Estimation of path lengths for diffusion of organic compounds through leafcuticles

The effect of waxes on the mobilities of organic solutes in isolated leaf c uticular membranes (CM) of six species was investigated using compounds wit h molar volumes ranging from 99 cm(3) mol(-1) (salicylic acid) to 349 cm(3) 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 solute s. The slopes of these graphs represent the size selectivity of the CM, Ext racting waxes from the CM using chloroform-methanol had no effect on size s electivity, but absolute values of mobilities increased by 28-fold (Citrus grandis L, and Camellia assamica L,) to 759-fold (Ilex paraguariensis St.-H il), Since the removal of waxes did not affect size selectivity it is argue d that extraction reduced the path lengths and these values are a measure f or tortuosity, that is the ratio of path lengths in CM and MX, Mobilities i n the polymer matrix of extracted cuticles did not differ much among specie s with the exception of C. assamica, Although the mobilities in CM of I. pa raguariensis and C. assamica were similar, the mobilities in the polymer ma trix of C. assamica were almost 40-fold lower than in I. paraguariensis, Ne ither the mobilities in CM and MX nor the tortuosity were correlated with t hickness of CM or amounts of wax.