Assessing Zinc Thresholds for Phytotoxicity and Potential Dietary Toxicity in Selected Vegetable Crops

Heavy metal pollution in soils influences crop yield and quality, and metal accumulation in vegetables may pose a human health risk when consumed. Zinc (Zn), one of the heavy metals, is an essential element for plants, animals, and humans, but it is toxic at high levels. In this study, bioavailability of added Zn in a vegetable garden soil and critical Zn concentrations for phytotoxicity and potential dietary toxicity were determined for Chinese cabbage (Brassica chinensis L.), pakchoi (Brassica chinensis L.), and celery (Apiumg graveolens L.). Different Zn levels (0, 100, 200, 300, 400 mg kg−1 soil, supplied as ZnSO4·7H2O) were added to the soil samples, based on Zn adsorption-desorption characteristics of the soil, the availability of added Zn in the soil decreased with time, with minimal change after 10–12 weeks of incubation. The results from sand and soil culture experiments showed that shoot growth was significantly inhibited at Zn concentrations above 25 mg L−1 in nutrient solution or at DTPA-Zn above 170 mg kg−1 in the soil. The sensitivity to Zn toxicity differed among the three vegetable crops, changing in the order: celery>Chinese cabbage>pakchoi. Zinc concentration in shoots and edible parts varied with Zn supply levels and type of the vegetables. Negative correlations were noted between Zn concentrations in shoots and dry matter yields (r=0.90–0.98, P<0.01), the critical Zn concentrations in plant tissues at 10% reduction of biomass yield (PT10) were 173.1, 167.5, 144.2 and 222.2 mg kg−1 (DW) for Chinese cabbage, pakchoi, celery (stem) and celery (leaf), respectively. Zinc concentrations in the edible parts were positively correlated with available and total Zn in the soil (r=0.91–0.99, P<0.01). Based on the threshold of human dietary toxicity for Zn (20 mg kg−1), the critical concentrations of total and available Zn in the soil were 413 and 244 mg kg−1 for Chinese cabbage, 224 and 75 mg kg−1 for pakchoi, and 272 and 101 mg kg−1 for celery, respectively. These results indicate that some vegetable species like pakchoi might accumulate Zn in edible parts over human dietary toxic threshold before the dry matter yield reduction was observed.