Changes in strain and deposition of cuticle in developing sweet cherry fruit

Changes in surface area, deposition and elastic strain of the cuticular membrane (CM) were monitored during development of sweet cherry (Prunus avium L.) fruit. Fruit mass and surface area (‘Sam’) increased in a sigmoidal pattern between 16 and 85 days after full bloom (DAFB) with maximum rates of 0.35 g day−1 and 0.62 cm2 day−1, respectively. Rates of total area strain, namely the sum of elastic plus plastic strain, were highest in cheek and stem cavity regions followed by stylar and suture regions. Rates of total uniaxial strain were higher in transverse, namely perpendicular to the stem/stylar axis, than in longitudinal direction, namely parallel to the stem/stylar axis. On a whole fruit basis CM mass remained essentially constant during fruit development. Mass of CM, dewaxed CM and wax per unit surface area decreased during development, particularly between 43 and 71 DAFB. There was no change in wax content of isolated CM. Up to 43 DAFB the surface area of isolated CM was similar to the area prior to excision indicating little elastic strain, but markedly decreased thereafter. Calculating elastic and plastic components of total strain of the CM revealed, that initial deformation up to 22 to 43 DAFB was mostly plastic. Thereafter, elastic strain was evident and both, elastic and plastic deformation, increased linearly with an increase in total strain. There was no consistent difference in the relative contribution of elastic strain to total strain between transverse and longitudinal directions, but both total and elastic strain were larger in the transverse direction. Abrading the CM had only little effect on fruit turgor. However, turgor decreased when the exocarp was cut indicating that the exocarp provided a significant structural shell of a mature sweet cherry fruit (‘Regina’). Our data demonstrate, that (1) surface area expansion in sweet cherry fruit causes elastic and plastic strain of the CM, and (2) the onset of elastic strain coincided with the cessation of CM formation.