Journal of Experimental Botany, Vol. 53, No. 375, pp. 1815±1823, August 2002
DOI: 10.1093/jxb/erf020
Semi-volatile organic compounds at the leaf/atmosphere
interface: numerical simulation of dispersal and foliar
uptake
Markus Riederer1,4, Andreas Daiû2, Norbert Gilbert2 and Harald KoÈhle3
1
Julius-von-Sachs-Institut fuÈr Biowissenschaften, Lehrstuhl fuÈr Botanik II, UniversitaÈt WuÈrzburg,
Julius-von-Sachs-Platz 3, D-97082 WuÈrzburg, Germany
2
BASF AG, Technische Entwicklung, Verfahrenstechnik, D-67056 Ludwigshafen, Germany
3
BASF AG, Landwirtschaftliche Versuchsstation, D-67114 Limburgerhof, Germany
Received 15 March 2002; Accepted 15 April 2002
Abstract
Introduction
The behaviour of (semi-)volatile organic compounds
at the interface between the leaf surface and the
atmosphere was investigated by ®nite-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 af®nity 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.
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 ¯ower 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
Key words: Floral scents, ¯uid mechanics, fungicides,
hormones, pheromones, plant cuticle, stomata, uptake,
volatiles.
4
To whom correspondence should be addressed. Fax: +49 931 888 62 35. e-mail: [email protected]
ã Society for Experimental Biology 2002
1816 Riederer et al.
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 suf®cient 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 in¯uence 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 af®nity 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 super®cial 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 in¯uence 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 simpli®ed conditions
ruled out analytical mathematical solutions. For this
reason, numerical ®nite-element simulations were applied
to this so far intractable problem in plant ecophysiology.
The ®nite-element approach has demonstrated its superior
power in solving complex engineering problems and has
also been used for the analysis of ¯uid ¯ow phenomena in
and around plants (Rand and Cooke, 1996; Finnigan, 2000;
Rand, 1983).
Materials and methods
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.
Model system I was designed for following the diffusive vapourphase 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 mm). No convection was allowed
within this hemisphere. A grid consisting of 3800 nodes was ®tted
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 waxcovered 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 in¯uenced 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 mm), the cuticular matrix (0.9 mm) and a
super®cial ®lm of cuticular wax (0.1 mm). At the centre of this disc
of leaf surface was a cylindrical deposit (diameter 50 mm, 25 mm
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 (90315 mm) 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 mm 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 physicochemical properties and wind speed. Over the width of the leaf
(0.5 cm) 20 small deposits and 20 stomatal pores were assumed to be
Simulation of foliar uptake of volatiles 1817
Fig. 1. Geometry, dimensions and node pattern of the model systems used in the numerical simulations. For further explanations see Materials
and methods.
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 ¯ow of air (1 m s±1) was assumed to pass over the
leaf perpendicular to its long axis. The leaf was assumed to be ¯at,
smooth, stiff, and ®xed 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 speci®cally 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 mm 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 ®ne 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 ®nite elements approach. The threedimensional 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 xax 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
xa px M x
pa Ma
…1†
The corresponding partial density of compound x in the phase j ( rjx)
can be obtained from the mass fraction and the overall density of the
phase rj by
xi ˆ ix i
…2†
The volume-based partition coef®cient of compound x between
phases i and j is then given by
1818 Riederer et al.
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 coef®cients (Kow) were provided by
BASF; the remaining parameters were estimated as follows: cuticle/water partition coef®cients (Kcw) from aqueous solubilities according to
Riederer (1995), cuticular matrix/air partition coef®cients (KMXa) from saturation vapour pressures according to Welke et al. (1998), cuticular
wax/air partition coef®cients (KWaxa) from KMXa according to Burghardt et al. (1998).
Compound
Fenpropimorph
Kresoxim-methyl
MW [g mol±1]
±3
303.54
343.36
K xij ˆ
Sw [mol l±1]
p° [Pa]
3.20310
2.30310±6
xj
jx j
ˆ
xi
ix i
±5
1.42310
6.39310±6
…3†
This relationship can be rearranged in order to calculate the mass
fraction of compound x in phase j when the partition coef®cient Kijx
and the mass fraction of compound x in phase i are known:
jx ˆ Kijx
xi x
xj i
…4†
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
i Dxi
jx
ix
ˆ ÿj Dxj
x
x
…5†
where Dx is the diffusion coef®cient 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-tolyloxymethyl)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 physicochemical 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.
