The effect of different temperatures on the fatty acid composition of Rhizobium leguminosarum bv. viciae in the faba bean symbiosis.

Fatty acid composition was determined in cells of strains CBhS and CBp7 of Rhizobium leguminosarum bv. viciae grown at four temperatures (10, 15, 22 and 30°C), and in bacteroids and nodules formed with faba bean (Vicia faba L.) grown at two day/night temperature regimes (22/15 and 15/10°C). Growth temperature markedly affected the fatty acid composition of free-living bacteria in both strains studied, and both showed similar variations at each temperature. The proportion of unsaturated fatty acids increased significantly with lowering of temperature. The major fatty acid found in bacteria and bacteroids was cis-vaccenic (C18: 1 ), which comprised up to c. 78 (bacteria) and 56% (bacteroids) of total fatty acids. The presence of polyunsaturated fatty acids (linoleic (C18: ) and linolenic (C18:3 ) acids) was noted only in bacteroids, indicating changes following the differentiation of bacteria into bacteroids in the nodules. The fatty acid composition of nodules was similar to that of bacteroids, although major differences were found in their proportions. The different day/night temperature regimes had contrasting effects in bacteroids and in nodules. In bacteroids of both strains, the proportions of stearic (C18:0) and linoleic (C18: ) acids decreased at the lower temperature regime. In nodules, the proportion of stearic (C18:0) acid decreased, while that of linolenic (C18:3 ) acid increased at the lower temperature regime. However, those of cis-vaccenic (C18:1 ), linoleic (C18: ) and palmitic (C16:0) acids increased or decreased depending on the rhizobial strain. The proportion of unsaturated fatty acids increased with the lowering of temperatures in bacteroids of both strains, and varied in whole nodules depending on the strain. Strain CBp7 showed a greater symbiotic efficiency (dry matter yield) than strain CBh5 under both temperature regimes, but no relationship was found with the proportion of unsaturated fatty acids of bacteria, bacteroids or nodules.