Tolerance and accumulation of cobalt in three species of Haumaniastrum and the influence of copper

The two metallophytes did not grow in the absence of supplements, i.e., H. katangense required high concentrations of copper (Cu) to develop and H. robertii growth was conditional with regard to both Cu and Co. This discrepancy in metal requirement for growth originates from differences in ecological niches between the facultative and the absolute metallophytes. The beneficial effect of Co on plant growth was observed for all three species of Haumaniastrum. In the non-metallophyte, Co was beneficial at low concentrations and became toxic above 20 μM. In the two metallophytes the positive effect of Co was conditional on supplementation of Cu, and no toxicity was observed in the range of tested Co concentrations (3-150 μM). Co was essential to the growth of the absolute metallophyte. Mineral bulk analysis of plant parts by ICP-MS showed that Co accumulated more and was more mobile than Cu in all species investigated. This was confirmed by LA-ICP-MS mapping of root and leaf tissues, showing localisation of the two trace elements. H. katangense hyperaccumulated Co when exposed to 6 μM and H. robertii when exposed to 20 μM. None of the species studied hyperaccumulated Cu. Furthermore, Co and Cu did not occur in the same root tissues in any of the three Haumaniastrum species studied. In leaves of H. villosum and H. robertii, Cu and Co accumulated in different tissues while in those of H. katangense, Cu and Co were found to co-localize in the mesophyll cells. In summary our study supports the existence of effective Co tolerance mechanisms in the two Haumaniastrum metallophytes, mechanisms which are at least partially different from those for Cu and involve fitness costs.