Sinorhizobium Meliloti Products Increase Carbon Metabolism in Alfalfa

It is well-known that roots colonized by microorganisms evolve more CO2 than sterile roots ( Meharg and Killham, 1991), but the source of the extra CO2 is difficult to determine when both roots and microbes are respiring (Cheng et al., 1993). The increase in root-plus-bacterial respiration may occur when soil microbes respire carbon compounds in root exudates (Meharg and Killham, 1991), but alternative explanations should also be considered. For example, cell wall fragments from pathogenic fungi directly increase plant cell respiration (Norman et al., 1994), and it is plausible that products from rhizosphere bacteria have similar effects.