Cadmium uptake and transport in vegetables near a zinc-lead mine: Novel insights from Cd isotope fractionation

In this study, Cd isotope analysis was conducted on drought-tolerant (cowpea and sesame) and less drought- tolerant vegetables (water spinach, green pepper, and mung bean) to elucidate the mechanisms underlying Cd uptake and transport. Cd isotopes in plants were identical to or lighter than those in the available pool and exhibited negative fractionation from roots to straws ( Delta 114/110 Cd =-0.22 %o to-0.17 %o ) in drought-tolerant vegetables, whereas contrasting results were obtained for less drought-tolerant vegetables ( Delta 114/110 Cd =-0.050 %o to 0.39 %o ). Positive Cd isotope fractionation from straws to fruits in drought-tolerant vegetables ( Delta 114/110 Cd = 0.33 %o f 0.03 %o and 0.10 %o f 0.03 %o , respectively) was observed, whereas negligible or negative fractionation was found in less drought-tolerant vegetables ( Delta 114/110 Cd = 0.01 %o f 0.04 %o and-0.34 %o f 0.02 %o , respectively). The vast secretion of organic acids might have led to positive available pool-to-roots and negative roots-to-straws isotope fractionation in drought-tolerant vegetables. In contrast, preferential xylem transport resulted in negative straws-to-fruits isotope fractionation in less drought-tolerant vegetables. This study demonstrated that Cd isotope fractionation in the soil-plant system is associated with plant drought tolerance, and drought-tolerant and less-tolerant plants developed a distinct Cd detoxification mechanism, corresponding to a reversed fractionation of Cd isotopes.