STUDIES OF SILICON DISTRIBUTION IN WOUNDED AND PYTHIUM-ULTIMUM INFECTED CUCUMBER PLANTS

The objective of this study was to investigate the deposition of silicon (Si) in relation to the ability of Si to reduce the severity of Pythium ultimum infection on cucumber. Roots, hypocotyls and leaves of cucumber plants grown in nutrient solutions unamended or amended with 1·7 mm (100 ppm) potassium silicate were inoculated with P. ultimum or pierced with a sharp needle. At 24, 48 and 72 h after treatment, the plants were examined for Si distribution using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) microanalysis. No Si was detected at sites of fungal penetration or in P. ultimum hyphae, regardless of the plant organ studied. Si was also absent from wounded roots and almost absent in wounded leaves and hypocotyls collected from plants maintained under high humidity in a growth chamber. By contrast, a specific and intense deposition of Si was found in cells surrounding the trichome hairs and in wounded leaves and hypocotyls of silicon amended plants. These results reinforce the idea that accumulation and polymerization of silica at fungal penetration sites or in epidermal cell walls has no role as a physical barrier against fungal attack. Thus Si deposition does not appear to be the mechanism by which fungal growth and penetration of plant tissues are hindered. © 1992.