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Journal of Experimental Botany, Vol. 53, No. 375, pp. 1815-1823, August 1, 2002
© 2002 Oxford University Press

Semi-volatile organic compounds at the leaf/atmosphere interface: numerical simulation of dispersal and foliar uptake

Received 15 March 2002; Accepted 15 April 2002

Markus Riederer4,1 , Andreas Daiß2 , Norbert Gilbert2 and Harald Köhle3

1 Julius-von-Sachs-Institut für Biowissenschaften, Lehrstuhl für Botanik II, Universität Würzburg, Julius-von-Sachs-Platz 3, D-97082 Würzburg, Germany
2 BASF AG, Technische Entwicklung, Verfahrenstechnik, D-67056 Ludwigshafen, Germany
3 BASF AG, Landwirtschaftliche Versuchsstation, D-67114 Limburgerhof, Germany

4 To whom correspondence should be addressed. Fax: +49 931 888 62 35. e-mail: riederer{at}botanik.uni-wuerzburg.de


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results and discussion
 Conclusions
 References
 
The behaviour of (semi-)volatile organic compounds at the interface between the leaf surface and the atmosphere was investigated by finite-element numerical simulation. Three model systems with increasing complexity and closeness to the real situation were studied. The three-dimensional model systems were translated into appropriate grid structures and diffusive and convective transport in the leaf/atmosphere interface was simulated. Fenpropimorph (cis-4-[3-(4-tert-butylphenyl)-2-methylpropyl]-2,6-dimethylmorpholine) and Kresoxim-methyl ((E)-methyl-2-methoxyimino-2-[2-(o-tolyloxy-methyl)phenyl] acetate) were used as model compounds. The simulation showed that under still and convective conditions the vapours emitted by a point source rapidly form stationary envelopes around the leaves. Vapour concentrations within these unstirred layers depend on the vapour pressure of the compound in question and on its affinity to the lipoid surface layers of the leaf (cuticular waxes, cutin). The rules deduced from the numerical simulation of organic vapour behaviour in the leaf/atmosphere interface are expected to help in assessing how (semi-)volatile plant products (e.g. hormones, pheromones, secondary metabolites) and xenobiotics (e.g. pesticides, pollutants) perform on plant surfaces.

Key words: Key words: Floral scents, fluid mechanics, fungicides, hormones, pheromones, plant cuticle, stomata, uptake, volatiles.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results and discussion
 Conclusions
 References
 
The occurrence, biological function and ecological consequences of (semi-)volatile organic substances in the boundary layers enveloping leaves and other aerial parts of plants has drawn considerable attention during recent years. Volatile plant hormones like methyl jasmonate and methyl salicylate have been discovered and numerous functions of these compounds in internal signalling and external communication have been described (McConn et al., 1997; Osawa et al., 2000; Reymond and Farmer, 1998). In addition, plants produce a wide array of volatile and semi-volatile organic compounds like flower scents, pheromones, attractants, and deterrents (Harborne, 1993; Pichersky and Gershenson, 2002) which are emitted into the atmosphere. Insects, fungi and other organisms living on plants also use organic compounds for communication or recognition purposes. A special role has been attributed to the physical and chemical properties of the leaf/atmosphere interface during the exchange of chemical signals between organisms (Harborne, 1993; Kessler and Baldwin, 2001).

Further sources of (semi-)volatile organic substances on plant surfaces and in the adjacent headspace are anthropogenic compounds like the active ingredients of pesticides or pollutants deposited on the aerial parts of plants. The behaviour of these compounds on the leaf surface and the leaf boundary layer will decide on whether the substance is taken up into the underlying leaf tissue or (re)dissipated into the environment. Depending on the actual situation and perspective either biological action and systemicity or contamination of plant material and loss to the atmosphere may occur.

