Effect of enhanced UV-B radiation and inoculated blast isolate (Magnaporthe grisea) on phenylalanine ammonial-yase activity and flavonoid content in seedlings of two rice cultivars

The effects of UV-B radiation and Magnaporthe grisea on the activities of phenylalanine ammonia-lyase and flavonoid contents in seedling leaves of two rice (Oryza sativa L.) cultivars (“Huangkenuo” and “Hexi 41”) were investigated under pot experimental conditions. The two rice cultivars were treated at three UV-B radiation levels (2.5 kJ·m^-2, 5.0 kJ·m^-2 and 7.5 kJ·m^-2) and two blast isolates (Y98-16T and Y99-63C). Changes in the activity of phenylalanine ammonia-lyase and flavonoid content of the rice seedling leaves were then determined. While the phenylalanine ammonia-lyase activities and flavonoid contents of “Huangkenuo” seedling leaves significantly increase under 2.5 kJ·m^-2 and 5.0 kJ·m^-2 UV-B radiation, they significantly decrease under 7.5 kJ·m^-2 UV-B radiation. Leaf flavonoid content of “Hexi 41” significantly increases under enhanced UV-B radiation from 2.5 kJ·m^-2 to 7.5 kJ·m^-2. While “Hexi 41” leaf phenylalanine ammonia-lyase activity significantly increases under 5.0 kJ·m^-2 UV-B radiation, it remains stable under other UV-B treatments. Whereas “Hexi 41” leaf phenylalanine ammonia-lyase activity and flavonoid content significantly increase, “Huangkenuo” leaf phenylalanine ammonia-lyase activity decreases against increasing flavonoid content under Y98-16T inoculation. Both “Huangkenuo” and “Hexi 41” leaf phenylalanine ammonia-lyase activities and flavonoid contents significantly decrease under Y99-63C inoculation. Under enhanced UV-B radiation and inoculated blast isolates, “Huangkenuo” phenylalanine ammonia-lyase activity and flavonoid content decrease, whereas those of “Hexi 41” either remain stable or increase. In general, “Huangkenuo” sensitivity and “Hexi 41” tolerance increase under enhanced UV-B radiation and inoculated blast isolates.