Journal of General MicrobioZogy (I977), 9, 19-27 Printed in Great Britain Water Status of Rhizobia in Relation to their Susceptibility to Desiccation

The greater susceptibility to desiccation of the Rhizobiurn legurninosarum group of rhizobia compared with the slow-growing rhizobia is related to the different amounts of water retained by these groups of bacteria at any relative vapour pressure, rather than differences in rates of water movement into or out of the bacteria. The higher retention of water by the R. legurninosarum group at any rela-tive vapour pressure is related to greater availability of adsorptive surface area, and to higher surface energies resulting in a greater affinity for water. Although the internal osmotic pressure is greater in the slow-growing group than in the R. leguminosarum group, it is concluded that differences in internal solute concentra-tions cannot account for the different adsorption isotherms found for these groups. Montmorillonitic clay protects the fast-growing R. leguminosarum group from the effects of desiccation, but not the slow-growing group. Ca-montmorillonite retains greater quantities of water at any relative vapour pressure than either group of bacteria. Possible mechanisms of protection of the bacteria by the clay are discussed in terms of relative affinities for water under desiccation conditions