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

Water conductance of the cuticular membrane (CM) of sweet cherry (Prunus avium L. cv. Sam) fruit during stages II and III (31–78 days after full bloom, DAFB) was investigated by gravimetrically monitoring water loss through segments of the exocarp. Segments were mounted in stainless-steel diffusion cells, filled with 0.5 ml of deionized water and incubated for 8 h at 25±2 °C over dry silica. Conductance was calculated by dividing the amount of water transpired per unit surface area and time by the difference in water vapor concentration across the segment (23.07 g m–3 at 25 °C). Fruit mass and fruit surface area increased 4.9- and 2.8-fold between 31 and 78 DAFB, respectively. However, CM mass per unit area decreased from 3.9 to 1.5 g m–2, and percentage of total wax content remained constant at about 31%. Stomatal density decreased from 0.8 to 0.2 mm–2 (31–78 DAFB). Total conductance of the CM on the fruit cheek (g tot. ) remained constant during stage II of development (approx. 1.38×10–4 m s–1 from 31 to 37 DAFB), increased to 1.73×10–4 m s–1 during early stage III of fruit growth (43–64 DAFB) then decreased to 0.95×10–4 m s–1 at maturity (78 DAFB). Partitioning g tot. into cuticular (g cut. ) and stomatal conductance (g sto. ) revealed that the relative contribution of g cut. to g tot. increased linearly from 30% to 87% of g tot. between 31 and 78 DAFB, respectively. On a whole-fruit basis, g tot. and g cut. consistently increased up to 64 DAFB, and decreased thereafter. A significant negative linear relationship was obtained between g cut. and CM thickness, but not between the permeability coefficient (p) and CM thickness. Further, p was positively related to strain rate, suggesting that strain associated with expansion of the fruit surface increased p.