Very little is known about the relations between aloes and soil metals, despite evidence that aloes often form dense populations on metalliferous soils. This study targeted eight rock outcrops where aloes dominated the vegetation to determine whether these succulents have a preference for soils rich in heavy metals and whether they accumulate any of these metals in their leaf tissue. Soil analyses suggested that densely populated rock outcrops are rarely characterised by high concentrations of heavy metals. Analyses of leaf material revealed no metal hyperaccumulators, with most species acting as excluders of most metals. The only metals to be reflected in leaf material to some degree across populations were copper (mean of 4 µg g-1), iron (mean of 208 µg g-1), manganese (mean of 342 µg g-1) and zinc (mean of 31 µg g-1)(all values are based on leaf dry weight). Aloe greatheadii plants from the ultramafic-peralkaline Koedoesfontein Complex in the Vredefort Dome contained the highest concentrations of Mn (558 µg g-1) and were subjected to further sampling across four geological substrates to determine the limits of its accumulation ability (mean of 855 µg g-1 on wehrlite). This aloe was found to be non-hyperaccumulating, but tolerant to elevated Mn concentrations in its tissue. The uptake of Mn from the soil was not so much influenced by soil available Mn, but rather by decreasing pH, and increasing electric conductivity and soil potassium.
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Vol. 2018 • No. 25