Screening methods to develop alfalfa germplasms tolerant of acid, aluminum toxic soils

Soil acidity and aluminum (Al) toxicity are major problems limiting performance of alfalfa (Medicago sativa L.) in many parts of the world, but neither an effective screening procedure nor a tolerant cultivar is available. The objective of this study was to evaluate different screening methods for selection of acid soil tolerant alfalfa germplasms in the greenhouse during 1991-1994. The general screening methods included selection in unlimed soil, selection in unlimed soil containing a limed germination layer, selection for either tolerance or sensitivity to acid soil stress with Al toxicity in tissue culture, selection in unlimed soil with tandem selection for Al tolerance in tissue culture, and selection in unlimed soil containing a limed, fertilized germination layer with tandem selection for Al tolerance in tissue culture. All selected populations and checks were evaluated during 1994 in greenhouse cups containing the following soil treatments: (i) cups filled with unlimed soil, (ii) cups filled with limed soil, and (iii) cups filled with unlimed soil containing a germination layer of limed soil. Most of the selected populations possessed better root and shoot growth than the original base population (GA-TE) in unlimed soil, but only the population selected in unlimed soil showed better mot and shoot growth in unlimed soil with a limed germination layer. No population had poorer performance than GA-TE in limed soil. Selection in cell culture for Al toxicity tolerance did not improve tolerance per se, but selection for Al sensitivity enhanced sensitivity. In terms of success, resources, and time, screening in unlimed soil was the most effective method to improve acid soil stress tolerance.