The Photosynthetic Activity of Cox's Orange Pippin Apple Flowers in Relation to Fruit Setting

The ability of green parts of Cox's apple flowers (sepals, receptacle and pedicel) to photosynthesise was demonstrated using 14CO, feed experiments and an infra-red gas analysis (IRGA) system. The sepals had the greatest ability, comparable with that of leaves, followed by the receptacle. Sepals and leaves fixed the same amount of CO2 in the dark. Photosynthesis and respiration rates in orchard flowers were measured in comparison to those of leaves. Flower gross photosynthesis (GP) was about a third of that of leaves at the balloon stage and 15 d after full bloom in 1987, but this ratio was much less at full bloom (FB) in the same year. GP in flowers decreased rapidly with fruitlet enlargement. It was estimated that flower photosynthesis significantly contributed to their carbohydrate balance (15-33%) during the flowering and fruit setting periods. The highest contributions were made at the green cluster stage (33%) and between 6 and 11 d after FB (27%), the period when the fruit set had been initiated. Removal of sepals, the main photosynthetic parts of the flower, at the pink bud stage decreased initial set. The results suggest that flower photosynthesis might play an important role in flower growth and fruit setting.