Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol16.1996:
Tree Physiology 16, 649--654
© 1996 Heron Publishing----Victoria, Canada
Field measurements of isoprene emission from trees in response to
temperature and light
THOMAS D. SHARKEY,1 ERIC L. SINGSAAS,1 PETER J. VANDERVEER1 and
CHRIS GERON2
1
Department of Botany, 430 Lincoln Drive, University of Wisconsin, Madison, WI 53706, USA
2
National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Research Triang
le Park, NC 27711, USA
Received July 10, 1995
Summary The atmospheric hydrocarbon budget is important for predicting ozone episodes and the effects of pollution
mitigation strategies. Isoprene emission from plants is an important part of the atmospheric hydrocarbon budget. We measured isoprene emission capacity at the bottom, middle, and top
of the canopies of a white oak (Quercus alba L.) tree and a red
oak (Quercus rubra L.) tree growing adjacent to a tower in the
Duke University Forest. Leaves at the top of the white oak tree
canopy had a three- to fivefold greater capacity for emitting
isoprene than leaves at the bottom of the tree canopy. Isoprene
emission rate increased with increasing temperature up to
about 42 °C. We conclude that leaves at the top of the white
oak tree canopy had higher isoprene emission rates because
they were exposed to more sunlight, reduced water availability,
and higher temperature than leaves at the bottom of the canopy.
Between 35 and 40 °C, white oak photosynthesis and stomatal
conductance declined, whereas red oak (Quercus rubra) photosynthesis and stomatal conductance increased over this
range. Red oak had lower rates of isoprene emission than white
oak, perhaps reflecting the higher stomatal conductance that
would keep leaves cool. The concentration of isoprene inside
the leaf was estimated with a simplified form of the equation
used to estimate CO2 inside leaves.
Keywords: isoprene, Quercus alba, Quercus rubra, red oak,
temperature, white oak.
Introduction
Isoprene is the dominant hydrocarbon released by vegetation
in most ecosystems (Guenther et al. 1994, Geron et al. 1994).
Isoprene release by vegetation, particularly trees (Rasmussen
1970), exceeds anthropogenic hydrocarbon release to the atmosphere (Lamb et al. 1987, 1993). Isoprene and other hydrocarbons can react to form ozone in sunlight when NOx is
present (Trainer et al. 1987, Chameides et al. 1988, Thompson,
1992). Ozone causes respiratory distress and reduces crop and
forest yields (Reich and Amundson 1985, Runeckles and
Chevone 1992).
Isoprene emission rate is strongly affected by light and
temperature (Sanadze and Kursanov 1966, Rasmussen and
Jones 1973, Tingey et al. 1979, Monson and Fall 1989, Loreto
and Sharkey 1990). In addition to short-term (up to 20 min)
effects of light intensity on isoprene emission rates, leaves that
develop in full sun emit isoprene at a higher rate than leaves
that develop in shade (Sharkey et al. 1991, Harley et al. 1994).
Carbon assimilated in photosynthesis rapidly appears in emitted isoprene (Delwiche and Sharkey 1993). Isoprene emission
typically accounts for 2% of the carbon fixed in photosynthesis
(Monson and Fall 1989, Loreto and Sharkey 1990) but in
kudzu (Pueraria lobata (Willd.) Ohwi) it can account for over
50% of the carbon fixed in high light following water stress
(Sharkey and Loreto 1993). Sharkey and Singsaas (1995) hypothesized that isoprene synthesis protects leaves against thermal stress that can occur when leaves are exposed to the radiant
heat load of full sunlight, especially in fluctuating light environments.
Current methods for scaling from leaf-level measurements
to canopy-scale fluxes of isoprene model the light and temperature dependence of isoprene emission (Geron et al., 1996).
However, these models, particularly BEIS2 (revision of the
Biogenic Emission Inventory System, see Pierce and Waldruff
1991), require many assumptions about how isoprene emission
rate is affected by micrometeorological variables such as light,
both instantaneous irradiance and irradiance during leaf development, and leaf temperature. Furthermore, it is not known
whether a big leaf model (as opposed to a layered-canopy
model) can give acceptably good fit to canopy flux measurements made by micrometeorological measurements, or how
sophisticated leaf energy balance calculations must be to obtain reliable estimates of canopy isoprene flux from leaf-level
measurements.
