Sources and Quantities of Yeast Extract for Growth of Rhizobia

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Abstract
Summary: The growth of 4 strains of rhizobia representing 4 inoculation groups (clover, medic, cowpea‐type and lupin) in liquid medium containing one of 4 commercially available sources of yeast extract (Vegemite, Oxoid powder, Oxoid paste and BBL yeast) was compared at 4 levels of each extract (1, 2, 4 and 8 g/l). Total cell numbers increased with increasing levels of yeast extract, but viable cell numbers were significantly depressed by some yeast sources. For the clover, cowpea‐type and lupin strains strong depressive effects of BBL yeast were recorded at the highest levels, but satisfactory growth was obtained at the lower levels. Oxoid powder depressed viable numbers slightly but Vegemite did not. All yeast sources increased viable cell numbers of the medic strain with increasing levels in the medium. Reasons for these depressive effects and their practical implication are discussed in relation to legume inoculant preparation.