Abstract
The behaviors of conidia of Helminthosporium oryzae Ito et Kuribayashi on coleoptiles of two rice varieties, resistant (Kameji) and susceptible (Shiga Asahi No. 27) were compared by using light microscope. Conidia germinated on the coleoptiles of both varieties to almost the same percentage, whereas the length of germ tubes was shorter and the percentage of appressorium formation was smaller on the coleoptile of resistant variety than those on the coleoptile of susceptible one.
On the other hand, the fine structure of a germ tube, appressorium, invading hypha of the causal fungus and the host cells (Shiga Asahi No. 27) were observed under the electron microscope. Mature germ tubes and appressoria formed on both varieties gained a two-layered cell-wall, 0.2μ in thickness. The outer layer was thin and electron-dense, and the inner layer thick and electron-transeparent when stained with potasium perma nganate and osmium tetroxide. The cell wall of immature germ tubes, however, possessed only a mucilaginous inner layer. This substance was further detectable around germ tubes and appressoria, and appears to serve for the attachment of these to the epidermal cell wall. When an appressorium firmly attaches the epidermal cell wall, the pectic layer is resolved and the cuticle layer is often ruptured just around the appressorium by being pulled up with the mucilage. The epidermal cell walls of the resistant variety was thicker than those of the susceptible one by about 27 per cet.
