Preillumination of excised spinach leaves with red light increases resistance of photosynthetic apparatus to UV radiation

Leaves of spinach (Spinacia oleracea, cv. Ispolinskii) were preilluminated by low-intensity light (1.0 and 1.5 W/m², 0.5?C3.0 h) with wavelengths ranging from 530 to 730 nm to study the effect of this pretreatment on the activity of photosystem II (PS II), content of photosynthetic pigments, and peroxidase activity in excised leaves exposed to UV-A irradiation. Irradiation of leaves with UV-A suppressed the activity of PS II, reduced the content of chlorophylls (a + b) and carotenoids, and increased the peroxidase activity. Preillumination of leaves with red light (RL, 620?C660 nm) alleviated the inhibitory action of UV-A on PS II activity and reduced the pigment losses but increased the peroxidase activity in leaves and thylakoid membrane preparations, as compared to the respective effects of UV-A light applied without preillumination. The preexposure of leaves to red light alternating with far-red light (FR, 730 nm) removed partly the influence of RL on the parameters under study, which indicates the involvement of phytochrome active form, P_FR into stress-induced defense responses in leaves. It is supposed that elevated resistance of photosynthetic apparatus to UV-A radiation was formed with the involvement of P_FR and the antioxidant system induced by oxidative stress after preillumination of leaves with red light