Estimation of path lengths for diffusion of organic compounds through leaf cuticles
The effect of waxes on the mobilities of organic solutes in isolated leaf cuticular membranes (CM) of six species was investigated using compounds with molar volumes ranging from 99 cm3 mol–1 (salicylic acid) to 349 cm3 mol–1 (cholesterol). When log(solute mobility) was plotted versus molar volumes straight lines were obtained for all plant species showing that the mobilities decreased exponentially with increasing molar volumes of solutes. The slopes of these graphs represent the size selectivity of the CM. Extracting waxes from the CM using chloroform–methanol had no effect on size selectivity, but absolute values of mobilities increased by 28-fold (Citrus grandis L. and Camellia assamica L.) to 759-fold (Ilex paraguariensis St.-Hil). Since the removal of waxes did not affect size selectivity it is argued that extraction reduced the path lengths and these values are a measure for tortuosity, that is the ratio of path lengths in CM and MX. Mobilities in the polymer matrix of extracted cuticles did not differ much among species with the exception of C. assamica. Although the mobilities in CM of I. paraguariensis and C. assamica were similar, the mobilities in the polymer matrix of C. assamica were almost 40-fold lower than in I. paraguariensis. Neither the mobilities in CM and MX nor the tortuosity were correlated with thickness of CM or amounts of wax.