We have measured isoprene emission, photosynthesis, and
stomatal conductance at three canopy levels in a white oak
(Quercus alba L.) tree and a red oak (Quercus rubra L.) tree,
including the topmost canopy leaves at a height of 30 m.
Measurements were made in half and full sunlight at 30, 35
and, in some cases, 40 °C.
650
SHARKEY, SINGSAAS, VANDERVEER AND GERON
Materials and methods
Site description
The study was conducted at the Duke University Research
Forest (35°58′25″ N and 79°06′05″ W) near Chapel Hill, North
Carolina. The forest is a mature, second-growth uneven-aged
stand with the oldest trees exceeding the age of 180 years. A
40-m walkup tower facilitated measurements within, and at the
top of, the forest canopy (Geron et al. 1996).
Most measurements were made during June 23--25 and
August 20--22, 1994. June was warm and dry, whereas August
was relatively wet. In both months, there were substantial rains
within 3 days of making the measurements and so the trees
were not under water stress during the measurements, although
the trees may have experienced water stress in the weeks
leading up to the June measurements.
During both periods, measurements were made at three
canopy levels. The leaf area index (LAI) measured with an LAI
2000 (Li-Cor, Inc., Lincoln, NE) was 4.1 at the bottom of the
canopy and 3.0, 1.1 and 0.3 at canopy heights of 6, 20 and
30 m, respectively. The leaves sampled at a height of 30 m
were not quite the highest in the canopy. The tower was not
excluded from the field of view of the LAI 2000. Although the
LAI determined by the LAI-2000 differs from the true LAI, it
provides a measure of the relative shadiness of each location.
Gas exchange measurements
Photosynthesis and stomatal conductance were measured with
a prototype of the Li-Cor 6400 gas exchange measuring system. The light source was a light emitting diode with peak
irradiance at 670 nm as recommended by Tennessen et al.
(1995), who showed that red light has a negligible effect on
measurements of photosynthesis and no effect on isoprene
emission relative to xenon-arc lamp light (Tennessen et al.
1994). The air supply for the gas exchange system was drawn
through a tube held about 3 m from the tower to avoid CO2
from investigator breath. Leaf temperature was controlled by
thermoelectric modules that are part of the Li-Cor 6400 system. In addition, we used a heat gun to obtain the high temperatures required and to overcome the low power supply to
the thermoelectric modules on the Li-Cor 6400. By directing
the heat gun onto the gas exchange cuvette, we could change
the leaf temperature by 5 °C in less than 2 min.
Isoprene measurements
Isoprene emission was measured by fitting a three-way valve
to the output port of the Li-Cor 6400 cuvette. Gas flow leaving
the cuvette was directed past a portable Scentoscreen gas
chromatograph (Sentex, Ridgefield, NJ) except when required
for matching the reference and measuring gas analyzers. The
gas chromatograph had a small pump that pumped 20 ml of air
leaving the leaf chamber through a small adsorbent tube containing Carbosieve B. To inject, the argon carrier gas flow was
directed through the preconcentrator while it was heated with
nichrome wire.
An argon ionization detector was used for detection. Tritium
decay excites the argon carrier gas causing ionization of hydro-
carbons coming from the column. In the field, single point
calibrations were made by mixing liquid isoprene in a 1-dm3
flask of N2 and mixing a small amount of that gas mixture in
another 1-dm3 flask of N2 to give a 256 ppb standard. The
accuracy of this method is limited by the accuracy of the
gas-tight syringes. Over one month, the standard deviation was
less than 5% of the mean calibration factor. When the standard
was made up seven times in a row the standard deviation of the
calibration factors was 4% of the mean. Most measurements
were in the range of 50 to 300 ppb and the ambient isoprene
concentration was 1--3 ppb. We have since discovered that
making the standards in air gives a 5% lower detector response.
We assume that standards in air are a better measure and so the
values reported here have been corrected for this effect. After
these measurements were completed we discovered that the
Sentex gas chromatograph output was nonlinear. The nonlinearity was extensively characterized by preparing and measuring several dilution series in the laboratory and comparing
results with a Shimadzu gas chromatograph with photoionization detection. The Shimadzu was linear across all dilutions
from 32 to 512 ppb isoprene giving us confidence that the
dilution series were correctly prepared.
