This study investigated the effects of shading on photosynthesis in 3-year-old potted apple trees. The trees were grown either in full sunlight or in a shade environment with about 50% of sunlight filtered through black plastic nets. Under the shade conditions, the net photosynthetic rate (PN), the stomatal conductance, the transpiration rate, and the intercellular CO2 concentration decreased before midday, which was due to low light energy but the PN of the shaded trees increased relative to the control trees during the afternoon. In addition, the concentrations of chlorophyll (Chl) a, b, and total Chl increased in the shaded trees. In shaded apple trees, the photochemical efficiency increased. There was a lower content of reactive oxygen species and activities of antioxidant enzymes. The results showed that moderate shading can improve the total photosynthetic efficiency, which is mainly attributed to alleviation of photodamage and low accumulation of reactive oxygen species.

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