Tomato (Lycopersicon esculentum Mill.) fruit growth and ripening as related to the biomechanical properties of fruit skin and isolated cuticle

The control of growth rate and the mechanical integrity of the tomato (Lycopersicon esculentum Mill.) fruit has been attributed to the exocarp. This study focused on the biomechanics of the fruit skin (FS) comprising cuticle, epidermis and a few subdermal cell layers, and the enzymatically isolated cuticular membrane (CM) during fruit growth and ripening. Morphology and mechanical properties of the FS and the CM of three cultivars were analysed separately at three distinct ripening stages by scanning electron microscopy (SEM) and one-dimensional tension testing, respectively. Both were subject to significant cultivar-specific changes. Thickness of the CM increased during ripening from 7.8-8.6 to 9.9-15.7 microm and exceeded by far that of the epidermal cell wall. The mechanical properties, such as modulus of elasticity, strength, and failure strain, were highest in the FS for all cultivars at any stage, with only one exception; however, the cuticle largely mirrored these properties throughout fruit maturation. Stiffness of both isolated CM and FS increased from immature to fully ripe fruits for all cultivars, while failure stress and failure strain displayed a tendency to decrease for two of them. Stress-strain behaviour of the CM could be described as strain softening, mostly linear elastic throughout, and strain hardening, and was subject to growth-related changes. The FS displayed strain hardening throughout. The results indicate evidence for the cuticle to become increasingly important as a structural component for the integrity of the tomato fruit in addition to the epidermis. A supplementary putative model for tomato fruit growth is proposed.