A Comparison of Hydroponics, Soil, and Root Staining Methods for Evaluation of Aluminum Tolerance in Medicago truncatula (Barrel Medic) Germplasm

Aluminum toxicity and soil acidity are major constraints in alfalfa (Medicago sativa subsp. sativa) production in the world. Despite intense research efforts, neither an effective screening procedure nor an Al-tolerant alfalfa germplasm is available. This dictates the need for identifying a new source of Al-tolerant genes in the closely related species M. truncatula (barrel medic). Our objectives were to compare three Al tolerance screening methods: (i) a seedling-based hydroponics method, (ii) a soil-based plant method, and (iii) an Al-stressed seedling-based lumogallion root staining method in 32 M. truncatula accessions. The soil system compared the genotypes for dry root and shoot weights in unlimed soil and for relative weights. The lumogallion root staining of Al-stressed seedlings compared the genotypes for fluorescence intensity of Al-bound lumogallion within the root tips. In the hydroponics system, the genotypes were compared for root elongation and relative growth. The three methods were different from each other, with altered rankings for genotypes across the methods. The soil assay demonstrated a higher capacity for discriminating Al response among genotypes with a higher reproducibility. Most of the genotypes that were Al-tolerant in soil were also Al-tolerant using the hydroponics and root staining methods. The results suggested that a combination of soil-based and hydroponics screening might be essential to identify Al-tolerant genotypes possessing multiple Al tolerance mechanisms.