Studies on the Relation between the Susceptibility of Pedicel of Panicle (“Hokubi”) of Rice Plant to Blast Disease caused by the Low Soil Temperature and its Anatomical Characters.

Abstract

1. In this paper, the results of the following investigations which were done on the pedicels of panicles (“Hokubi”) of the rice plants grown on the flooded soil with the temperatures, 15.3°-28.2°C.(the lot of low temperature), and 16.5°-34.8°C(the lot of natural temperature), are mentioned:- (1) Inoculation experiments of Piricularia Oryzae CAVARA, (2) Appresorium formation of the fungus on the epidermal cells (3) Entrance of the fungus into the epidermal cells. (4) Infection (determined by the appearance of necrosis) of the epidermal cells by the fungus., (5) Microscopical examinations on some anatomical characters of the pedicels of panicles.
2. Throughout all the experiments, “Kameji” and “Mubo-aikoku” were used as a resistant variety and “Omachi” and “Kokuryomiyako” as a susceptible one. The rice plants of these varieties in the lot of low temperature continued very weak growth, and their panicles appeared about ten days later than those in the lot of natural temperature which did very strong and vigorous development.
3. The pedicels of panicles in the both lots were inoculated with the spore suspension from the pure culture of the fungus when the plants in the lot of low temperature began to bloom. From the macroscopic observation of the experiments, the disease development in the lot of low temperature showed 75-100 per cent. for the both susceptible and resistant varieties, while that in the lot of natural temperature did 0-13.3 per cent. for the resistant varieties and 20-33.3 per cent for the susceptible ones.
4. After the epidermal tissues of the pedicels of panicles inoculated with the fungus had been treated with 0.01 per cent. of safranin and concentric phenol, the appressorium formation, the entrance and the infection were microscopically examined on the epidermal cells. These examinations gave the results that without regard to the varieties and the kind of the cells tested, the number of the epidermal cells, on which the appressoria were formed, as well as that of the cells penetrated and infected, were larger in the lot of low temperature than in that of natural temperature. Regardless of the kind of the cells examined, they were also larger in the susceptible varieties than in the resistant ones, and this difference was more obviously in the lot of natural temperature than in the other lot. Furthermore, it was found that regardless of the varieties, the lot, and the kind of the cells, the epidermal cells, on which the appressoria were formed, gave the largest number, those into which the fungus entered the next, and those infected by the fungus the smallest, and whatever the varieties and the lots were concerned, the numbers were the greatest in the long cells in the non-stomatal zones of the tissues, the next in the long and short cells in the stomatal zones, and the smallest in the accessory cells of stomata, although this difference was obscurely observed in the lot of low temperature as compared with the other lot.
5. It was comfirmed that the low temperature of the soil, on which the plants are grown, seems to decrease the resistance of the pedicels of panicles to the disease owing to the reason that it makes the fungus to form abundant appressoria on the host cells in addition to easily enter into the cells and to easily infect them.
6. The comparative anatomical studies were made on the epidermis of the healthy panicles in the both lots which were not inoculated. The thickness of the outer wall of the epidermal cells was measured on the transversal sections by the usual method, while that of the silicated layer of the wall was done by the method of mounting the sections in concentric phenol.