The rate of isoprene emission was calculated as the concentration of isoprene in the airstream multiplied by the flow rate,
normally 250 µmol s −1. The Li-Cor 6400 cuvette encloses 6
cm2 of leaf.
Estimating isoprene concentration inside the leaf
We estimated the concentration of isoprene in the air phase in
the leaf by equations originally developed for estimating the
CO2 concentration inside leaves.
Ii = Ia + 1.94(JI /g),
where Ii is the isoprene concentration inside the leaf, Ia is the
isoprene concentration in the air outside the leaf, 1.94 is the
square root of the ratio of molecular weights of isoprene to
water, since diffusivity is related to the kinetic energy of a
molecule, JI is the flux of isoprene from the leaf, and g is the
conductance for water vapor. To estimate CO2 concentration
inside leaves it is common practice to use the ratio of the
mutual diffusion coefficients of CO2 in air and water in air
which fortuitously is close to the square root of the ratio of
molecular weights (von Caemmerer and Farquhar 1981). Because the mutual diffusion coefficient of isoprene in air is not
known, we resorted to the simpler formulation but recognize it
may need correction in the future. Other effects that caused
errors of 1 to 3% are not accounted for in our estimation of leaf
isoprene concentration.
Dry weights
Leaf punches of approximately 1 cm2 were taken from the
leaves used for isoprene emission rate measurements in June,
dried at 60 °C for two days, and then weighed.
Xanthophyll and chlorophyll measurements
In August, samples for analysis of chlorophyll and xantho-
ISOPRENE EMISSION
phylls were taken from leaves adjacent to the leaves used to
measure isoprene emission rates. Adjacent leaves were used
because we increased leaf temperature to the damage threshold
for many leaves during the August measurements of isoprene
emission rates. Leaf samples were extracted by grinding in
acetone. The acetone extract from three grindings was combined for each sample. The extraction technique gave reproducible results despite the toughness of the oak leaves.
Xanthophyll cycle intermediates, lutein, chlorophylls, and carotenoids were quantified by HPLC on a special column (Zorbax ODS non-endcapped 4.6 × 250 mm made by Dupont and
supplied by MACMOD Analytical, Inc., Chadds Ford, PA) as
described by Tennessen et al. (1995).
Results
In June, isoprene emission rates were high (Table 1) and
increased with increasing temperature from 30 to 35 °C at
irradiances of both 1000 and 2000 µmol m −2 s −1. The energies
of activation determined over this temperature range by the
Arrhenius equation were 68 and 55 kJ mol −1 at 1000 and 2000
µmol m −2 s −1, respectively. Although isoprene emission rates
increased with increasing temperature from 30 to 35 °C, assimilation rates decreased. As a result, the proportion of photosynthate being converted to isoprene almost doubled to
nearly 8%. The decline in photosynthetic rate with increasing
temperature was correlated with reduced stomatal conductance and could be a consequence of stomatal closure. However, stomatal closure does not affect isoprene emission rate
because isoprene accumulates in the leaf until the flux is the
same before and after stomatal closure (Fall and Monson
1992). The calculated isoprene concentration inside the leaf
doubled between 30 and 35 °C. At both 30 and 35 °C, doubling
the irradiance from 1000 to 2000 µmol m −2 s −1 increased rates
of isoprene emission and photosynthesis by about 10%.
Under standard conditions of 1000 µmol m −2 s −1 and 30 °C,
isoprene emission rates were over 4 times greater at the top of
the canopy (LAI = 0.3) than near the bottom of the canopy
651
(LAI = 3.0) (Figure 1). Photosynthetic rates were 1.8 times
greater at the top of the canopy than at the bottom of the canopy
(data not shown). Leaf weight per leaf area was 52 and 133 g
m −2 at the bottom and top of the canopy, respectively. Consequently, when isoprene emission rates were expressed on a leaf
mass basis, differences between the top and the bottom of the
canopy were reduced but not eliminated.
