Changes in the structure of the skin of kiwifruit in relation to water loss

SUMMARY Fruit water loss and calcium inflow decrease in kiwifruit (Actinidia deliciosa ‘Hayward’) after the first few weeks of growth, but the relationship between fruit transpiration and the structure of the skin is not clear. Changes in skin structure during fruit development were studied using cryo-scanning electron microscopy and the concentrations and types of waxes present in the skin were measured. Young fruit were covered in turgid unicellular and multicellular hairs, which grew on an amorphous and continuous layer of wax platelets. However, 5 weeks after fruit set, the hairs began to collapse and the wax layer began to crack. The epidermal cells dehydrated and suberised to form a peridermlike layer. The degeneration of the hairs and the wax layer continued as the fruit matured. Fruit surface area increased from 6.9 cm 2 at 4 weeks after fruit set, to 98.2 cm 2 at 20 weeks after fruit set (i.e., at harvest), while the total wax content increased from 0.33 to 2.40 mg per fruit. Total wax density peaked at 91 µg cm ‐2 5 weeks after fruit set, then decreased to 24 µg cm ‐2 at harvest. Alkyl alkanoates were the main waxes [approx. 70% (w/w)] in young fruit, while hydrocarbons and alkyl alkanoates (approx. 50% each) were the main waxes in mature fruit. The decline in fruit water loss in kiwifruit was consistent with degeneration of the hairs on the skin, the evolution of the suberised outer layers, and the death of outer cells associated with wax biosynthesis.