ENDOPHYTIC ESTABLISHMENT OF AZORHIZOBIUM-CAULINODANS IN WHEAT

Nitrogen fixing nodules are formed on the roots and stems of the tropi cal legume Sesbania rostrata by Azorhizobium caulinodans as a result o f crack entry invasion of emerging lateral roots. Advantage was taken of this invasion capability of A. caulinodans to determine whether ino culation of the non-legume wheat with A. caulinodans would result in t he endophytic establishment of azorhizobia within wheat roots. Advanta ge was also taken of the oxygen tolerance of the nitrogenase of free-l iving azorhizobia to assess the extent to which the endophytic establi shment of azorhizobia in wheat roots would provide a niche for nitroge n fixation of benefit to the plant. Wheat was inoculated with A. cauli nodans and grown in pots under controlled conditions, without added gr owth regulators and without addition of fixed nitrogen. Microscopic ex amination of the short lateral roots of inoculated wheat showed invasi on of azorhizobia between cells of the cortex, within the xylem and th e root meristem. Acetylene reduction assays combined with analysis of tissue nitrogen levels indicated the likelihood that colonization led to nitrogenase activity. Inoculated wheat showed significant increases in dry weight and nitrogen content as compared with uninoculated cont rols. We discuss the extent to which this nitrogen fixation is likely to involve symbiotic nitrogen fixation, and we indicate the need for f ield trials to determine the extent to which inoculation of wheat with A. caulinodans will reduce the requirement for inputs of nitrogenous fertilizers.