Differential responses of photosystem II activity to photooxidation in red and green tissues of Amaranthus tricolor leaves

In order to study the antioxidative potential of amaranthine and its relationships with photoprotection, changes of PS II activity of red and green tissue in the same edible amaranth leaf were compared under photooxidation treatment induced by MV (methyl viologen) or H₂O₂. In the first 90 min of MV treatment, PS II maintained stable activity; the chlorophyll-fluorescence parameters were close to those of the controls. However, with the extension of treatment time, during 90-300 min of continuous photooxidative stress, F_v/F_m, Y (II) and Y (NPQ) of leaf discs decreased significantly, while Y (NO) and F_o dramatically increased, which showed that the activity of PS II suffered irreversible photooxidative damage, suppressed or even completely inhibited. Compared with the control, under 50 mmol·L^-1 of H₂O₂ stress for 360 min, Y(NO) and F_o increased, accompanied by Y(NPQ) and the F_v/F_m decreases, while Y(II), qP and ETR were slightly increased, reflecting that treatment with 50 mmol·L^-1 H₂O₂ promoted distribution of light energy in photochemistry reaction. After photooxidative treatments, a lower F_o and Y (NO) but higher F_v/F_m, Y (II), qP and Y (NPQ) was found in the red leaf discs compared with the green ones. The decrease and the decline rate of the amaranthine content in the green and red leaf discs implied that the PS II of green leaf discs have a higher sensitivity to photooxidative damage than the red ones. It is suggested that the higher content of amaranthine may be used as a detoxification mechanism to counteract photooxidation in red leaf.