Studies on water transport through the sweet cherry fruit surface: III. Conductance of the cuticle in relation to fruit size

Rain-cracking of sweet cherry fruit has been related to water absorption through the fruit surface and large fruit has been reported to be more susceptible to cracking than small fruit. Therefore, the effect of fruit size on water conductance of the cuticular membrane (CM) of exocarp segments excised from cheek, suture or stylar end region of mature sweet cherry fruit (Prunus avium L. cv. Sam) was investigated. Segments consisting of epidermis, hypodermis and several layers of mesocarp cells were mounted in diffusion cells filled with deionized water. Mass loss due to transpiration was monitored gravimetrically during an 8-h incubation period (25 +/- 2degreesC) over dry silica in the dark. Conductance was calculated from the amount of water transpired per unit surface area and time divided by the difference in water vapour concentration across the segment. For an average size cv. Sam sweet cherry CM conductance was k1.06 x 10(-4), 0.91 x 10(-4) and 2.09 x 10(-4) m s(-1) in cheek, suture and stylar end region, respectively. Fruit size had no significant effect on conductance in cheek or suture regions, but for the stylar end region conductance was positively related to fruit size: Stomatal density in the cheek, but not the suture or stylar end region increased as fruit size increased. The area of the stylar scar was positively related to fruit size. Conductance of the stylar scar averaged 37.6 +/- 4.0 x 10(-4) m s(-1) and was 54-fold higher than that of the CM between stomata in the cheek region (mean 0.69 x 10(-4) m s(-1)). Conductance calculated on a whole fruit basis is estimated to increase by 108% as fruit size increases from 6 to 12 g. Increased conductance on a whole fruit basis may be attributed to increased fruit surface area and increased conductance per unit fruit surface area, particularly in the stylar end region.