Abstract: Diurnal variations in photosynthesis，chlorophyll fluorescence，antioxidant enzymes and antioxidant metabolism in leaves in response to low sink demand caused by fruit removal were studied in ‘Sweet Charlie’strawberry. Compared with the retained fruit treatment（control），the fruit removal treatment resulted in a significantly lower photosynthetic rate，stomatal conductance and transpiration rate，but generally higher intercellular CO2 concentration. Compared with the control plants，leaves of fruit removal plants had significantly lower actual efficiency of PSⅡ（ΦPSⅡ），photochemical quenching coefficient（qP），efficiency of excitation capture by open PSⅡreaction centers（Fv’/Fm’），but higher non-photochemical quenching（NPQ）. The fruit removal treatment resulted in higher hydrogen peroxide （H2O2），superoxide radical（）and malondialdehyde（MDA）concentrations compared with the control, indicating existence of photo-oxidative damage. Compared with the control plants，leaves of fruit removal plants had significantly higher activities of antioxidant enzymes，including superoxide dismutase（SOD），catalase（CAT）and a higher reduction state of ascorbate（AsA）and glutathione（GSH）. These results indicated that the antioxidant system was up-regulated providing protection from reactive oxygen species damage to leaves during low sink demand.

Key words: strawberry, photosynthesis, photoprotection, antioxidant systems, source-sink relationship