Genotypes of lucerne (Medicago sativa L.)show differential tolerance to manganese deficiency and toxicity when grown in bauxite residue sand

The physical and chemical characteristics of bauxite residue sand (BRS) affect the availability of a number of nutrients to plants, especially manganese (Mn). Lucerne (Medicago sativa L.) has been chosen as a BRS revegetation species because of its deep-rooting habit and tendency to tolerate moderately alkaline and saline soils, even though it is still prone to Mn deficiency stress. Sixteen commercially available lucerne genotypes were grown in BRS after addition of 5, 50 or 500 mug Mn g(-1) BRS in a glasshouse. Manganese deficiency and toxicity symptoms were observed in 5 and 500 mug g(-1) treatments, respectively. Symptom expression varied in severity between genotypes. Relative tolerance to Mn deficiency was defined by shoot dry weight at 5 mug Mn g(-1) as a percentage of shoot dry weight at 50 mug Mn g(-1). Salado, a genotype tolerant to Mn deficiency, and Sirosal, a genotype intolerant to Mn deficiency, were then grown with 0, 10, 20, 50, 100, 200 or 800 mug Mn g(-1) BRS and found to have critical shoot Mn concentrations of 17.7 (Salado) and 26.6 mug g(-1) (Sirosal). The use of genotypes with high relative Mn deficiency tolerance is recommended to help improve sustainability of BRS revegetation as well as to improve productivity on Mn-fixing agricultural soils.