Morphological Changes Associated with Bruising in Strawberry Fruit Surface and Internal Tissues

Strawberry (Fragaria ananassa Duch.) is among the most widely consumed fruits worldwide. The soft and juicy texture of ripe strawberries is highly favored by a wide range of consumers. Despite significant improvements in packaging and transport, deterioration of strawberry fruit quality during distribution remains a major problem (Tatara et al., 1999). The fruit is subject to bruising from its own weight as well as external forces, such as contact with other fruits and packages. Fruit bruising is a serious problem, as it cannot be efficiently detected using common color machine vision systems (Nagata et al., 2006), resulting in consumer complaints and economic losses. The segregation of bruised fruits from non-bruised fruits can be both laborand time-intensive. Therefore, effective preventing fruit bruising is essential for improving economic benefits and reducing labor requirements for the distribution of strawberries. However, the precise mechanism of bruising in strawberries is poorly understood. The location and the morphological structure of the bruise in strawberries are controversial. Previous studies showed that firmness enhancement of the fruit surface by pre-cooling and cold chain processing cannot adequately prevent bruise occurrence but considerably reduces the occurrence of scratches (Ootake and Tanaka, 1988). Accordingly, internal tissues are speculated to be the primary site of bruising, whereas the skin surface is thought to be that of scratching in strawberries. Cell rupture and juice release have been observed in bruised apple (Holt and Schoorl, 1977; Ingle and Hyde, 1968), which has also been predicted in bruised strawberry fruit (Holt and Schoorl, 1982). However, the results of previous studies and precise mechanism of bruising in strawberries remain inconclusive, as most did not directly examine the skin and flesh of the bruised fruit in detail. Because the skin of strawberry fruit is very thin and fragile, it may become bruised with or without external force to the internal tissues. In addition, tensile tests showed that strawberry cells can be isolated without fracturing (Harker et al., 2000), suggesting that flesh or pith cells are less likely to rupture. It is important to directly determine whether morphological changes in the fruit skin, flesh, and pith are associated with bruising. Determining the mechanism of bruising will contribute to the development of strawberry breeding, cultivation, and post-harvest techniques. If the skin over the bruised internal tissue is not transformed, the role of skin firmness may be less important in bruising. In addition, if bruised tissue cells are not ruptured, then the robustness of tissue cells does not need to be increased further in strawberry breeding or cultivation. Another study showed that the generation of wound volatile compounds, which play an important role in plant–fungus interactions, is related to the morphological structure of fruit skin (Myung et al., 2006). This implies that changes in the three-dimensional structure of the fruit surface are involved in microbial attachment, infection, and removal (Liao and Sapers, 2000; Wang et al., 2009). Therefore, morphological assessment of the fruit skin overlaying bruised tissues is important for determining whether large amounts of fungicides can be used to cultivate bruised fruits. We performed confocal laser scanning microscopy to determine the morphological structure of the skin and other tissues in bruised strawberry fruit of cultivars with different mechanical properties. It is difficult to observe the sur-