The diffusion and convection of CO2, O2 and water vapour across the leaf/atmosphere interface and within the leaf boundary layer are well understood (Monteith, 1981; Schuepp, 1993; Gates, 1980; Thornley and Johnsen, 1990). However, the theoretical tools used for analysing and predicting the behaviour of small inorganic molecules are not fully sufficient when vapours of larger organic compounds are to be considered. In the latter case, in addition to properties governing diffusion and convection, the volatility and lipophilicity of the compound drastically influence overall behaviour. With all other properties constant, the vapour pressure of the compound controls the kinetics of release from a point source into the boundary layer. Sources may be a stomatal pore or a gland for endogenous substances or a solid or liquid deposit on the leaf surface for compounds of exogenous origin. The affinity of the compound to lipoid materials like cuticular waxes, cutin and membrane lipids counteracts its escaping tendency into the atmosphere. For organic substances with appreciable lipophilicity, the cuticle opens up as a pathway to and from the interior leaf tissues in addition to the stomatal route (Riederer, 1995). The combination of volatility and lipophilicity, together with the degree of stomatal opening or closure, will decide on whether release or uptake of organic vapours occurs through the cuticle, the stomatal pores or both.

The objectives of the present work were to model the movement of organic vapours from a superficial point source into the boundary layer adjacent to a leaf and to estimate the resulting uptake into the leaf as well as the loss into the turbulent atmosphere. The influence of stomata and of convection on these processes has also been studied. The dependence on the physico-chemical properties of organic vapour behaviour at the leaf/atmosphere interface was illustrated by using two well-characterized model compounds.

In order to answer these questions, the behaviour of organic vapours in the vicinity of the leaf surface has to be studied on a microscopic scale and with a high spatial and temporal resolution. There are no appropriate experimental tools suitable for this purpose. Therefore, the processes had to be simulated mathematically. The inherent complexity of the processes involved even under simplified conditions ruled out analytical mathematical solutions. For this reason, numerical finite-element simulations were applied to this so far intractable problem in plant ecophysiology. The finite-element approach has demonstrated its superior power in solving complex engineering problems and has also been used for the analysis of fluid flow phenomena in and around plants (Rand and Cooke, 1996; Finnigan, 2000; Rand, 1983).


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results and discussion
 Conclusions
 References
 
Model systems
The assumptions concerning the properties and the geometry of the leaf/atmosphere interface were fairly simplifying in order to keep the resulting simulations tractable. Three model systems (Fig. 1) with increasing complexity and closeness to the real situation were studied.



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  		Fig. 1. Geometry, dimensions and node pattern of the model systems used in the numerical simulations. For further explanations see Materials and methods.

 
Model system I was designed for following the diffusive vapour-phase movement of a semi-volatile organic compound away from a solid deposit over a wax-covered surface. The model system consisted of a hemisphere of air (radius 240 mm) with an impermeable bottom plate. A circular deposit of the compound (diameter 2 mm) was assumed to be situated at the centre of this plate. The deposit was surrounded by a concentric ring of wax (diameter 80 mm, thickness 50 µm). No convection was allowed within this hemisphere. A grid consisting of 3800 nodes was fitted into this system with node densities increasing toward the central deposit. Model system I allows the dispersion of semi-volatile compounds from a point source into the air-space above a wax-covered and an inert surface, respectively, to be simulated. This system is intended to mimic a situation where a compound volatilizes on a (wax-covered) leaf which is surrounded by air. It is further assumed that the compound is not taken up into the leaf.

Model system II was devized in order to assess how the geometry of the leaf surface influenced the transfer of a compound from the vapour phase into the leaf. The system was assumed to consist of a cylinder of air (diameter 1 mm, 5 mm high) on top of a circular part of leaf surface. The surface was treated as a multi-laminate structure made up of the cell wall (5 µm), the cuticular matrix (0.9 µm) and a superficial film of cuticular wax (0.1 µm). At the centre of this disc of leaf surface was a cylindrical deposit (diameter 50 µm, 25 µm high) of the semi-volatile compound in question.

