Variation in Cadmium Accumulation among 30 Cultivars and Cadmium Subcellular Distribution in 2 Selected Cultivars of Water Spinach (Ipomoea aquatica Forsk.)

To reduce the influx of cadmium (Cd), a toxic heavy metal, into the human food chain through vegetable intake, a pot experiment for the selection of a pollution-safe cultivar (PSC) of water spinach (Ipomoea aquatica Forsk.) was carried out. The experiment with 30 tested cultivars revealed that the maximum differences in Cd concentration between the cultivars containing the highest and the lowest Cd were 3.0-3.9-fold under low-Cd treatment (soil Cd = 0.593 mg kg(-1)), 2.7-3.5-fold under middle-Cd treatment (soil Cd 1.091 mg kg(-1)), and 2.6-2.7-fold under high-Cd treatment (soil Cd = 1.824 mg kg(-1)), large enough to define the Ccl-PSCs. Concentrations of Cd in edible parts of six cultivars, cv. Daxingbaigu, Huifengqing, Qiangkunbaigu, Qiangkunqinggu, Shenniuliuye, and Xingtianqinggu, were lower than 0.2 mg kg(-1), the maximum level (ML) of Cd allowed by the Codex Alimentarius Commission (CAC) standard, even under middle-Cd treatment. Accordingly, these cultivars were treated as typical Cd-PSCs. Four cultivars, cv. Jieyangbaigeng, Xianggangdaye, Sannongbaigeng, and Taiwan 308, contained Cd in edible parts exceeding the ML even under low-Cd treatment, and they were defined as typical non-Cd-PSCs. The correlations of the Cd concentrations among the tested cultivars between the three treatments were significant at the p < 0.05 level. A conspicuous difference in Cd subcellular distribution in hydroponic plant tissues between cv. Oiangkunqinggu (a typical Ccl-PSC) and cv. Taiwan 308 (a typical non-Cd-PSC) were observed. Cd absorbed by cv. Oiangkunqinggu seemed to be well-compartmentalized in root and in cell wall fragment, which may be one of the mechanisms leading to its low Cd accumulating property. The results indicated that water spinach, a leafy vegetable, could be easily polluted by soils contaminated with Cd, as 80% of the tested cultivars had exceeded the ML of Cd according to the CAC standard even under the middle-Cd treatment. Much of the evidence obtained from the present study proved that the high Cd-accumulating ability of water spinach is a stable biological property at cultivar level and, thus, is genotype dependent. Therefore, application of the PSC strategy to produce water spinach that is safer to consume is feasible and necessary.