Different stoichiometric ratios of Ca and Cd affect the Cd tolerance of Capsicum annuum L. by regulating the subcellular distribution and chemical forms of Cd

The effect of calcium (Ca)-cadmium (Cd) interactions on the plant Cd bioaccumulation process may be closely related to the ecological Ca/Cd stoichiometry in the substrate. However, owing to the complexity of plant absorption, accumulation mechanisms and influencing factors, the mechanism of Ca-mediated Cd bioaccumulation and Cd tolerance in Capsicum is still unclear. In this study, the bioaccumulation, subcellular distribution and chemical forms of Cd in Capsicum were analysed via pot experiments to reveal the Ca-mediated Cd bioaccumulation process and its detoxification mechanism under different Ca/Cd stoichiometric ratios. The results revealed that an increase in the substrate Ca/Cd ratio promoted the accumulation of Cd in the roots; restricted the transport of Cd to the stems, leaves and peppers; and promoted the accumulation of Cd in the aboveground leaves but decreased its accumulation in edible parts. Cd was enriched mainly in the cell wall and cell-soluble fraction in each tissue and was enriched in only 1 %-13 % of the organelles. The accumulation of Cd in the cell wall and cell-soluble fractions of roots treated with different Ca concentrations increased by 56.57 %-236.98 % and 64.41 %-442.14 %, respectively. The carboxyl, hydroxyl and amino groups on the root cell wall play important roles in binding and fixing Cd2+. Moreover, the increase in the Ca content also increased the proportion of pectin and protein-bound Cd (F-(NaCl)), insoluble phosphate-bound Cd (F-(C)) and insoluble oxalate-bound Cd (F-(HCl)) in the roots, stems and leaves and reduced the proportion of highly active chemical forms such as inorganic acid salt-bound Cd (F-(E)) and water-soluble phosphate-bound Cd (F-(W)). Our study revealed that the bioaccumulation of Cd in Capsicum was influenced by the Ca/Cd ratio and that Ca could alleviate Cd stress by regulating the subcellular distribution and chemical form ratio of Cd in different tissues where the cell wall plays an important role in Cd tolerance and detoxification.