Comparison of Two Selection Methods for Tolerance to Acidic, Aluminum-rich Soil in Alfalfa

In acid soils (pH(water) <5), Al becomes toxic, affecting alfalfa (Medicago sativa L.) root growth and development. In the southeastern United States, Al toxicity and its associated deficiencies are the most significant factors limiting alfalfa production. This study aimed to compare genetic gain for acid or Al tolerance as assessed by seedling biomass production in acidic soil under green-house conditions using phenotypic recurrent selection with gridding (PR SG) and among and within family selection (AWFS) in 'Bulldog 805' and PR SG in the germplasm 'Cultivated Alfalfa at the Diploid Level' (CADL) for two cycles. Selection was based on aerial biomass production or visual score 60 d after germination in Al-rich acidic (unlimed, UL) soil and limed soil. Cycles 0, 1, and 2 from each method and population were evaluated for their root and shoot dry weight (RDW and SDW) after 60 d in UL and limed soils. Unlimed/limed soil RDW and SDW ratios were computed. CADL did not respond to selection, probably because of the lack of initial variability for Al tolerance. Bulldog 805 selected in limed soil did not result in any improvement in limed or UL conditions but selection in UL soil improved performance in acidic soil after two cycles and did not negatively affect growth in limed soil. In Bulldog 805, PR SG had the largest response per cycle: over 20% over Cycle 0 (C0). Direct selection in UL soil was the best way to improve growth in acidic Al-rich soils. These results need to be validated in the field.