The rate of isoprene emission in August was less than that
observed in June (Table 2), although the activation energies at
2000 and 1000 µmol m −2 s −1 were similar to those determined
in June at 55 and 80 kJ mol −1, respectively. In August, isoprene
emission rate increased with increasing temperature until the
temperature was above 40 °C. As in June, photosynthetic rate
and stomatal conductance fell with increasing temperature so
that, at 40 °C and in full sunlight, isoprene accounted for
13.7% of the carbon fixed in photosynthesis and the isoprene
concentration was 2.6 ppm inside the leaf. Biosynthesis of
isoprene from photosynthetic precursors likely results in four
carbons being lost as CO2, and so in addition to the 13.7% of
photosynthesis lost as isoprene, another 11% of potential photosynthate is lost as a by-product of isoprene synthesis giving
a total carbon cost of about 25% of photosynthesis.
The gradient in leaf-area based isoprene emission capacity
from the top to the bottom of the canopy was about 3-fold at
30 °C and 5-fold at 35 °C (Table 2). Both assimilation capacity
and leaf chlorophyll concentration were highest in the middle
of the canopy (Table 3), whereas the chlorophyll a/b ratio was
highest at the top of the canopy and declined with depth in the
canopy paralleling the decrease in irradiance from the top to
the bottom of the canopy.
Contents of the xanthophyll cycle intermediates, violaxanthin (V), antheraxanthin (A), and zeaxanthin (Z), were high in
leaves at the top of the canopy and declined with depth in the
canopy. In addition, the epoxidation state ((A+0.5V)/(Z + V +
A)) (Demmig-Adams and Adams 1992) was lowest at the top
of the canopy and increased down the canopy. Lutein, which
does not participate in the light-protecting xanthophyll cycle,
Table 1. Isoprene emission rate and other gas exchange parameters
measured in white oak (Quercus alba) leaves at the top of the canopy
in June 1994. Values are given as means ± SE (n = 3 measurements on
each of three leaves). Units are: isoprene emission rate, nmol m −2 s −1;
photosynthetic CO2 assimilation rate, µmol m −2 s −1; I/A is the ratio of
carbon atoms emitted as isoprene to carbon atoms assimilated in
photosynthesis expressed as %; conductance to water vapor exchange,
mol m−2 s−1, and isoprene concentration, nmol mol−1 air inside the leaf.
Measurements were made at either 1000 or 2000 µmol m −2 s −1 and at
30 or 35 °C.
Light/temp
1000/30
Isoprene
87 ± 9
Assimilation 11.2 ± 0.4
I/A
3.9 ± 0.4
Conductance 0.23 ± 0.01
Isoprene conc. 759 ± 80
1000/35
2000/30
135 ± 8
9.1 ± 0.3
7.4 ± 0.2
0.19 ± 0.00
1363 ± 56
108 ± 8.3
12.9 ± 0.7
4.2 ± 0.1
0.28 ± 0.02
746 ± 17
2000/35
155 ± 12
10.0 ± 0.2
7.8 ± 0.5
0.21 ± 0.01
1471 ± 69
Figure 1. Isoprene emission rate from canopy leaves of white oak,
Q. alba. Leaf temperature was 35 °C and irradiance was 2000 µmol
m −2 s −1 . Error bars represent standard errors of the mean, n = 3
measurements on each of three to five leaves.
652
SHARKEY, SINGSAAS, VANDERVEER AND GERON
Table 2. Isoprene emission rate and other gas exchange parameters measured in white oak (Quercus alba) leaves in August 1994. Values given are
means ± SE (n = 1 measurement on each of five leaves). Units are given in Table 1. Measurements were made atthe top (LAI = 0.3), middle (LAI
= 1) and near the bottom (LAI = 3) of the tree canopy. Measurements were made at either 1000 or2000 µmol m −2 s −1 and at 30 or 35 °C.
