Abstract:

【Objective】 The objective of this study is to understand the characteristics of photoinhibition and mechanisms of photoprotection in broccoli leaves. 【Method】 The roles of D1 protein turnover and xanthophylls cycle in protecting photosynthetic apparatus of broccoli leaves against photodamage were studied by using their inhibitors and chlorophyll fluorescence parameters.【Result】 Significant decreases in Fv/Fm and increases in Fo were observed in broccoli leaves exposed to strong light stress (light intensity 1 800 μmol?m-2?s-1) for 1-4 h or a natural high temperature and strong light condition at noon in summer, the changes of Fv/Fm or Fo showed recovery in darkness for 5h or when sunlight intensity decreased after 14:00. SM and DTT resulted in stronger degree of reducement in Fv/Fm, qP, F’v/F’m and ΦPSⅡwhen broccoli leaves were treated with strong light. Compared to the control, ΦPSⅡ and F’v/F’m after chlorophyll fluorescence induction with light intensity 1 000 μmol?m-2?s-1 for 660 seconds in SM-treated leaves decreased by about 85.71%, 80.31% in broccoli leaves treated under 1 800 μmol?m-2?s-1 for 4h and then recovered in the dark for 12h, and in DTT-treated leaves reduced by about 22.45% and 11.48%, respectively. 【Conclusion】 These results suggested that there were perfect mechanisms of photoprotection in broccoli leaves, and that photosynthetic apparatus in SM- or DTT-treated leaves were photodamaged under strong light. More severe photodamage occurred in SM-treated leaves than that in DTT-treated ones, indicating that D1 protein turnover might play more important role in protecting photosynthetic apparatus of broccoli leaves against photodamage than xanthophylls cycle.

Key words: broccoli, strong light, D1 protein turnover, xanthophylls cycle, SM, DTT