| Broccoli (Brassica oleracea L. Var Italicap) is one of nutritious and popular vegetables. Strong light and high temperature have become important factors to restrict the yield and quality of the cultivation of broccoli in summer. Photoinhibition of photosynthesis is an universal phenomenon occuring in higher plants under strong light. However, there are not reports of photoinhibition and photoprotection mechanisms in broccoli leaves. It is known that zinc fertilization can improve the photosynthetic capacity and the product quality of broccoli. But the mechanisms of zinc for enhancement photosynthetic capacity of broccoli remains unclear.The objective of this study is to understand the characteristics of photoinhibition, mechanisms of photoprotection, and the effects of zinc on them in photosystemⅡof broccoli leaves, using chlorophyll fluorescence and CO2 exchange parameters. The photoinhibition characteristics were studied through diurnal variations of chlorophyll fluorescence on sunny day in summer and through variations of chlorophyll fluorescence under artificial high-irradiance and (or) high-temperature conditions. The roles of D1 protein turnover and xanthophylls cycle in protecting photosynthetic apparatus of broccoli leaves against photodamage were studied by using their inhibitors. The effects of zinc on photosynthesis and photoinhibition characteristics in leaves of broccoli were studied by applying 0.5mg·L-1 Zn2+concentration, 0.05mg·L-1 Zn2+ concentration as control. The results are as follow:1. There were perfect mechanisms for energy dissipation and photoprotection in broccoli leaves under strong light and high temperature conditions, among them, the reversible inactivation of PSⅡwas more important.With the increases in sunlight intensity and leaf temperature in the morning on sunny day in summer, Fv/Fm, Fv'/Fm',ФPSⅡand qP of PSⅡin attached broccoli leaves decreased, but Fo increased. During the midday, the Fv/Fm, Fv'/Fm',ФPSⅡand qP reached their minimum, Fo reached its maximum, when the leaves were exposed to a combination of high sunlight intensity and high temperature stress for more than two hours. Significant decreases in Fv/Fm, Fv'/Fm',ФPSⅡ, and increases in Fo were observed in broccoli leaf discs exposed to artificial high irradiance stress at 25℃, 33℃, 37℃or 40℃. The changes of Fv/Fm, Fv'/Fm',ФPSⅡ, qP and Fo showed recovery in darkness for 5h or when sunlight intensity decreased after 14:00. These results suggested that there were perfect mechanisms for energy dissipation in broccoli leaves, and the reversible inactivation of PSⅡplayed an important role in protecting broccoli leaves against photodamage under strong light and high temperature conditions.2. Photosynthetic apparatus in SM- or DTT-treated leaves were photodamaged under high irradiance or high irradiance and high temperature. More severe photodamage occurred in SM-treated leaves than that in DTT-treated ones, indicating that D1 protein turnover might play a more important role in protecting photosynthetic apparatus of broccoli leaves against photodamage than xanthophylls cycle.SM or DTT resulted in stronger degree of decrement in Fv/Fm, Fv'/Fm'andФPSⅡwhen broccoli leaves were exposed to high irradiance for 14h at 25℃, 33℃, 37℃or 40℃. The Fv/Fm in SM- or DTT-treated broccoli leaves under high irradiance for 4h at 25℃, 33℃, 37℃or 40℃recovered incompletely in followed darkness for 5h, indicating that photosynthetic apparatus in SM- or DTT-treated leaves were photodamaged under high irradiance or high irradiance and high temperature. Compared to the control, the Fv'/Fm',ФPSⅡand qP after chlorophyll fluorescence induction with light intensity 1000μmol·m-2·s-1 for 660 seconds in SM- or DTT-treated leaves decreased significantly in broccoli leaves treated under 1800μmol·m-2·s-1 at 25℃, 33℃, 37℃or 40℃for 4h and then recovered in darkness for 12h. These results further validated that photosynthetic apparatus in SM- or DTT-treated leaves were photodamaged under high irradiance or high irradiance and high temperature. More severe photodamage occurred in SM-treated leaves than that in DTT-treated ones, indicating that D1 protein turnover might play a more important role in protecting photosynthetic apparatus of broccoli leaves against photodamage than xanthophylls cycle.3. Zinc enhanced the photosynthetic capacity in broccoli leaves, but accelerated photoinhibition in broccoli leaves exposed to high irradiance or high irradiance and high temperature.Zinc fertilization elevated the content of zinc in broccoli leaves and improved the growth of broccoli seedlings. Compared to the control plants, the content of chla and chlb increased by 9.56%, 11.32% respectively, and CA activity increased by 152.94% in broccoli leaves treated by adding 0.5mg·L-1 Zn2+. The Pn, Gs and Ci were higher in leaves treated by adding 0.5mg·L-1 Zn2+ than that in leaves of control plants. AQY increased by 12.14%, CE by 7.59% in leaves treated by adding 0.5mg·L-1 Zn2+, compared to that in control leaves. It was proposed that zinc could decrease stomatal limitation of photosynthesis and improve photosynthetic capacity by increasing CA activity in broccoli leaves. But zinc accelerated photoinhibition in broccoli leaves exposed to high irradiance or high irradiance and high temperature, and accelerated its recovery in darkness either. Such results might be attributed to the participation of zinc in D1 protein degradation.4. The experimental system for photoinhibition study was established.The system was composed of sulfur lamp, water bath, glass beaker with water and leaf discs. The light intensity was controlled by adjusting the distance between the sulfur lamp and the experimental material, the temperature was controlled by water bath. Several treatments seperated by glass beaker could be carried out at the same time.
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