Theoretical phytoextraction rates evaluating the application potential of two Cd accumulators from Crassulaceae for cleaning Cd-contaminated farmland

Phytoextraction based on heavy metal accumulators is a promising strategy for cleaning heavy metal-contaminated arable soils for food safety. In this study, the cadmium (Cd) tolerance and accumulation capacities of two Crassulaceae species were explored via pot experiments. The morphological and physiological results revealed that both Phedimus aizoon and Kalanchoe blossfeldiana could cope with heavy soil Cd contamination (50 mg·kg−1) at least by triggering an antioxidant system for Cd detoxification. After 3 months of growth, the Cd concentrations in the dry leaves, stems, and roots of P. aizoon were 212.0, 104.0, and 83.5 mg·kg−1, and of K. blossfeldiana were 101.5, 53.7, and 125.0 mg·kg−1, respectively. The bioconcentration factor (BCF) and translocation factor (TF) in P. aizoon leaves and stems were greater than the threshold of Cd hyperaccumulators, whereas the TFs in K. blossfeldiana leaves and stems was lower than this threshold. These results revealed that P. aizoon is a potential Cd hyperaccumulator and K. blossfeldiana is a high-Cd-accumulating species. To evaluate the Cd phytoextraction potentials of P. aizoon and K. blossfeldiana in the field, a new index, the theoretical phytoextraction rate (TPR), was introduced through the integration of the Cd accumulation level, plant biomass, and plant shape and size. Interestingly, it was found that K. blossfeldiana rather than P. aizoon had a higher TPR value. These findings demonstrate the decisive effect of plant shape on planting density is of importance for screening ideal phytoextraction resources in addition to plant biomass and heavy metal accumulation levels. This study offers new resources for phytoextraction of Cd-contaminated soils and a novel theoretical method to evaluate phytoextraction efficiency.