Tolerance of Soil Acidity in Symbioses of Mung Bean with Rhizobia¹

Abstract

Variation 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.

Rhizobium -Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

“…High (not extreme) soil temperatures will delay nodulation or restrict it to the subsurface region (133). Munns et al (221) found that alfalfa plants grown in desert environments in California maintained few nodules in the top 5 cm of soil but were extensively nodulated below this depth. Nodulation of soybean was markedly inhibited at 42 and 45°C during 12-h and 9-h days, respectively (186), with no correlation between the ability of plant strains to grow at high temperature and to induce nodulation under temperature stress.…”

“…The host cultivar-rhizobial strain interaction at acid pH has also been investigated. Munns et al (221) noted that nodulation and nitrogen fixation by some strains of Bradyrhizobium at acidic pH differ with the cultivar of mung bean used. Vargas and Graham (327) examined the cultivar and pH effects on competition for nodule sites between isolates of Rhizobium in beans (P. vulgaris) under acidic conditions.…”

Influence of the Size of Indigenous Rhizobial Populations on Establishment and Symbiotic Performance of Introduced Rhizobia on Field-Grown Legumes

“…Inoculation of legumes with exotic strains of rhizobia is a common agricultural practice intended to promote nitrogen fixation and increase crop yield. Despite improvements in inoculation methods (3,13,31,34) and selection of rhizobial strains for increased nitrogen fixation capacity (16), competitive ability (1), and ability to withstand environmental stress (15,17,19), inoculation does not always lead to increased plant growth and crop yield.…”

Effect of Acidity on the Composition of an Indigenous Soil Population of Rhizobium trifolii Found in Nodules of Trifolium subterraneum L

“…Evidence has been presented that many legumes are more sensitive to acidity when dependent on N2 fixation than when grown on combined nitrogen (2). This has been explained as being due to the sensitivity to acid conditions of either nodule function (21,22) or the initiation and development of nodulation (14,18). It is not surprising, therefore, that amelioration of soil acidity with lime is often (though not always) beneficial to the growth of many legumes (20).…”