Libertyʼ) were monitored in the fi eld over a diurnal course at about 3 months after full bloom. Compared with leaves, sun-exposed peel of apple fruit had much lower photosystem II operating effi ciency at any given photon fl ux density (PFD) and a larger xanthophyll cycle pool size on a chlorophyll basis. Zeaxanthin (Z) level increased with rising PFD in the morning, reached the highest level during midday, and then decreased with falling PFD for the rest of the day. At noon, Z accounted for >90% of the xanthophyll cycle pool in the fruit peel compared with only 53% in leaves. Effi ciency of excitation transfer to PSII reaction centers (Fv'/Fm') was negatively related to the level of Z in fruit peel and leaves throughout the day. In fruit peel, the antioxidant enzymes in the ascorbate-glutathione cycle, ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) showed a diurnal pattern similar to that of incident PFD. In contrast, the activities of APX and GR in leaves did not change signifi cantly during the day although activities of both MDAR and DHAR were higher in the afternoon than in the morning. In both fruit peel and leaves, superoxide dismutase activity did not change signifi cantly during the day; catalase activity showed a diurnal pattern opposite to that of PFD with much lower activity in fruit peel than in leaves. The total ascorbate pool was much smaller in fruit peel than in leaves on an area basis, but the ratio of reduced ascorbate to oxidized ascorbate reached a maximum in the early afternoon in both fruit peel and leaves. The total glutathione pool, reduced glutathione and the ratio of reduced glutathione to oxidized glutathione in both fruit peel and leaves also peaked in the early afternoon. We conclude that the antioxidant system as well as the xanthophyll cycle responds to changing PFD over the course of a day to protect fruit peel from photooxidative damage.

梨树（Pyrus bretschneideri Regd.）自然休眠和休眠解除时，花芽的活性氧代谢发生变化。O2-·产生速率和H2O2的含量在休眠期间上升，在休眠后期下降。抗氧化系统中SOD活性在自然休眠期呈下降趋势，自然休眠结束活性上升。POD和CAT活性在自然休眠期上升。抗氧化物质ASA和GSH的含量随休眠进行而下降，休眠解除过程中重新升高。APX和GR的活性在休眠期间活性下降，休眠结束活性迅速上升。这些结果表明：花芽的休眠与活性氧的代谢有很大关系。

Approximately 80 days after full bloom, well-exposed fruit on the south part of the canopy of mature Liberty/M9 apple (Malus domestica Borkh.) trees were randomly assigned to one of the following two treatments. Some fruits were turned about 180◦ to expose the original shaded side to full sun, whereas the rest served as untreated controls. On days 0, 1, 2, 4, 7 and 10 after treatment, fruit peel samples were taken from the original shaded side of the treated fruit and both the sun-exposed side and the shaded side of the control fruit at midday, to determine photosynthetic pigments, enzymatic and non-enzymatic antioxidants. Maximum photosystem II (PSII) efficiency, measured as Fv/Fm (maximum fluorescence) of chlorophyll fluorescence at pre-dawn, was higher in the shaded side than the sun-exposed side of the control fruit. Fv/Fm of the original shaded side decreased sharply after 1 day exposure to full sun, and then gradually recovered to a similar value of the sun-exposed side of the control fruit by day 10. The shaded side of the control fruit had much lower xanthophyll cycle pool size, conversion and antioxidant enzymes and also soluble antioxidants of the ascorbate–glutathione cycle than the sun-exposed side. Xanthophyll cycle pool size of the original shaded side increased in response to full sun exposure, reaching a similar value of the sun-exposed side by day 10. Almost all the xanthophyll cycle pool was converted to zeaxanthin and antheraxanthin starting from the first day of exposure to full sun. In contrast, chlorophyll content decreased whereas lutein or neoxanthin content remained unchanged. In response to full sun exposure, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase and total pool size and reduction state of both ascorbate and glutathione, all increased to the corresponding values found in the sun-exposed side of the control fruit over a 10-day period. It is concluded that both xanthophyll cycle and the ascorbate-glutathione cycle in the original shaded side are up-regulated in response to full sun exposure to minimize photo-oxidative damage and contributes to its acclimation to full sun.