutrient intake for Zn. However, via the consumption of pakchoi with foliar application of ZnSO₄ or high-level ZnNa₂ EDTA, the BEDI values of Zn were higher than that of the recommended nutrient intake and met the human needs for Zn from vegetables. Under the ZnSO₄ treatment, the BEDI values of Zn from pakchoi were the highest. In conclusion, foliar zinc application could significantly reduce the bioaccessibilities of Cd and Zn in pakchoi and the BEDI values of Cd and increase the BEDI values of Zn. Foliar application with ZnSO₄ was the most suitable treatment to reduce Cd intake and increase Zn intake from pakchoi.

Keywords: Brassica chinensis; bioaccessibility; cadmium; foliar zinc fertilization; human health risk; in vitro digestion method.