Differences between cereal crop cultivars in root-associated nitrogen fixation,possible causes of variable yield response to seed inoculation

Large differences in N₂-ase activity with fractions of active plants from 3–67% and maximal activities from 3–35 nmol C₂H₄.h^-1 were found between sterile, Azospirillum-inoculated seedlings of 14 German cereal cultivars. Examples of similar cultivar differences in gnotobiotic or unsterile cereals in response to Azospirillum inoculation, in root exudation and the specificity of bacteria-root interactions are reviewed. As possible causes of yield responses to bacterial seed inoculation N₂-fixation, plant growth regulating metabolites and bacterial interaction with root pathogens are discussed. The need for suitable screening methods to select and breed cultivars with desirable responses to beneficial rhizosphere bacteria is pointed out.