An investigation of zinc isotope fractionation in cacao (Theobroma cacao L.) and comparison of zinc and cadmium isotope compositions in hydroponic plant systems under high cadmium stress

This study aims to establish whether zinc (Zn) and cadmium (Cd) share similar physiological mechanisms for uptake and translocation in cacao plants (Theobroma cacao L.). Multiple-collector ICP-MS was used to determine the Zn stable isotope compositions in the roots, stems and leaves of 19 diverse cacao genotypes grown in hydroponics with 20 mu mol L-1 CdCl2. Additional plants of one genotype were grown in hydroponic solutions containing lower Cd concentrations (0 and 5 mu mol L-1 added CdCl2). Regardless of the Cd concentration used in the exposures, the Zn stable isotope compositions show the same systematic patterns in plant organs, with delta Zn-66(root) > delta Zn-66(stem) > delta Zn-66(leaf) (delta Zn-66 denotes relative differences in Zn-66/Zn-64 ratios in parts per thousand). The mean Zn stable isotope fractionation between the plants and the hydroponic solutions was epsilon Zn-66(uptake) = -1.15 +/- 0.36 parts per thousand (2SD), indicating preferential uptake of isotopically light Zn by plants from the hydroponic solution. The mean stable isotope fractionation factor associated with translocation of Zn from roots to shoots, epsilon Zn-66(seq-mob) = + 0.52 +/- 0.36 parts per thousand (2SD), shows that isotopically heavy Zn is preferentially sequestered in the cacao roots, whilst isotopically light Zn is mobilised to the leaves. A comparison with the Cd stable isotope compositions of the same plants shows that both isotopically light Zn and Cd are preferentially taken up by cacao plants. In contrast to Zn, however, the cacao roots retain isotopically light Cd and transfer isotopically heavy Cd to the leaves.