PHYSIOLOGICAL CHARACTERIZATION OF INDIGENOUS RHIZOBIA NODULATING VIGNA-UNGUICULATA IN ZIMBABWEAN SOILS

Cowpea (Vigna unguiculata [L.] Walp.) nodule isolates from 14 Zimbabwe an soils included similar proportions of both fast- (49%) and slow-gro wing (51%) rhizobia. Colony morphologies ranged from small (< 1-2 mm) 'dry' or gummy in slow-growing isolates, to large (> 3 mm) 'wet' water y/slimy types of fast growing rhizobia. All fast-growing isolates prod uced an acid reaction while slow-growers produced acid, alkaline or ne utral reactions on YEM. The majority of both fast and slow-growing iso lates tolerated high acidity (growth at pH 4.0) and salt (up to 5.5% N aCl). Phosphatase activity ranged from undetectable to 0.02 mu mol p-n itrophenol h(-1) 10(-6) cells and was generally higher among fast- com pared to slow-growing isolates. Intrinsic antibiotic sensitivity was g enerally higher in fast compared to slow-growers. The least utilized o f 12 sole carbon sources tested were p-hydroxybenzoic acid (31%) and x ylose (78%). Despite some similarities with established rhizobial taxa , the indigenous strains showed considerable cultural and physiologica l diversity and probably include unique types belonging to several, as yet undefined species.