Light/temperature
1000/30
1000/35
2000/30
2000/35
Top of the canopy
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
65 ± 3
8.3 ± 0.6
4.0 ± 0.4
0.18 ± 0.03
703 ± 29
107 ± 5
7.1 ± 0.8
8.2 ± 1.5
0.15 ± 0.02
1379 ± 71
76 ± 5
8.6 ± 0.8
4.6 ± 0.6
0.16 ± 0.03
916 ± 62
126 ± 7
7.9 ± 0.9
8.5 ± 1.2
0.16 ± 0.03
1536 ± 91
Middle of the canopy
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
47 ± 2
10.3 ± 0.5
2.3 ± 0.08
0.33 ± 0.05
273 ± 12
69 ± 2
9.2 ± 0.2
3.7 ± 0.2
0.28 ± 0.01
479 ± 16
53 ± 3
11.4 ± 0.5
2.3 ± 0.1
0.35 ± 0.04
294 ± 14
81 ± 3
9.7 ± 0.5
4.2 ± 0.2
0.26 ± 0.03
603 ± 17
Bottom of the canopy
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
19 ± 0.8
3.8 ± 0.5
2.7 ± 0.3
0.10 ± 0.01
372 ± 15
22 ± 2
3.7 ± 0.5
3.3 ± 0.1
0.10 ± 0.01
435 ± 48
21 0.2
4.9 ± 0.5
2.2 ± 0.2
0.11 ± 0.01
367 ± 3
27 ± 0.9
4.0 ± 0.4
3.4 ± 0.3
0.09 ± 0.01
576 ± 20
2000/40
180 ± 10
6.7 ± 0.6
13.7 ± 1.0
0.13 ± 0.01
2681 ± 155
Table 3. Light environment and chlorophyll and xanthophyll contents of Q. alba leaves at three heights in the canopy of a single tree. Values are
means and standard errors, n = 3.
LAI
Total chlorophyll, µmol m −2
Chl a/b
Xanthophyll cycle intermediates, µmol m −2
Epoxidation, %
Lutein, µmol m −2
β-Carotene, µmol m −2
Top
Middle
Bottom
0.3
454 ± 18
3.15 ± 0.05
16.8 ± 0.8
31 ± 1
33 ± 2
49 ± 8
1.0
477 ± 16
3.04 ± 0.01
4.8 ± 0.5
62 ± 10
32 ± 1
58 ± 1
3.0
383 ± 6
2.63 ± 0.03
1.02 ± 0.3
100 ± 0
30 ± 1
45 ± 2
and β-carotene contents did not vary through the canopy,
indicating that the differences in chl a/b ratio, contents of
xanthophyll cycle intermediates, and epoxidation state were
specific responses, presumably to light environment, and not
simply a reflection of the general health or specific leaf area of
the leaves.
In August, we measured isoprene emission and gas exchange on leaves of several branches of red oak (Quercus
rubra) that were cut from the top of a tree near the tower. The
rates of isoprene emission were less than for white oak when
measured at a PPFD of 1000 µmol m −2 s −1 but the difference
was less obvious at a PPFD of 2000 µmol m −2 s −1. Assimilation rate of red oak was also less than that of white oak at a
PPFD of 1000 mol µmol m −2 s −1 but not at a PPFD of 2000
µmol m −2 s −1, an effect that could be explained by lower
stomatal conductance at low irradiances in red oak compared
with white oak. Because stomata were more closed at low
irradiances in red oak than in white oak, the concentration of
isoprene was nearly constant across treatments despite the
large change in isoprene emission rate. In red oak, both stomatal conductance and photosynthetic rate increased when the
temperature was increased from 35 to 40 °C, whereas in white
oak both stomatal conductance and photosynthetic rate declined over this temperature range (Table 4).
We tested whether isoprene emission rate increased when
the temperature was above 40 °C in one white oak leaf and
three red oak leaves. In all cases, isoprene emission rate increased as temperature increased to 42 °C. In one red oak leaf,
isoprene emission rate increased up to 44 °C. For two red oak
leaves and the white oak leaf the temperature was held at 42 °C
for 5 to 20 min and isoprene emission rate fell over this time.
Discussion
Isoprene emission rates were much higher from leaves at the
top of the canopy of a white oak tree than from leaves lower in
ISOPRENE EMISSION
653
Table 4. Isoprene emission rate and other gas exchange parameters measured in red oak (Quercus rubra) leaves in August 1994. Values given are
means ± SE (n = 3). Units are given in Table 1. Measurements were made on leaves attached to a branch cut from the top of a 30-m tree and placed
in water. Measurements were made at either 1000 or 2000 µmol m −2 s −1 and at 30 or 35 °C.
