Assessment of Al³⁺ availability in callus culture media for screening tolerant genotypes of Cynodon dactylon

Aluminium-tolerant genotypes of Cynodon dactylon are potential candidates for the vegetation of gold mine tailings in South Africa. As a prerequisite to in vitro selection of tolerant genotypes, this work aimed at assessing and adapting micropropagation media to ensure Al³⁺ activity and toxicity. This was investigated using MINTEQA2, a chemical equilibrium speciation model. The maximum Al³⁺ activity achieved in any medium was 7.5 μM. Of the seven published media investigated, four never achieved an activity greater than 4 μM at 3-4 mM aluminium. The most appropriate medium was that of Yamamoto et al. (1996) (modified MS without KH₂PO₄ and EDTA), as it showed an increasing range of Al³⁺ activities from 2 to 7.5 μM at aluminium concentrations from 0.25-2.5 mM. An improved modified MS formulation retaining phosphate was investigated because phosphate is an MINTEQA2, no reduction in Al³⁺ activity by phosphate was detected in standard MS medium at pH 4. Through further simulations a new modified MS medium was derived with 1 mM SO₄^2- and no EDTA at pH 4, which gave the maximum Al³⁺ activity (7.5 μM) at 2 mM aluminium. This medium gave the highest Al³⁺ activities for the 0.25-2 mM concentration range of all the tested formulations, including the seven published media. It also resulted in significantly higher callus growth rates than standard MS media and other tested media. This new medium is currently being used to screen C. dactylon for aluminium tolerance at pH 4.