Comparisons of cadmium subcellular distribution and chemical forms between low-Cd and high-Cd accumulation genotypes of watercress (Nasturtium officinale L. R. Br.)

Mechanisms of low cadmium (Cd) accumulations in genotypes of watercress are poorly investigated. In order to improve the understanding, genotype differences in subcellular and biochemical properties of Cd were studied. Two hydroponic experiments were conducted to compare the subcellular distribution and chemical forms of Cd in shoots and roots between low-Cd genotypes (G18 and G20) and high-Cd genotypes (G4 and G5) of watercress under Cd exposures of T1 (8.93 mu mol L-1), T2 (44.64 mu mol L-1) and T3 (89.29 mu mol L-1). High shoot Cd concentration exceeding 100 mg kg(-1)DW even under T1 treatment indicated a high risk of Cd pollution in watercress. Differences in subcellular distributions and chemical forms between low-Cd and high-Cd genotypes were significant under T1 and T2 treatments. Proportions of Cd in cell wall fraction were the highest in both shoots (48-62 %) and roots (40.1-83.3 %) and were higher in high-Cd group than in low-Cd group under T1 treatment, but reversed under T2 and T3 treatments. It was found that to convert Cd into insoluble phosphate precipitation and pectate- or protein-bound forms may be the primary means for reducing Cd mobility and toxicity in watercress. Genotype differences in Cd subcellular distributions responding to the Cd stresses were more significant than those in Cd chemical forms. The intense Cd accumulation of watercress demonstrated the necessity of adopting and breeding low-Cd cultivars to reduce Cd contamination in the crop. Cd subcellular distribution and chemical forms were related to the genotype-dependent differences in Cd accumulation of watercress.