The most important feature of this system was that both stomatal and cuticular pathways of uptake were considered. The dimensions of the stomatal pores (90x15 µm) were chosen so as to represent typical values for the leaves of crop species such as wheat. However, simulating the real situation with a large number of tiny stomatal pores scattered over the leaf surface would have exceeded reasonable processing times. Therefore, the total pore area of the stomata was combined into a single structure without affecting the typical ratio of total pore to leaf surface area. The structure representing the total of all stomata was thus a circular slit 10 µm wide and with an inner radius of 0.15 mm surrounding the central deposit.

In system II, it was assumed again that no other mechanism than diffusion contributed to mass transfer and that the compound was rapidly and quantitatively removed by metabolism or translocation within the interior of the leaf. Thus, its concentration at the cell wall/plasmalemma interface was kept at zero all the time. A further assumption was that the partial pressure of the compound at the site of the deposit equalled its saturation vapour pressure over a period of 8 d. Afterwards, the deposit was considered to be exhausted completely. The compound was further assumed to leave the system only via the basal plate made up by the leaf surface. Losses across the remaining boundaries of the system were set to zero as the boundaries were either far away (upper lid of the cylinder) or periodical and any losses, therefore, would have been either negligible or symmetrical. The mesh size of the grid consisting of 7900 points of intersection decreased towards the leaf surface and the circular slit in order to enhance resolution.

Model system III was again a further step closer to the real situation as it was intended to represent a two-dimensional abstraction of a wheat leaf. Thus, it allowed cuticular and stomatal uptake of an organic vapour to be studied as functions of physico-chemical properties and wind speed. Over the width of the leaf (0.5 cm) 20 small deposits and 20 stomatal pores were assumed to be situated at equal distance. The properties and dimensions of the leaf surface were analogous to those used in system II. A 2 cm thick layer of air above the leaf was included into the model system. Either no wind or a laminar flow of air (1 m s–1) was assumed to pass over the leaf perpendicular to its long axis. The leaf was assumed to be flat, smooth, stiff, and fixed horizontally. The wind speed of 1 m s–1 is a representative wind velocity encountered close to a leaf surface within a dense stand of plants not specifically exposed to strong winds (Campbell and Norman, 1998). This is a realistic scenario for the volatilization of a compound from a leaf surface within a dense stand of plants. A width of 10 µm was assumed for the stomata, so as to assure a relationship between stomatal pore area and total leaf surface area equivalent to the real situation.

Instead of treating stomata and deposits in detail simple assumptions were made. At the sites of the deposits the mass ratio of the compound in the air was set equivalent to its saturation vapour pressure. The concentration of the compound within the stomatal pore was again assumed to be negligible. Node-density of the grid (20 500 points of intersection) was highest close to the leaf surface providing a very fine resolution in the vicinity of the stomata. Due to the extremely low partial pressures of semi-volatile organics in air, the effect of water vapour diffusing out of the stomatal pores on the inward directed movement of other molecules (Jarman, 1974; von Caemmerer and Farquhar, 1981) is quantitatively irrelevant in the context of the present simulation.

Modelling mass transport across the leaf/atmosphere interface
Diffusive and convective transport in the leaf/atmosphere interface was simulated using a finite elements approach. The three-dimensional model systems were translated into appropriate grid structures and numerical simulations were performed using FIDAP version 7.6 (Fluent Inc., 1992).

