The problems for agriculture on acidic soils are especially significant in relation to legume nodulation on such soils. The low rainfall Mediterranean climate of the eastern wheat-belt of Western Australia represents one particular type of problem. Here the nitrogen input for wheat crops needs to come from N2 fixation from pasture legumes, but the growing season is inadequate for normal annual clovers and the acidity of the soils is too great for normal strains of Sinorhizobium meliloti to persist and nodulate the annual species of Medicago with suitable short growing season requirements. Introduction of strains of S. meliloti isolated from annual Medicago spp. growing on acid soils in Mediterranean Europe (Howieson, Ewing 1986) have, however, helped establish wide areas (ca. 400 000 ha) of medic pastures in this environment. Our laboratory is trying to understand why these introduced strains of S. meliloti are able to colonize, persist and nodulate in such soils when so many others cannot.

The initial question was whether strains were acid-tolerant because they carried genes, which others did not, or whether they simply had more effective versions of genes which were of general occurrence. Other questions concern how many genes are specifically required for acid tolerance, how the acid environment is perceived, how the acid-tolerant cells respond to it, and what physiological mechanisms are brought into play. Some of these questions are addressed in this brief review.

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