Endophytic rhizobia in barley, wheat and canola roots

Endophytic rhizobia have been shown to improve the nutrition of nonlegume crops. The objective of this work was to investigate the effects of field pea (Pisum sativum)-based crop rotations on endophytic rhizobia in roots of cereal and oilseed crops. Barley (Hordeum vulgare), wheat (Triticum aestivum) and canola (Brassica rapa) were each grown (a) following inoculated peas, (b) following uninoculated peas or (c) in monoculture. At flag-leaf or flowering growth stage, populations of endophytic rhizobia were usually in the order: crop following uninoculated peas (up to 7244 cells g -1 root DM) > crop following inoculated peas (up to 1660 cells g -1 root DM) > crop grown in monoculture (< 10 cells g -1 root DM). At one of three sites, there were significant positive correlations between endophytic rhizobia and crop N and yield. Populations of rhizobia in bulk soil, rhizosphere, or rhizoplane of nonlegume roots were greater where nonlegume crops were preceded by peas (inoculated or uninoculated) than where they were grown in monoculture. Significant positive correlations between populations of these rhizobia outside roots and crop N or yields were observed at each site. Key words: Crop rotation, inoculation, nitrogen fixation, plant growth-promoting rhizobacteria (PGPR), rhizosphere