The concentration of a compound in a given phase of the leaf/atmosphere system was expressed by its dimensionless mass fraction. The mass fraction of a given compound x in the air phase adjacent to the leaf surface {xi}ax is related to the vapour pressure (px) and the molar mass (Mx) of the compound, and to the atmospheric pressure (pa) and the molar mass of dry air (Ma 28.97 g mol–1) according to

The corresponding partial density of compound x in the phase j ({rho}jx) can be obtained from the mass fraction and the overall density of the phase {rho}j by

The volume-based partition coefficient of compound x between phases i and j is then given by

This relationship can be rearranged in order to calculate the mass fraction of compound x in phase j when the partition coefficient Kijx and the mass fraction of compound x in phase i are known:

Further, it was assumed that transfer over the boundary between any two phases in the system investigated occurred by diffusion only. The corresponding mass balance at the interface is

where Dx is the diffusion coefficient of compound x in the phases i and j, respectively. The gradients of the mass fractions were considered along the z-coordinate which was assumed to be perpendicular to the leaf surface.

The output of the simulations were data for mass ratios in the various phases as a function of distance and time. In some cases, the mean mass ratio averaged over the total thickness of a phase or the total amount present in a given phase were also calculated.

Physico-chemical properties of chemicals and leaves
The simulations were based on the physico-chemical properties of the two model compounds Fenpropimorph (cis-4-[3-(4-tert-butylphenyl)-2-methylpropyl]-2,6-dimethylmorpholine, FPM) and Kresoxim-methyl ((E)-methyl-2-methoxyimino-2-[2-(o-tolyloxy-methyl)phenyl] acetate, KM). These compounds are commonly used as active ingredients in fungicidal preparations. They were chosen as model compounds for this work because (1) their physico-chemical properties are well characterized and (2) because both compounds being similar in molecular mass differ by three orders of magnitude in the saturation vapour pressure and by a factor of 5 in lipophilicity (Table 1). The physico-chemical properties of the compounds were either determined experimentally or estimated from quantitative property/property relationships.


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  		Table 1. Physico-chemical properties at 25 °C of the model compounds used Experimental data on saturation vapour pressures (p°), aqueous solubilities (Sw) and 1-octanol/water partition coefficients (Kow) were provided by BASF; the remaining parameters were estimated as follows: cuticle/water partition coefficients (Kcw) from aqueous solubilities according to Riederer (1995), cuticular matrix/air partition coefficients (KMXa) from saturation vapour pressures according to Welke et al. (1998), cuticular wax/air partition coefficients (KWaxa) from KMXa according to Burghardt et al. (1998).
 
Diffusion coefficients (at 25 °C) of 1x10–10 m2 s–1, 1x10–13 m2 s–1, 1x10–18 m2 s–1, and 1x10–5 m2 s–1 were assumed for the predominantly aqueous cell wall (Nobel, 1991), the polymer matrix of the cuticle consisting of the amorphous polymer cutin (Riederer and Schreiber, 1995), the layer of semi-crystalline cuticular wax (Riederer and Schreiber, 1995) and the atmosphere (Nobel, 1991), respectively. Since the molecular sizes of the reference compounds vary only within a very narrow range, the same estimates for diffusion coefficients were used for the two compounds. For all simulations, isothermal (298 K) and isobaric conditions (101 325 Pa) were assumed.


   Results and discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
Conclusions
 References
 
Homogenous wax surface without convection
Using model system I the instationary diffusional spreading of FPM and KM was simulated for a period of 8 d. Zero time was the moment when a pure solid sample of the compound was assumed to be deposited in the centre of the system. The mass fraction of the compounds in the air just above the deposits was supposed to remain unchanged over this period of time ({xi}a=3.3x10–7 for FPM and 2.7x10–10 for KM).

As expected, the mass fraction of both compounds in the air just above (1 mm) the wax layer rapidly decreased with distance from the central deposit. When the diffusional spreading was allowed to proceed for 0.5 d, the mass fraction of FPM at the outer rim of the wax disc (r=40 mm) was lower by a factor of 41 (Fig. 2A). This difference decreased to a factor of approximately 6 after 8 d. With KM, the simulation yielded principally the same results. A minimum of the mass fraction (factor of 9000 lower than over the deposit) was predicted at a distance approximately 35 mm from the centre. At larger distances, and especially over the surrounding glass surface, {xi}a of KM increased again (Fig. 2B). After 8 d, the mass fraction at the minimum was one order of magnitude higher while the general pattern of the dependence of {xi}a on distance remained essentially unchanged.



