EFFECT OF ACID DETERGENT LIGNIN CONCENTRATION IN ALFALFA LEAVES ON 3 COMPONENTS OF RESISTANCE TO ALFALFA RUST

As plant breeders select alfalfa (Medicago sativa) genotypes for impro ved digestibility by ruminants, there may be an increased risk of yiel d losses due to plant disease. This is because increases in digestibil ity are often associated with a decrease in lignin content and lignin has been shown to play an important role in plant defense mechanisms a gainst disease and pests. The method most often employed by public and private alfalfa-breeding programs to assess digestibility is acid det ergent lignin (ADL) analysis. ADL concentration was determined for ind ividual alfalfa plants from two different alfalfa populations. Plants representing a range of ADL concentrations within each population were arbitrarily selected, cloned, and used in experiments to quantify the relationship between leaf ADL concentration and components of resista nce to Uromyces striatus, the causal agent of alfalfa rust. Three comp onents of resistance were quantified: infection efficiency (pustules p er cm(2)leaf area), latent period (the time from inoculation to when 5 0% of the pustules were visible), and sporulation capacity (the number of urediniospores produced per pustule). Although analysis of varianc e found significant differences among clones for infection efficiency, latent period, and sporulation capacity, regression analysis revealed little or no relationship between ADL concentration and components of alfalfa rust resistance. F statistics for regression equations and t statistics for slope parameters generally were not statistically signi ficant and when these statistics were significant, coefficients of det ermination (r(2)) values indicated that ADL concentration explained on ly 23% or less of the variation in resistance components.