Diffusion coef®cients (at 25 °C) of 1310±10 m2 s±1, 1310±13 m2 s±1,
1310±18 m2 s±1, and 1310±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 coef®cients were used for the two compounds. For all
simulations, isothermal (298 K) and isobaric conditions (101 325 Pa)
were assumed.
log Kow
log Kcw
log KMXa
log KWaxa
4.1
3.4
3.9
4.1
8.5
11.3
7.5
10.3
Results and discussion
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 (xa=3.3310±7 for FPM and
2.7310±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, xa 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 xa on distance
remained essentially unchanged.
Two major differences in the behaviour of both
compounds can be recognized: (i) the estimates of xa of
KM were considerably lower than those of FPM and (ii)
the dependence of xa 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 coef®cients
(KWaxa) which both differ by almost three orders of
magnitude (Table 1). The lower saturation vapour pressure
of KM results in a smaller xa above the deposit. This effect
is further enhanced by the much higher af®nity 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
Simulation of foliar uptake of volatiles 1819
Fig. 2. One-dimensional spatial pro®les of mass fractions (x) 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
in cuticular wax (1 mm 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 af®nity KM
exhibits to wax than FPM does (wax/air partition coef®cients 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 af®nity
to wax the former compound is ef®ciently scavenged from
the gas phase close to the source. Therefore, at larger
Fig. 3. One-dimensional spatial pro®les of mass fractions (x) of
Fenpropimorph (A) and Kresoxim-methyl (B) in the wax-covered
regions of model system I as a function of time.
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 coef®cients of
molecules in wax essentially excludes any equilibration by
lateral diffusion.
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
1820 Riederer et al.
Fig. 4. Amounts of Fenpropimorph and Kresoxim-methyl in the air
and wax compartments of model system I, respectively, as a function
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.
Simpli®ed 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 xr according to
r ˆ
p
…6†
where xp 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
Fig. 5. Isolines of the reduced mass fractions of (xr) of Kresoximmethyl (A, C) and Fenpropimorph (B, D) in model system II after 16 s
and 1600 s from t=0.
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).
After 1600 s (»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 mm 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).
Simpli®ed leaf analogue with convection
Finally, model system III was devized in order to mimic
the dispersal of semi-volatile compounds from multiple
Simulation of foliar uptake of volatiles 1821
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 6.03103 s and 1.23106 s for
Fenpropimorph and Kresoxim-methyl, respectively).
Fraction of total amount (%)
Uptake into leaf
Stomatal pathway
Convection (v=1 m s±1)
No convection
Fenpropimorph
Kresoxim-methyl
Fenpropimorph
Kresoxim-methyl
92
54
> 99
37
25
> 99
65
19
Fig. 6. Isolines of the reduced mass fractions of (xr) Kresoxim-methyl
in the stationary boundary layers adjacent to a leaf (model system III)
without (A) and with wind (B).
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 in¯uenced by the presence of sources
and sinks resulting in drastic differences in vapour
concentrations over very short distances.
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
Fig. 7. Isolines of the reduced mass fractions of (xr) Kresoxim-methyl
in the boundary layers adjacent to a leaf (model system III) with
convection. A stationary state was approached after 260 s.
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 suf®ciently high
biological activity this mechanism may lead to longdistance 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 xr 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
1822 Riederer et al.
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
af®nity 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 af®nity to lipoid material will
almost exclusively take the cuticular pathway, while
compounds with an opposite combination of physicochemical 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.
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 6.03103 s and 1.23106 s
for FPM and KM, respectively.
No qualitative differences in the boundary layer pro®les
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 ®nd 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).
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 in¯uenced 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).
Parts of this work have been supported by grants from the BASF AG,
the Sonderforschungsbereich 567 `Mechanismen der interspezi®schen Interaktion von Organismen' and the Fonds der chemischen
Industrie.
Conclusions
References
The ®nite-element simulation approach has identi®ed the
principal factors determining the behaviour of organic
vapours at the leaf/atmosphere interface. Under still and
under laminar ¯ow 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 af®nity 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
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