Preillumination of lettuce seedlings with red light enhances the resistance of photosynthetic apparatus to UV-A

Seedlings of 10-day-old lettuce (Lactuca sativa L., cultivar Berlin) were preilluminated by low intensity red light (λ_max = 660 nm, 10 min, 5 μmol quanta m^-2 s^-1) and far-red light (λ_max = 730 nm, 10 min, 5 μmol quanta m^-2 s ^-1) to study the effect of pre-treatment on photosynthesis, photochemical activity of photosystem II (PSII), the contents of photosynthetic and UV-A-absorbing pigments (UAPs) and H₂O₂, as well as total and ascorbate peroxidase activities in cotyledonary leaves of seedlings exposed to UV-A. UV radiation reduced the photosynthetic rate (P_n), the activity of PSII, and the contents of Chl a and b, carotenoids and UAPs in the leaves, but increased the content of H₂O₂ and the total peroxidase activity. Preillumination with red light removed these effects of UV. In turn, the illumination with red light, then far-red light removed the effect of the red light. Illumination with red light alone increased the content of UAPs, as well as peroxidase activity. It is suggested that higher resistance of the lettuce photosynthetic apparatus to UV-A radiation is associated with involvement of the active form of phytochrome B, thereby increasing peroxidase activities as well as UAPs and saving preservation of photosynthetic pigment contents due to pre-illumination with red light.