DIFFERENTIAL TOLERANCE OF GENOTYPES OF MEDICAGO-TRUNCATULA TO LOW PH

Growth and nodulation of five Moroccan ecotypes of Medicago truncatula Gaertn. collected from soils of different pH, Medicago truncatula cv. Cyprus and Medicago murex Willd. cv. Zodiac, were compared in soil and solution at different pH values. The seven genotypes tested varied greatly in their ability to grow and nodulate on acid soils. Increasing soil pH from 4.5 to 5.4 increased the shoot dry weight of Cyprus and three M. truncatula ecotypes but not Zodiac and two M. truncatula ecotypes. Cultivar Cyprus, with a shoot dry weight at pH 4.5 only 58% of that at pH 5.4, was the most affected by acidity. Nodulation was the most sensitive step as nodule numbers decreased with increasing acidity for sensitive genotypes. Several genotypes were more able to nodulate at low pH than the commercial cultivar Cyprus. Acid tolerance was not restricted to genotypes occurring naturally in acid soils.