An in vitro screening system to assess aluminum toxicity in sugarcane (Saccharum spp.) cultivars

Mutagenic breeding is an approach being developed especially for those characteristics that are not inherited through conventional genetic hybridization. Soil acidity and aluminum (Al³⁺) toxicity limit sugarcane production, particularly in leached soils in the coastal cultivation regions in South Africa. An in vitro screening procedure was developed based on plantlet re-rooting in Al³⁺-containing medium to select putative Al³⁺ tolerant variants produced by induced mutagenesis. A preliminary test did not show any difference among three cultivars in their ability to re-root in the presence of Al³⁺. Subsequent work focused on Saccharum sp. 'NCo376' and two Al³⁺ concentrations (15 and 45 mM), representing LC ₅₀ and LC₉₀ re-rooting inhibition. In an attempt to increase the stringency of the selection conditions, the possible effects of inorganic nitrogen (N) forms in the culture media on Al³⁺ toxicity were investigated. The form of N affected root regrowth and citrate exudation. Sugarcane plantlets showed an uptake preference for ammonium (NH₄⁺) over nitrate (NO₃⁻), and 10 and 20 mM NH₄⁺ appeared to alleviate Al³⁺ toxicity compared with NO₃⁻. Although in agricultural soils NO₃⁻ is the dominant form of N, it can readily leach into the sub-soil, in which deep roots encounter Al³⁺ toxicity in the presence of NO₃⁻. Therefore, the ability to withstand the toxic effects of Al³⁺ in the presence of NO₃⁻ could be beneficial for N use efficiency and access to water at depths during drought.