Silicon deprivation enhances localized autofluorescent responses and phenylalanine ammonia-lyase activity in oat attacked byBlumeria graminis

The influence of silicon (Si) nutrition on resistance responses of the susceptible oat cv. Selma to attack byBlumeria graminis (Erysiphe graminis)f.sp.avenaewas investigated. Using modified Shive's hydroponic solutions, Si deprived (Si-) and Si supplied (Si+) plants were obtained. First-formed leaves were used for experimentation. Use of energy dispersive X-ray microanalysis revealed that in Si+ leaves abundant Si accumulated in leaf trichomes and at sites of epidermal cell response (halo/papilla regions) toB. graminisappressoria. By contrast, Si was barely detectable in comparable analyses from trichomes and attacked epidermal cells in Si- plants. In Si+ leaves there was a consistent but not statistically significant tendency for fewer appressoria to produce successful penetrations. Haustorium formation was unaffected by Si deprivation; however, Si deprivation was associated with a substantial increase in the frequency and intensity of localized autofluorescence responses (phenolic compound accumulation) in epidermal cells attacked by appressoria. In uninoculated Si- leaves there was an increase in the specific activity of phenylalanine ammonia lyase (PAL) compared to Si+ leaves. Enzyme specific activity was also determined 18 h after inoculation. In both Si+ and Si- leaves inoculation led to increased PAL activity, but this increased enzyme activity was far greater in the Si- treatment. It is suggested that this rise in PAL activity compensated for the lack of Si, and this compensation contributed toB. graminispenetration resistance in Si- leaves.