Zinc uptake and shoot partitioning between zinc efficient and inefficient exacum genotypes

Interspecific hybrids of exacum (Exacum L.) display variable responses to zinc nutrition. Our research compared two genotypes with contrasting zinc efficiency phenotypes in terms of root cation exchange capacity (CEC), whole plant ⁶⁵Zn uptake, and the effects of Cu⁺² and Mg ⁺² on ⁶⁵Zn uptake and partitioning to shoot tissues. Results show that the zinc efficient and inefficient genotypes had significantly different root CEC [27.2 and 16.9 cmol(+)·kg^-1 root dry weight (DW), respectively] and whole plant ⁶⁵Zn uptake rates (0.048 and 0.026 μmol·h^-1·g^-1 DW, respectively). In equimolar concentrations to Zn⁺², Cu⁺² reduced Zn ⁺² uptake by approximately 50% in both genotypes while supplemental Mg⁺² enhanced Zn⁺² uptake. In addition, Mg⁺² facilitated a larger proportion of absorbed ⁶⁵Zn to the upper shoot of the efficient genotype. We conclude zinc is absorbed through a specific Zn⁺²/Cu⁺² transporter and that zinc efficiency in exacum is based on a combination of apoplastic and symplastic traits. In addition, a secondary Mg⁺² x Zn⁺² interaction may contribute to the zinc efficiency phenotype.