Tolerance of Soil Acidity in Symbioses of Mung Bean with Rhizobia1

Abstract

AbstractVariation in tolerance of soil acidity among 40 rhizobial strains was assessed in greenhouse trials in which the strains were applied as separate seed inoculants (5 ✕104 cells per seed) to two cultivars of mung bean (Vigna radiata L.) and performance was measured by nodulation, growth, and N‐yield of the host plant. The plants grew in a low N, low Ca acid subsoil, Goldridge fine sandy loam (Typic Hapludult, fine loamy, mixed, mesic), left at its natural pH of 5.0 (saturation paste) or limed with CaCO3 to pH 6.3. Each pH ✕ strain treatment was triplicated in separate pots.Strains demonstrated a large and perhaps continuous variation in acid tolerance. A few were very sensitive: they failed to nodulate at pH 5.0. About half were moderately sensitive: nodulation and growth were significantly impaired at pH 5.0. The remainder were tolerant: like NH4NO3 they supported similar plant growth at both soil pH values. Some strains combined high tolerance with high effectiveness.A strain's acid tolerance could not be predicted from the abundance or effectiveness with which it nodulated at favorable pH, or from its growth rate or acid production in conventional yeast mannitol medium.A few strains were sensitive on one host cultivar and tolerant on the other, implying that acid tolerances of symbiotic legumes cannot be compared validly in trials with only one inoculant.