Shoot Accumulation and Subcellular Distribution of Zinc and Cadmium in Lettuce Root at Various Cd2+ and Zn2+ Activities in the Root-Growing Media

Abstract

Background

Zinc (Zn) and cadmium (Cd) compete with each other for adsorption on the cell wall–charged sites, translocation via plasma membrane transporters, and storage in vacuoles. Therefore, the subcellular distribution of Cd is suggested to be under the influence of Cd:Zn ratio in root-growing media.

Aim

Subcellular fractionation of Cd and Zn in lettuce roots and its contribution to shoot Cd and Zn accumulation at low to phytotoxic levels of Cd²⁺ and Zn²⁺ activities was investigated.

Methods

An EGTA-buffered nutrient solution was used to provide the desired activity of free metal cations. Higher distributions of Cd in root cell wall in comparison with soluble and organelle fractions demonstrated Cd retention capacity of root apoplasmic spaces.

Results

At each level of Cd²⁺ activity, the elevated activity of Zn²⁺ in the root growth solution caused a significant reduction of Cd concentration in cell wall and soluble fractions, whereas organelles’ Cd fraction increased. In contrast, at each constant Zn²⁺ activity, by increasing Cd²⁺ activity in nutrient solution, cell wall Zn fraction was decreased, whereas the soluble and organelles’ Zn fractions were increased or remained unchanged. The highest concentrations of Cd in the cell wall (34.6 µg g⁻¹ FW), organelles (5.64 µg g⁻¹ FW), and soluble (12.3 µg g⁻¹ FW) fractions were found at the Cd-to-Zn ratios of 0.8, 0.1, and 0.8, respectively.

Conclusions

The subcellular partitioning of Cd in lettuce root is highly influenced by Cd:Zn ratio in the root media, and increasing Zn substantially reduced the overall uptake-translocation of Cd to shoots.