Self-inhibition in the uredospore germination of Puccinia coronata

Abstract

Some inconsistent results from germination studies of rust spores have been recently explained on the basis of self-inhibitors. The present study was performed to make clear the self-inhibition in the uredospore germination of Puccinia coronata Corda. The samples of spores were harvested by shaking the rusted leaves which had been collected from field-grown oat-plants, washed with sterilized water, and then held in large glass vessels for 2∼3 days at a saturated humidity. Germination tests were made by floating spores over the surface of dist. water contained in 30 or 50ml flasks.1. The germination percentage of uredospores decreases as the spore density is increased. With very large numbers there is no germination at all. However, even in these samples excellent germination occurs when transferred to fresh water with small populations. With a definite spore density the germination percentage decreases as the amount of water in the flasks is decreased. As germination increases, the mats become very white, and the brownish-red color of the original spore film disappears entirely.2. There is no difference in germination percentage among the closed, cotton-sealed, open, and aerating flasks. When the solutions, produced by floating spores, are removed by inserting a pipette beneath the floating film of spores and transferring a sample of the spore free solution to another flask, the germination percentage on this solution is very higher than that in the original solution. These facts indicate that the lack of O2, the toxicity of gaseous bodies such as CO2, and the accumulation of toxic metabolite in the solutions are not the principal cause of the unfavourable germination in large populations.3. 4.4g samples of the spores were killed by heating at 100°C for 5min. and extracted with water at 5°C for 10 days. The extract was filtered, decolorized by treating twice with active carbon, and concentrated to 2ml. Then 20ml of ethanol was added to the material and the resulting white precipitate was filtered off. The filtrate was concentrated to a small volume to yield a colorless oily material. This substance prevented spore germination completely, even when diluted with water corresponding to the original weight of spores. This result clearly indicates that the substance in question is in sufficient quantities to account for the self-inhibitor of uredospore germination.4. Presumably the unfavourable germination in large populations of spores should be closely related to this self-inhibitor. In addition, the cause that uredospores of this fungus germinate better in water than in air also may probably be due to this substance.