Low cuticle deposition rate in 'Apple' mango increases elastic strain, weakens the cuticle and increases russet.

Russeting compromises appearance and downgrades the market value of many fruitcrops, including of the mango cv. ‘Apple’. The objective was to identify the mechanistic basis of ‘Apple’ mango’s high susceptibility to russeting. We focused on fruit growth, cuticle deposition, stress/strain relaxation analysis and the mechanical properties of the cuticle. The non-susceptible mango cv. ‘Tommy Atkins’ served for comparison. Compared with ‘Tommy Atkins’, fruit of ‘Apple’ had a lower mass, a smaller surface area and a lower growth rate. There were little differences between the epidermal and hypodermal cells of ‘Apple’ and ‘Tommy Atkins’ including cell size, cell orientation and cell number. Lenticel density decreased during development, being lower in ‘Apple’ than in ‘Tommy Atkins’. The mean lenticel area increased during development but was consistently greater in ‘Apple’ than in ‘Tommy Atkins’. The deposition rate of the cuticular membrane was initially rapid but later slowed till it matched the area expansion rate, thereafter mass per unit area was effectively constant. The cuticle of ‘Apple’ is thinner than that of ‘Tommy Atkins’. Cumulative strain increased sigmoidally with fruit growth. Strains released stepwise on excision and isolation (eexc+iso), and on wax extraction (eextr) were higher in ‘Apple’ than in ‘Tommy Atkins’. Membrane stiffness increased during development being consistently lower in ‘Apple’ than in ‘Tommy Atkins’. Membrane fracture force (Fmax) was low and constant in developing ‘Apple’ but increased in ‘Tommy Atkin’. Membrane strain at fracture (emax) decreased linearly during development but was lower in ‘Apple’ than in ‘Tommy Atkins’. Frequency of membrane failure associated with lenticels increased during development and was consistently higher in ‘Apple’ than in ‘Tommy Atkins’. The lower rate of cuticular deposition, the higher strain releases on excision, isolation and wax extraction and the weaker cuticle account for the high russet susceptibility of ‘Apple’ mango.