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 av ium L. cv. Sam) fruit during stages II and III (31-78 days after full bloom , DAFB) was investigated by gravimetrically monitoring water loss through s egments 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 degreesC over dry silica. Conductance was calculated by dividing the am ount of water transpired per unit surface area and time by the difference i n water vapor concentration across the segment (23.07 g m(-3) at 25 degrees C). Fruit mass and fruit surface area increased 4.9- and 2.8-fold between 3 1 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 a t 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 constan t during stage II of development (approx. 1.38x10(-4) in s(-1) from 31 to 3 7 DAFB), increased to 1.73x10(-4) in s-1 during early stage III of fruit gr owth (43-64 DAFB) then decreased to 0.95x 10(-4) in s(-1) at maturity (78 D AFB). Partitioning g(tot) into cuticular (g(cut)) and stomatal conductance (g,,) 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 permeabil ity 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.