Further Studies on the Relationship between Glyceollin Accumulation and the Resistance of Soybean Leaves toPseudomonas syringaepv.glycinea

Abstract

Several isolates of P. syringae patovar glycinea (Psg) previously classified as new races or not race typed were inoculated into the standard differential soybean cultivars. Based on the observed visible reaction types, the only clearly defined races in the collection were the previously described races 1, 4, 5 and 6. Certain of the other isolates fell into these existing races, but some gave ambiguous intermediate reactions or otherwise failed to justify the status of unique races. Plants of soybean cultivars Hardee and Peking gave unique reaction with Psg races 1, 4, 5 and 6 and accordingly are useful supplements to the standard differentials. The production of glyceollin in several newly tested soybean-Psg race combinations was consistently associated only with reactions visually classified as hypersensitive resistant (HR). Bacterial multiplication in inoculated leaves of several cultivars was negatively correlated with glyceollin levels fromed in the leaves (r = -0.94). Significantly, some cultivar-race reactions typified by a visible HR resulted in considerably lower levels of glyceollin accumulation and higher bacterial multiplication than other hypersensitive interactions. Unlike certain other plants, inoculated soybean leaves maintained under high humidity exhibited visible HR, glyceollin production, and inhibition of bacterial populations similar to the same leaves incubated at ambient humidity. Mixed inocula of incompatible and compatible races of Psg gave a visible HR and glyceollin accumulation similar to the incompatible race alone and partial, but not complete, restriction of bacterial populations to the level of the incompatible race inoculated alone. The results, therefore, indicate that the incompatible race is physiologically dominant and that the soybean HR and glyceollin accumulation are associated with restricted bacterial multiplication.