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 fru it (Prunus avium L. cv. Sam) was investigated by monitoring water loss from segments of the outer pericarp excised from the cheek of the fruit. Segmen ts consisted of epidermis, hypodermis and several cell layers of the mesoca rp. Segments were mounted in stainless-steel diffusion cells with the mesoc arp surface in contact with water, while the outer cuticular surface was ex posed to dry silica (22 +/- 1 degreesC). Conductance was calculated by divi ding the amount of water transpired per unit area and time by the differenc e 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 carbonylc yanide 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 n o consistent effect on conductance. Conductance of the CM increased from ch eek (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 st omatal density. From this relationship the cuticular conductance of a hypot hetical astomatous CM was estimated to be 0.97 +/- 0.09 x 10(-4) m s(-1). R emoval of epicuticular wax by stripping with cellulose acetate or extractin g epicuticular plus cuticular wax by dipping in CHCl3/methanol increased co nductance 3.6- and 48.6-fold, respectively. Water fluxes increased with inc reasing temperature (range 10-39 degreesC) and energies of activation, calc ulated 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 conduct ance, respectively.