Light/temperature
1000/30
1000/35
2000/30
2000/35
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
43 ± 7
3.0 ± 0.4
7.8 ± 2.0
0.07 ± 0.02
1385 ± 511
102 ± 5
5.2 ± 0.6
8.9 ± 1.3
0.15 ± 0.02
1398 ± 119
66 ± 6
5.3 ± 0.9
5.7 ± 1.4
0.11 ± 0.03
1325 ± 485
100 ± 6
6.0 ± 0.5
8.5 ± 1.1
0.17 ± 0.02
1213 ± 175
the canopy. Sun leaves are known to have a greater capacity for
isoprene emission than shade leaves (here capacity means the
rate of emission under standard conditions, most often at a
PPFD of 1000 µmol m −2 s −1 and 30 °C) (Sharkey et al. 1991,
Harley et al. 1994). Leaves at the tops of trees will also have
the lowest water potential. The white oak tree was about 30 m
tall and so the gravity component of the water potential is
about 0.3 MPa; however, resistance to water flow would reduce the water potential below −0.3 MPa in the top of the tree.
Water stress is known to increase the capacity for isoprene
emission in kudzu (Sharkey and Loreto 1993). Finally, high
temperature can increase the capacity for isoprene emission
(Sharkey and Loreto 1993) and high leaf temperature is more
likely at the top of the tree than elsewhere because of both the
higher radiation load and the reduced water supply relative to
other parts of the tree. All of these factors may have contributed
to the high isoprene emission capacity at the top of the white
oak tree.
Our results support the thermal protection hypothesis put
forward by Sharkey and Singsaas (1995) to account for plant
isoprene emission. The estimates of isoprene concentration
inside the leaves confirmed that the concentration of isoprene
required to provide thermal protection occurs in natural conditions. Leaves subjected to the highest temperatures had the
greatest capacity for isoprene emission. The distribution of
isoprene emission capacity through the canopy was similar to
the distribution of xanthophyll cycle intermediates. Xanthophylls are believed to protect leaves against excess radiation
damage (Demmig-Adams and Adams 1992), whereas isoprene
is believed to protect leaves against excess heat damage
(Sharkey and Singsaas 1995). The contents of lutein, a xanthophyll that does not participate in the light-protecting xanthophyll cycle, and β-carotene were constant throughout the
canopy.
The increase in isoprene emission rate with increasing temperature continued to higher temperatures in the field than had
been assumed based on laboratory studies (Guenther et al.
1993). For example, Guenther et al. (1991) concluded that
39 °C was the peak temperature for isoprene emission. However, although isoprene emission becomes unstable above
40 °C, we observed very high rates for short periods at above
40 °C. Such high temperatures are likely on hot days with little
wind, the same conditions that lead to ozone pollution episodes. Because leaf temperature tends to be variable, even on
still days, it is possible that the transient very high rate of
2000/40
165 ± 21
6.9 ± 0.1
11.9 ± 1.4
0.21 ± 0.01
1534 ± 196
isoprene emission seen immediately after raising the temperature above 40 °C is relevant to modeling isoprene emission for
isoprene emission inventories. If so, it may be useful to use the
Arrhenius equation to describe the temperature response of
isoprene emission because only two parameters would be
required, the basal emission rate and the physiologically relevant activation energy. The current temperature response models require basal emission rate plus three empirical coefficients
(Guenther et al. 1991).
Red oak leaves exhibited a higher temperature optimum for
photosynthesis than white oak leaves. Unfortunately we were
restricted to using cut branches of red oak because the tree was
between 5 and 10 m from the tower. Cutting branches may
affect isoprene emission (Loreto and Sharkey 1993) and the
degree to which it did so in this study is unknown. In white oak,
the stomata closed at high temperatures, whereas in red oak
photosynthetic rate and stomatal conductance increased with
increasing temperatures between 35 and 40 °C. This difference
in stomatal behavior would keep red oak leaves cooler than
white oak leaves under natural conditions and could explain
why the red oak leaves had a slightly lower capacity for
isoprene emission than the white oak leaves.
We made measurements in June and August expecting to
find a developmental increase in isoprene emission (Grinspoon
et al. 1991, Kuzma and Fall 1993, F. Loreto and T.D. Sharkey
unpublished data with red oak); however, isoprene emission
rates were higher in June than in August, indicating that oak
leaves are fully competent for isoprene emission early in the
season.
We conclude that most isoprene will be lost from directly
sunlit leaves. Isoprene emission is temperature and light dependent and not completely saturated even at a PPFD of 1000
µmol m −2 s −1. Sunlit leaves are likely to be warmer than shaded
leaves. In addition, leaves that are often in the sun have as
much as a 5-fold greater capacity for isoprene emission than
leaves lower in the canopy that are only rarely sunlit.
