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Comparative Morphology and Enzyme Histochemistry in Root Knot Resistant and Susceptible Soybeans. Joseph A. Veech, Plant Physiologist, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705; Burton Y. Endo, Nematologist, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 60:896-902. Accepted for publication 31 December 1969. DOI: 10.1094/Phyto-60-896.

A comparison was made of certain enzyme histochemical and morphological responses of susceptible (Lee) and resistant (Delmar) soybeans to infection by Meloidogyne incognita acrita. The activity of certain oxidoreductive, hydrolytic, and oxidative enzymes of the susceptible variety were increased significantly primarily within the syncytium. Initially, galling response to infection was similar in both susceptible and resistant plants, i.e., slight galling was observed prior to the microscopic detection of syncytia. During the first few days after inoculation, both cultivars showed a similar, slight increase of host enzyme activity at the nematode feeding site. With further disease development, however, the responses of the two cultivars differed. In the susceptible host, syncytia developed at the site of nematode feeding. These syncytia generally developed into extensive units that contained increased levels of enzyme activity. The syncytia usually were sufficient to support the development of the nematode to maturity. At a time corresponding to syncytia induction in the susceptible host, the most common resistant host response was cell necrosis. Often after inducing necrosis, the nematode migrated to nonnecrotic cells and commenced feeding; these cells subsequently became necrotic. An increase in enzyme activity was seldom found in resistant-host cells associated with nematode feeding. Unlike the susceptible plant response to the nematode, the resistant plant rarely produced syncytia. In rare cases, however, where the nematode induced syncytia in the resistant plant, the syncytia contained increased levels of enzyme activity.