Abstract
1. Tissues of a potato petiole and slices of potato tuber were heated at 45°-53.5°C for five minutes, and then inoculated with Phytophthora infestans. The resistance of them to P. infestans was reduced remakably.
2. No influence of the heat-treatment on the time required for penetration of the fungus into the host cell, nor on the growth rate of the intracellular hypae was found by microscopical observations on the cortex cells of the petioles.
3. The necrogenous reaction of potato-plant tissue to infection by incompatible race of Phytophthora infestans was delayed remarkably by pre-infectional heat-treatment. The delay of the hypersensitive death of an infected cell was ascertained by microscopical observation on the cortex cells of the petioles.
4. The appearance of protoplasmic strands, in the cut-surface cell of a petiole was reduced remarkably by the heat-treatment.
5. The R.Q. of the potato tuber slice was not affected by the heat-treatment, but the respiratory rate was depressed remarkably by the same treatment. The rate of respiratory depression seems to go along with that of the reduction of resistance.
6. Basis was found for the supposition that the metabolic activity in the cell is closely associated with the hyper-sensitive death of infected cell.