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  		Fig. 2. One-dimensional spatial profiles of mass fractions ({xi}) of Fenpropimorph (A) and Kresoxim-methyl (B) in the gas phase over glass and wax-covered regions of model system I as a function of time.

 
Two major differences in the behaviour of both compounds can be recognized: (i) the estimates of {xi}a of KM were considerably lower than those of FPM and (ii) the dependence of {xi}a on distance from the deposit was much steeper for the former than for the latter compound. This can be explained by the differences in the saturation vapour pressures and the wax/air partition coefficients (KWaxa) which both differ by almost three orders of magnitude (Table 1). The lower saturation vapour pressure of KM results in a smaller {xi}a above the deposit. This effect is further enhanced by the much higher affinity of KM toward the wax which removes larger amounts of the compound from the air phase.

This difference in the partitioning between wax and the adjacent atmosphere also becomes evident when the simulations of the mass fractions of the two compounds in cuticular wax (1 µm below the surface) are analysed (Fig. 3). Again, the mass fraction of KM is lower than that of FPM. While the mass fractions in air differed by about three orders of magnitude (Fig. 2), the differences predicted for the wax phase amount to only two orders of magnitude. This is due to the much higher affinity KM exhibits to wax than FPM does (wax/air partition coefficients differ by approximately a factor of 600, Table 1). Similar to the situation in the gas phase, with increasing distance from the source the content of KM in the wax decreases much faster than that of FPM. Due to its affinity to wax the former compound is efficiently scavenged from the gas phase close to the source. Therefore, at larger distances only very low concentrations in the air phase occur which again results in low KM mass fractions in the wax phase. The extremely low diffusion coefficients of molecules in wax essentially excludes any equilibration by lateral diffusion.



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  		Fig. 3. One-dimensional spatial profiles of mass fractions ({xi}) of Fenpropimorph (A) and Kresoxim-methyl (B) in the wax-covered regions of model system I as a function of time.

 
The essential difference in the behaviour of both model compounds becomes even more evident when the amounts lost to the air and accumulated in the cuticular wax phase of system I are integrated over prolonged periods of time (Fig. 4). After approximately 8 d, more than 99% of the KM that diffused away from the source was associated with the wax (i.e. the leaf in the corresponding real situation). For the much more volatile FPM the opposite behaviour is predicted: within 8 d 88% of the total amount evaporated from the source will be lost to the atmosphere while only a small proportion is expected to be associated with the wax. Yet, due to its higher volatility, FPM reaches a maximum amount sorbed in the wax phase which is almost 70-fold higher than that of KM. At the same time, the amount of the latter model compound dissipated into the air is almost 4 orders of magnitude lower than that of the former.



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  		Fig. 4. Amounts of Fenpropimorph and Kresoxim-methyl in the air and wax compartments of model system I, respectively, as a function of time.

 
Simplified leaf surface analogue without convection
Model system II approaches the in planta situation in as far as more realistic assumptions concerning the geometric and material properties of a leaf surface are incorporated. This system primarily serves for investigating the diffusive dispersal of a semi-volatile compound from a source on a leaf surface in close vicinity to an open stoma in calm air. In order to emphasize the effect of vapour–leaf surface interactions on dispersal kinetics the (otherwise predominating) effect of the vapour pressure differences between the two model compounds has been eliminated by using the reduced mass fraction {xi}r according to

where {xi}p is the mass fraction of the compound at saturation vapour pressure.

