Screening Methods to Develop Alfalfa Germplasms Tolerant of Acid, Aluminum Toxic Soils

Abstract

Soil acidity and aluminum (Al) toxicity are major problems limiting performance of alfalfa (Medicago saliva 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 germimation 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 root and shoot growth in unlimed soil with a limed germination layer. No population had poorer performance than GATE 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.

Screening Methods for Aluminum Tolerance in Alfalfa

“…Our goal was to develop rapid and reliable methods to characterize and quantify Al tolerance that could be scaled to evaluate alfalfa populations. Previously used methods were time and labor intensive (Bouton, 1996;Dall' Agnol et al, 1996;Pan et al, 2008;Scott et al, 2008;Sledge et al, 2005) and not suitable for high-throughput evaluations. We developed two whole-plant assays, one in media and one in soil, to assess performance in acid and Altoxic conditions.…”

“…Both limed and unlimed soils were fertilized with 0.340 g kg (Bouton, 1996). The soil was air dried and sieved using a 3-mm screen (Dall' Agnol et al, 1996) before fi lling the conetainers. Conetainers were watered every 1 to 2 d using distilled water.…”

“…Previously described methods to evaluate Al tolerance in alfalfa include a callus-based assay in tissue culture (Parrot and Bouton, 1990), hydroponic solution assays (Pan et al, 2008;Scott et al, 2008;Sledge et al, 2005), and a soil-based assay (Bouton, 1996;Dall' Agnol et al, 1996).…”

“…This limitation may partially explain the low correlation of this cell-based assay with results from soil-based assays (Dall' Agnol et al, 1996). The soil-based assay evaluates Al tolerance as the ratio of root growth in limed soil to the root growth in unlimed soil of plants grown in Styrofoam cups but is time consuming and labor intensive (Bouton, 1996;Dall' Agnol et al, 1996) and therefore not amenable for phenotyping large numbers of genotypes. Hydroponic systems to screen for Al responses have been implemented in legumes (Pan et al, 2008;Sledge et al, 2005;Zhang et al, 2007) and other crops species (Ma et al, 1997;Magalhaes et al, 2004;Raman et al, 2002).…”

Breeding lucerne for persistence

“…The screening and selection protocols employed a direct use of acid, Al-toxic soil and whole-plant expression was mainly based on herbage yield and/or root growth to identify tolerant genotypes to use as parents for synthetics (Devine et al 1990;Dall'agnol et al 1996); although later, screening in cell culture (Parrot and Bouton 1990;Bouton and Parrott 1997) and solution culture (Zhang et al 2007) were also found to be useful as screening protocols.…”