THE ASSOCIATION BETWEEN HEAT-INDUCED SUSCEPTIBILITY OF TOMATO TO MELOIDOGYNE-INCOGNITA AND PEROXIDASE-ACTIVITY

Resistant tomato seedlings (cultivar VFN8) were tested for their reaction to the root-knot nematode Meloidogyne incognita at five temperatures between 27 and 38 degrees C. The data obtained showed that the seedlings became increasingly susceptible to M. incognita above 30 degrees C with an optimum at 34 degrees C. Seedlings incubated and inoculated at 34 degrees C, after 2 days showed a susceptible response, i.e., reduced necrosis and a higher number of galls. A temporary break of resistance could also be induced by conditioning uninoculated seedlings al higher temperatures for 6 days. The plants remained susceptible for 1-2 days when the temperature was shifted from 34 to 27 degrees C before inoculation. Chromatographic and electrophoretic analyses demonstrated that enhanced peroxidase activity accompanied nematode infection at 27 degrees C. High-temperature-induced susceptibility of tomato to M. incognita was correlated with low to no increase in peroxidase activity. The increase of peroxidases at 27 degrees C was due to an increase in activity of constitutive enzymes. Heat treatment reduced the lignin level of both infected and uninfected roots, indicating that the process of lignification was partially blocked by high temperature. The data provide evidence that heat-induced susceptibility is associated with a reduced hypersensitive reaction, and that there is a correlation between hypersensitivity and reduced peroxidase and lignin levels.