Russeting in apple and pear: a plastic periderm replaces a stiff cuticle

BACKGROUND AND AIMS Russeting in apples (Malus × domestica Borkh.) and pears (Pyrus communis L.) is a disorder of the fruit skin that results from microscopic cracks in the cuticle and the subsequent formation of a periderm. To better understand russeting, rheological properties of cuticular membranes (CM) and periderm membranes (PM) were studied from the russet-sensitive apple 'Karmijn de Sonnaville' and from 'Conference' pear. METHODOLOGY The CM and PM were isolated enzymatically, investigated by microscopy and subjected to tensile tests, creep/relaxation tests and to stepwise creep tests using a material testing machine. PRINCIPAL RESULTS The isolated CM formed a continuous polymer, whereas the PM represented a cellular structure of stacked cork cells. Tensile tests revealed higher plasticity of the hydrated PM compared with the CM, as indicated by a higher strain at the maximum force (ɛ(max)) and a lower modulus of elasticity (E). In apple, the maximum force (F(max)) was higher in the CM than in the PM but in pear the higher F(max) value was found for the PM. In specimens obtained from the CM : PM transition zone, the weak point in apple was found to be at the CM : PM borderline but in pear it was within the CM. In both apple and pear, creep/relaxation tests revealed elastic strain, creep strain, viscoelastic strain and viscous strain components in both the PM and CM. For any particular force, strains were always greater in the PM than in the CM and were also greater in pear than in apple. The ɛ(max) and F(max) values of the CM and PM were lower than those of non-russeted and russeted whole-fruit skin segments, which included adhering tissue. CONCLUSIONS In russeting, stiff CM are replaced by more plastic PM. Further, the cell layers underlying the CM and PM represent the load-bearing structure in the fruit skin in apple and pear.