The simulation showed that within a relatively short period of time (16 s after t=0) a hemispherical plume of vapour diffuses into the air space surrounding the source deposited on a leaf surface (Fig. 5A, B). Close to the point source, the mass fractions of both compounds approach 50–80% of the value equivalent to the saturation vapour pressure. Due to the enhanced volatility of FPM its relative mass fractions at locations more distant to the source are higher than that of KM (Fig. 5A, B). In absolute concentrations this discrepancy would be further enhanced by about three order of magnitudes by the large differences in the saturations vapour pressures of both compounds (Table 1).



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  		Fig. 5. Isolines of the reduced mass fractions of ({xi}r) of Kresoxim-methyl (A, C) and Fenpropimorph (B, D) in model system II after 16 s and 1600 s from t=0.

 
After 1600 s ({approx}27 min) the differences already seen at 16 s are even more obvious with partial pressures reaching 20–30% of the saturation value at 150 µm distance from the source (i.e. in the airspace above the stomata, Fig. 5C, D). The snapshots at both times also demonstrate the effect that both substances are assumed to be removed completely by metabolic and transport processes in the interior of the leaf. This leads to courts of depleted airspace emerging from the stomatal openings and progressing into the adjacent boundary layer. The interplay between closely spaced epicuticular point sources and stomatal sinks, consequently, leads to a complex three-dimensional pattern of organic vapour concentrations close to a leaf surface which may be of practical (pesticide application) or biological importance (e.g. for biotic interactions).

Simplified leaf analogue with convection
Finally, model system III was devized in order to mimic the dispersal of semi-volatile compounds from multiple sources on the surface of a grass leaf without wind and at a wind speed of 1 m s–1. The simulations illustrated the pronounced effect of convective transport on the build-up of vapour concentrations in the boundary layer of a leaf (Fig. 6). In the absence of convection, thick stationary boundary layers enriched with the vapours of the model compounds develop. Under wind-still conditions a continuous envelope of vapour at about 60% of its saturation value forms over the leaf surface (Fig. 6). In the immediate vicinity of the surface the three-dimensional distribution of partial pressures is influenced by the presence of sources and sinks resulting in drastic differences in vapour concentrations over very short distances.



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  		Fig. 6. Isolines of the reduced mass fractions of ({xi}r) Kresoxim-methyl in the stationary boundary layers adjacent to a leaf (model system III) without (A) and with wind (B).

 
At a wind speed of 1 m s–1, the thickness of the vapour envelope of the leaf is drastically reduced while the pattern of high- and low-concentration regions directly adjacent to the cuticular surface essentially remains intact (Fig. 6). The simulations assuming convective transport also show that a plume of air enriched with organic vapour is blown away across the leeward margin of the leaf. The material thus carried away will reach the turbulent parts of the atmosphere within a canopy and consequently be dispersed and diluted. With substances having a sufficiently high biological activity this mechanism may lead to long-distance effects like host recognition by insects or pesticidal activity on untreated plants. With real leaves agitated by the wind much thinner boundary layers will develop and increased amounts of organic vapour are expected to be removed by convection. The same is true for higher wind speeds, intensely turbulent conditions and elevated temperatures.

Calculating isolines of reduced mass fractions {xi}r of FPM and KM in the air adjacent to a leaf surface for consecutive points on the time axis revealed that the system fairly rapidly reached a stationary state (Fig. 7). In the case of FPM steady-state mass fractions were reached after only 3 s while the same process took 260 s to occur with KM. In the stationary state, removal by convection and uptake into the leaf was equal to the volatilization from the source. This steady-state situation was estimated to end with the depletion of the sources after 6.0x103 s and 1.2x106 s for FPM and KM, respectively.



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  		Fig. 7. Isolines of the reduced mass fractions of ({xi}r) Kresoxim-methyl in the boundary layers adjacent to a leaf (model system III) with convection. A stationary state was approached after 260 s.

