Pak. J. Bot., 36(4): 725-731, 2004. THE EPIDERMAL CHARACTERISTICS OF FRUIT SKIN OF SOME SWEET CHERRY CULTIVARS IN RELATION TO FRUIT CRACKING LEYLA DEMIRSOY AND HUSNU DEMIRSOY Department of Horticulture, Faculty of Agriculture, Ondokuz Mayıs University, 55139 Kurupelit-Samsun, Turkey. Abstract The epidermal characteristics of fruit skin of 8 sweet cherry cultivars were determined in terms of susceptibility to cracking. The cultivars 0900 Ziraat and Bella di Pistoia having the thickest cuticle, had the lowest cracking index. There was a negative correlation between the cuticle thickness and fruit cracking. There were no correlations relationship between fruit cracking and thickness of epidermis, thickness of subepidermis and number of subepidermal cell layers. Introduction Fruit cracking caused by rainfalls before harvest is a major problem in sweet cherry growing in many parts of the world. In spite of the fact that many researches have been carried out on the phenomenon but the cracking mechanism has not yet been completely understood. The prevailing theories on cracking mechanism have been based on the water penetrating the fruit surface causing an increase in volume of the fruit that itself causes cracking (Sawada, 1931; Verner & Blodget, 1931; Christensen, 1972, 1976, 1994). Some other studies indicated that the water uptake by the fruit from the root system, causing internal turgor pressure buildup, plays an important role in cracking mechanism (Kertesz & Nebel, 1935; Considine & Kriedeman, 1972; Yamamoto et al., 1990). Recently according to a new theory, the properties and function of the fruit cuticle and water conducting tissues (xylem and phloem) in the tree, the fruit pedicel and fruit itself are crucial to the sweet cherry cracking mechanism (Sekse, 1995a,b; 1998). There is a great difference among the sweet cherry cultivars in terms of susceptibility to cracking. Several theories have been put forward by researches as to these differences eg., i. The varieties that take up more water from their fruit surface are likely to be more sensitive to cracking. ii. The varieties that have skins with epidermal cells that are more strongly bound together are likely to resist cracking. The fruit skin, its cuticle and epidermis may be more of a barrier to water uptake into fruit in the resistant varieties (Webster & Cline, 1994). Sweet cherry fruit cuticle acts as an efficient water barrier against water uptake from the fruit surface, but contains pores that allow some water penetration. Another important property of the sweet cherry fruit cuticle influencing fruit cracking is its rigidity due to its wax-like structure. Irregular water supply to the fruits causes formation of parenchymatous tissue, thereby fruit grows irregularly. Concurrent with this the waxed cuticle does not stretch and fractures develop (Sekse, 1995b). Such cuticular fractures would allow greater water penetration (Glenn & Poovaiah, 1989).