EXTERNAL PHOSPHATE AND CALCIUM CONCENTRATIONS, AND PH, BUT NOT THE PRODUCTS OF RHIZOBIAL NODULATION GENES, AFFECT THE ATTACHMENT OF RHIZOBIUM-MELILOTI TO ROOTS OF ANNUAL MEDICS

The attachment of cells of Rhizobium meliloti to roots of seedlings of Medicago polymorpha L. and M. murex Willd. was examined for an acid s oil-tolerant strain (WSM540) and an acid soil-sensitive strain (CC169) in nutrient solutions. Both external P and Ca concentrations markedly affected the number of cells which attached in a 4h period. Increasin g external P concentration from 20 to 2000 mum at pH 5.9 caused an 11- fold increase in the attachment of CC 169 to M. polymorpha, and a 2.5- fold increase for WSM540. Increasing the Ca concentration from 1.0 to 10.0 mm at pH 5.9 increased the attachment of CC169 7-fold, and that o f WSM540 4-fold. Increasing acidity decreased the attachment of cells to roots, however, the acid soil-tolerant WSM540 attached in greater n umbers at low pH to both M. polymorpha and M. murex than did the acid soil-sensitive CC169. The attachment of WSM540 was in general less aff ected by changes in pH, Ca and P concentrations than CC169, and it is therefore hypothesized that acid soil-tolerance or -sensitivity relate s to the surface ionogenic properties of these rhizobia. The induction of nodulation genes in rapidly growing cultures of R. meliloti for 6 h prior to the attachment assay did not affect the number of cells whi ch attached to M. murex.