Mobilities of organic compounds in plant cuticles as affected by structure and molar volumes of chemicals and plant species

Abstract

Abstract. Solute mobilities of 28 compounds in isolated cuticular membranes (CM) from Capsicum annuum L. fruit, Citrus aurantium L. and Pyrus communis L. leaves were studied using unilateral desorption from the outer surface. First-order rate constants of desorption (k*), which are directly proportional to the diffusion coefficient in the waxy outer limiting skins of cuticles were measured. When log k* was plotted vs. molar volumes of test compounds linear graphs were obtained. The y-intercepts of these graphs (k *~ represent the mobility of a hypothetical molecule having zero molar volume and the slopes of the graphs (13') represent the size selectivity of the barrier and are related to the free volume available for diffusion. Thus, solute mobilities in cuticles are composed of two independent terms which are subtractive. If k *~ and 13' are known, k* can be estimated for any solute from its molar volume (Vx) using the equation log k*= log k *~ 13'Vx. These parameters were used to analyse the effects of plant species, extraction of cuticular waxes and molecular structure of solutes on solute mobilities in plant cuticles. For aliphatic solutes, k *~ was a factor of 10 smaller than for cyclic compounds, while 13' was 0.011 and 0.012, respectively. The k*-values for CM of the three species were very similar, but 13' was higher for bitter-orange CM (0.012) than for those 9 f pepper fruits and pear leaves (0.009). This has the consequence that differences in solute mobilities (k *~ among cuticles from different plan species increase with increasing molar volumes of solutes. Our data and our analysis provide evidence that constituents of cuticular waxes are mobile, at least in the solid amorphous wax fraction, but mobility decreases rapidly with increasing molar volume. For instance, if 13' amounts to 0.01, mobilities of wax monomers decrease by a factor of 10 for every increase in molar volume of 100 cm 3. mol-1. Thus, hexaAbbreviations: CM = cuticular membrane(s); 2,4-D = 2,4-dichlorophenoxyacetic acid; LAB = lactic acid buffer; MX = polymer matrix membranes; UDOS = unilateral desorption from the outer surface Correspondence to: P. Baur; FAX: 49 (5066) 82 61 1 l decanoic acid is quite mobile in the amorphous wax fraction of Citrus (k* = 1.5 x 10 -6. s-l), but for dotriacontane having twice the molar volume, k* was only 2.5 x 10-9. s-a, which is almost three orders of magnitude smaller. Wax esters have even higher molar volumes and their mobilities will be even smaller (about 4 x 10-12. s-1 for a C4s-ester). Since low chain mobilities are a prerequisite for low mobilities and permeabilities, the selective advantage of high-molecular-weight wax monomers in plant cuticular waxes becomes obvious. Extracting cuticular waxes from pear leaf CM increased solute mobilities by a factor of 182, but it had no effect on size selectivity. We interpret this result as evidence to the effect that cuticular waxes reduce mobility by increasing tortuosity of the diffusion path, rather than by decreasing the mean free path of diffusiona...

Diffusion in plant cuticles as affected by temperature and size of organic solutes: similarity and diversity among species

“…In agreetnent with earlier studies (Bauer & Schonherr 1992;Baur et al 1996b), we found a veiy pronounced dependence of solute mobilities on molar volumes, especially at low temperatures, while at 60-70 °C differences almost vanished (Fig. 4).…”

“…The mobility of a solute depends on its size (Bauer & Schonhen-1992), and we have recently shown that tnobilities in CM of different plant species decrease exponentially with size of .solutes as expressed in tnolar volumes (Schonherr & Baur 1994;Baur et at. 1996b).…”

“…Solutions of cholesterol eontained 0-4; 100 (mass;volume) polyethylene glycol (PEG 400; Roth, Karlsruhe, Germany) in a 30;70 (volume;volume) mixture of ethanol;water. This does not change the transport properties of CM, as has been shown by Baur et al (1996b). WLI10547 was dissolved in a 40;60 (volume;volume) mixture of ethanol;water.…”

Lognormal distribution of water permeability and organic solute mobility in plant cuticles

“…However, to keep the data comparable to recently published results, especially concerning organic solute mobility in plant cuticles (e.g. Bauer & Schonherr 1992;Schonherr & Baur 1994;Baur et al 1996), this option was not used. Statistical tests and further data analysis were carried out using SPSS for Windows 5-0 (1993; Chicago, USA) and Multigraf 3-1 (1994; G-logic, Heidelberg, Germany).…”

Effect of surface waxes on the persistence of chlorpyrifos-methyl in apples, strawberries and grapefruits

“…The higher the pesticide persistence in the fruit, the higher the potential risk to the consumer via food intake. For this reason, studies of the effects of the cuticle and waxes on pesticide persistence can provide important data for the refinement of consumer risk assessment, and inform possible options for the mitigation of pesticide residues in the transport limiting barrier of the cuticle has a large tortuosity which is reduced or removed upon the removal of waxes (Schönherr and Baur, 1994;and Baur et al, 1996).…”