Studies on water transport through the sweet cherry fruit surface: characterizing conductance of the cuticular membrane using pericarp segments

Water conductance of the cuticular membrane (CM) of mature sweet cherry fruit (Prunus avium L. cv. Sam) was investigated by monitoring water loss from segments of the outer pericarp excised from the cheek of the fruit. Segments consisted of epidermis, hypodermis and several cell layers of the mesocarp. Segments were mounted in stainless-steel diffusion cells with the mesocarp surface in contact with water, while the outer cuticular surface was exposed to dry silica (22 +/- 1 degreesC). Conductance was calculated by dividing the amount of water transpired per unit area and time by the difference in water vapour concentration across the segment. Conductance values had a log normal distribution with a median of 1.15 x 10(-4) m s(-1) (n = 357). Transpiration increased linearly with time. Conductance remained constant and was not affected by metabolic inhibitors (1 mM NaN3 or 0.1 mM carbonylcyanide m-chlorophenylhydrazone) or thickness of segments (range 0.8-2.8 mm). Storing fruit (up to 42 d, 1 degreesC) used as a source of segments had no consistent effect on conductance. Conductance of the CM increased from cheek (1.16 +/- 0.10 x 10(-4) ms(-1)) to ventral suture (1.32 +/- 0.07 x 10(-4) m s(-1)) and to stylar end (2.53 +/- 0.17 x 10(-4) m s(-1)). There was a positive relationship (r(2) = 0.066**; n = 108) between conductance and stomatal density. From this relationship the cuticular conductance of a hypothetical astomatous CM was estimated to be 0.97 +/- 0.09 x 10(-4) m s(-1). Removal of epicuticular wax by stripping with cellulose acetate or extracting epicuticular plus cuticular wax by dipping in CHCl3/methanol increased conductance 3.6- and 48.6-fold, respectively. Water fluxes increased with increasing temperature (range 10-39 degreesC) and energies of activation, calculated for the temperature range from 10 to 30 degreesC, were 64.8 +/- 5.8 and 22.2 +/- 5.0 kJ mol(-1) for flux and vapour-concentration-based conductance, respectively.