 
No qualitative differences in the boundary layer profiles could be detected for both model compounds even though they differ considerably in their physico-chemical properties. Quantitatively, however, big differences are predicted for the fraction of total material taken up into the leaf versus that lost to the atmosphere. Under still conditions, almost all FPM and KM is taken up into the leaf (Table 2). With convection, the two model compounds are expected to behave completely differently: while for KM more than 60% of the total amount will find its way into the leaf interior, three quarters of the total amount of Fenpropimorh is predicted to being dissipated into the atmosphere. This behaviour correlates with the saturation vapour pressures and the lipophilicities (e.g. log Kow) of both compounds (Table 1).


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  		Table 2. Distribution and foliar uptake of model compounds in model system III Fractions given relate to total deposits. Simulations were extended until sources were depleted (after 6.0x103 s and 1.2x106 s for Fenpropimorph and Kresoxim-methyl, respectively).
 
The relative importance of the stomatal versus the cuticular pathway for the uptake of semi-volatile substances from leaf surfaces is also illustrated by the simulations using model system III. This behaviour is again influenced by volatility and lipophilicity which leads to a clear distinction between the two model compounds: FPM enters the leaf preferentially via the stomata while 60–80% of KM (still conditions and convection, respectively) diffuse across the cuticle (Table 2).


   Conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 Conclusions
References
 
The finite-element simulation approach has identified the principal factors determining the behaviour of organic vapours at the leaf/atmosphere interface. Under still and under laminar flow conditions the vapours emitted by a point source rapidly form stationary envelopes around the leaves. At a given temperature and atmospheric pressure, vapour concentrations within these unstirred layers depend on the vapour pressure of the compound in question and on its affinity to the lipoid surface layers of the leaf (cuticular waxes, cutin). The balance between both properties will determine whether a given compound (e.g. hormone, pheromone, secondary metabolite, active ingredient) will dissipate into the turbulent atmosphere or remain closely associated with the leaf surface. This can be considered as an analogy to the gas chromatographic process where both vapour pressure and affinity to the stationary phase of the column govern the relative residence times of a molecule in the gas and the stationary phases, respectively. The longer a molecule stays dissolved in or adsorbed to the stationary phase the longer will be its retention time.

This behaviour is in striking contrast to that of the gases usually studied in plant ecophysiology like carbon dioxide, oxygen or water. Under natural conditions, their partial pressures are in the kPa range and thus 6–9 orders of magnitude higher than those of the semi-volatile model compounds studied here. In addition, small inorganic gases do not accumulate preferentially in lipoid material. Low vapour pressures (resulting in reduced volatility) and the affinity to lipoid waxes and cutin result in an association with leaf surfaces much closer than that of inorganic gases.

Uptake into the leaf or release from the interior tissue may occur via both the stomatal and the cuticular pathway. Which of the two routes an organic compound actually takes again depends on vapour pressure and lipophilicity (when stomata are open). An organic compound with low vapour pressure and high affinity to lipoid material will almost exclusively take the cuticular pathway, while compounds with an opposite combination of physico-chemical properties will mostly diffuse through the stomatal pores. These rules, deduced from the numerical simulation of organic vapour behaviour in the leaf/atmosphere interface, will help to assess how (semi-) volatile plant products (e.g. hormones, pheromones, secondary metabolites) and xenobiotics (e.g. pesticides, pollutants) perform on plant surfaces.


   Acknowledgements
 
Parts of this work have been supported by grants from the BASF AG, the Sonderforschungsbereich 567 ‘Mechanismen der interspezifischen Interaktion von Organismen’ and the Fonds der chemischen Industrie.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 Conclusions
 References
 
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Riederer M. 1995. Partitioning and transport of organic chemicals between the atmospheric environment and leaves. In: Trapp S, McFarlane JC, eds. Plant contamination. Modeling and simulation of organic chemical processes. Boca Raton: Lewis Publishers, 153–190.

Riederer M, Schreiber L. 1995. Waxes—the transport barriers of plant cuticles. In: Hamilton RJ, ed. Waxes: chemistry, molecular biology and functions. West Ferry, Dundee, Scotland: The Oily Press, 130–156.

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