EFFECTS OF ALUMINUM ON THE PEANUT (ARACHIS-HYPOGAEA L) BRADYRHIZOBIUMSYMBIOSIS

The effects of low activities of the monomeric Al species, Al3+, Al(OH )+ and Al(OH)2+, on the peanut/Bradyrhizobium symbiosis were examined in solution culture. In flowing solution culture, growth of the host p lant was depressed at activities greater-than-or-equal-to 5 M. Neither shoot dry weight, root dry weight nor root length were inhibited by 3 muM Al, an activity which reduced nodule number by 70%. Low nodule nu mber was compensated for, at this activity, by an increase in weight p er nodule. In non-flowing solution culture of similar composition, sur vival of a streptomycin resistant mutant of Bradyrhizobium spp. NC92 i n the bulk solution or in the rhizosphere of peanut roots was unaffect ed by 20 muM Al. The site of infection by Bradyrhizobium was examined by scanning electron microscopy. Lateral root axils of plants exposed to greater-than-or-equal-to 2 muM Al did not display the rosette of mu lticellular root hairs which is characteristic in normal plants. The d etrimental effects of Al on nodulation appear to be related to structu ral changes at the site of infection which are observed at Al activiti es too low to cause any depression in growth of the host plant, includ ing root length, and at activities of Al which do not affect survival of the free-living Bradyrhizobium.