Protective mechanism in photoinhibition of photosynthesis in Citrus unshiu leaves

By using a portable PAM-2000 fluorometer and a spectrophotometer (722 Model), the diurnal variation in main chlorophyll fluorescence parameters such as initial fluorescence (F 0), maximal fluorescence (Fm), ratio of variable fluorescence to maximal fluorescence (Fv/Fm), non-photochemical quenching (qN), fast-phase (qN f) and slow-phase (qNs) of non-photochemical quenching, electron transport rate (ETR), and relative content of xeaxanthin in Citrus unshiu leaves were observed on fine days. The results showed that ETR, qN, qN f, qN s and relative content of xeaxanthin increased with increasing light intensity, but Fm, F 0, photo-chemical efficiency of PSⅡ(Fv/Fm) decreased. After treated by DTT, qN s was much lower, but F 0 was much higher than control. These results indicated that xanthophyll cycle and △pH existed,and played an important protective role against photo-damage to the photosynthetic apparatus in Citrus unshiu leaves during diurnal variation of photosynthetic efficiency.