Influence of soil pH on the development of symbiosis in field-grown acid-sensitive and acid-tolerant annual medics

Abstract

Relationships between soil pH, resident population of Rhizobium meliloti in the soil, rhizobial colonisation of the rhizosphere, and nodulation of annual species of Medicago were examined. Established swards of M. truncatula (acid-sensitive) and of M. murex and M. polymorpha (acid-tolerant) growing at 6 different locations in central and western New South Wales were sampled to measure soil pH, to enumerate the populations of R. meliloti resident in the soil and colonising plant rhizospheres, and to assess the extent of nodulation. Soil pH (0.01 mol CaCl2/L) varied from 4.70 to 7.25, soil populations from 3 to 100000 rhizobialg, and rhizosphere populations from 10 to >1 x 106 rhizobia/plant. Nodulation varied from poor to good, depending on growing period and location, but was consistently poorer on M. truncatula than on M. murex or M. polymorpha. Regression analysis showed a consistent and significant (P<0.01) relationship between rhizobial colonisation of the rhizosphere and extent of nodulation. There was also a significant (P<0.001) correlation over all 44 sampling sites between soil pH and the population of R. meliloti resident in the soil. The relationships between soil pH and rhizosphere colonisation, and between soil pH and nodulation, were significant for M. truncatula (P<0.01, P<0.02, respectively) but not for either M. murex or M. polymorpha. These data suggest that the range of soil pH at which M. truncatula was growing (4.80-7.25, but mainly 4.8-5.6) spanned the threshold at which unfavourable soil pH began to obstruct normal expression of its symbiosis, whereas, the critical soil pH levels for symbiotic development of M. murex and M. polymorpha were below this range.