Diffusion in plant cuticles as affected by temperature and size of organic solutes : similarity and diversity among species
Mobilities of lipophilic organic solutes in cuticular membranes (CM) isolated from mature leaves of Citrus aurantium L., Citrus grandis L., Hedera helix L., IIex aquifolium L., Ilex paraguariensis St.-Hil., Mains domestica Borkh., Prunus armeniaca L., Primus laurocerasus L., Pyrus communis L., Pyrus pyrifolia (Burm. f.) Nakai, Stephanotis florihunda Brongn. and Strophantus gratus Baill. were measured over a temperature range of 15–78°C. In this range, solute mobilities increased up to 1000-fold, which corresponds to temperature coefficients Q10 of 3 (IAA in P. armeniaca) to 14 (cholesterol in H. helix). For most species, Arrhenius graphs showed good linearity up to 40°C, and up to 78°C for some species, while for others activation energies declined with increasing temperature. However, no distinct phase transitions caused by sudden structural changes in the CM were observed. In three species we examined whether heating to 70°C changed solute mobility irreversibly by comparing Arrhenius graphs for two successive experiments with the same CM. The two graphs were very similar for P. laurocerasus, while mobilities in the second graph were somewhat reduced for C. aurantium and greatly increased (at 25 and 35°C) for H. helix. This indicates rearrangements of at least some wax constituents when heated to high temperatures. The activation energies of diffusion (ED) ranged from 75 to 189 KJ mol−11 depending on species and solute size. Size selectivity and variability between cuticles decreased with increasing temperature, and this is caused by differences in (ED). An excellent correlation between the pre-exponential factor of the Arrhenius equation and ED was observed, which is evidence that organic solutes differing greatly in molecular size (130–349 cm3 mol−1) and cuticle/water partition coefficient (25–108) use similar diffusion paths in the CM of all 12 plant species tested. Diffusion occurs in regions with identical physicochemical properties and differs only in magnitude.