Note added in proof
We now estimate the ratio of diffusivities of water to isoprene to be
2.83 based on an analysis given by Lyman, W.J., W.F. Reehl and D.H.
Rosenblatt (1990) In Estimation Methods. Amer. Chem. Soc. Washington, DC, pp 17-9--17-17.
654
SHARKEY, SINGSAAS, VANDERVEER AND GERON
Acknowledgments
We thank Professor John Norman for discussions on leaf micrometereology and on the use of the LAI 2000. Research supported by U.S.
Environmental Protection Agency cooperative research agreement CR
823791-01-1 and U.S. National Science Foundation grant IBN9317900.
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© 1996 Heron Publishing----Victoria, Canada
Field measurements of isoprene emission from trees in response to
temperature and light
THOMAS D. SHARKEY,1 ERIC L. SINGSAAS,1 PETER J. VANDERVEER1 and
CHRIS GERON2
1
Department of Botany, 430 Lincoln Drive, University of Wisconsin, Madison, WI 53706, USA
2
National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Research Triang
le Park, NC 27711, USA
Received July 10, 1995
Summary The atmospheric hydrocarbon budget is important for predicting ozone episodes and the effects of pollution
mitigation strategies. Isoprene emission from plants is an important part of the atmospheric hydrocarbon budget. We measured isoprene emission capacity at the bottom, middle, and top
of the canopies of a white oak (Quercus alba L.) tree and a red
oak (Quercus rubra L.) tree growing adjacent to a tower in the
Duke University Forest. Leaves at the top of the white oak tree
canopy had a three- to fivefold greater capacity for emitting
isoprene than leaves at the bottom of the tree canopy. Isoprene
emission rate increased with increasing temperature up to
about 42 °C. We conclude that leaves at the top of the white
oak tree canopy had higher isoprene emission rates because
they were exposed to more sunlight, reduced water availability,
and higher temperature than leaves at the bottom of the canopy.
Between 35 and 40 °C, white oak photosynthesis and stomatal
conductance declined, whereas red oak (Quercus rubra) photosynthesis and stomatal conductance increased over this
range. Red oak had lower rates of isoprene emission than white
oak, perhaps reflecting the higher stomatal conductance that
would keep leaves cool. The concentration of isoprene inside
the leaf was estimated with a simplified form of the equation
used to estimate CO2 inside leaves.
Keywords: isoprene, Quercus alba, Quercus rubra, red oak,
temperature, white oak.
Introduction
Isoprene is the dominant hydrocarbon released by vegetation
in most ecosystems (Guenther et al. 1994, Geron et al. 1994).
Isoprene release by vegetation, particularly trees (Rasmussen
1970), exceeds anthropogenic hydrocarbon release to the atmosphere (Lamb et al. 1987, 1993). Isoprene and other hydrocarbons can react to form ozone in sunlight when NOx is
present (Trainer et al. 1987, Chameides et al. 1988, Thompson,
1992). Ozone causes respiratory distress and reduces crop and
forest yields (Reich and Amundson 1985, Runeckles and
Chevone 1992).
Isoprene emission rate is strongly affected by light and
temperature (Sanadze and Kursanov 1966, Rasmussen and
Jones 1973, Tingey et al. 1979, Monson and Fall 1989, Loreto
and Sharkey 1990). In addition to short-term (up to 20 min)
effects of light intensity on isoprene emission rates, leaves that
develop in full sun emit isoprene at a higher rate than leaves
that develop in shade (Sharkey et al. 1991, Harley et al. 1994).
Carbon assimilated in photosynthesis rapidly appears in emitted isoprene (Delwiche and Sharkey 1993). Isoprene emission
typically accounts for 2% of the carbon fixed in photosynthesis
(Monson and Fall 1989, Loreto and Sharkey 1990) but in
kudzu (Pueraria lobata (Willd.) Ohwi) it can account for over
50% of the carbon fixed in high light following water stress
(Sharkey and Loreto 1993). Sharkey and Singsaas (1995) hypothesized that isoprene synthesis protects leaves against thermal stress that can occur when leaves are exposed to the radiant
heat load of full sunlight, especially in fluctuating light environments.
Current methods for scaling from leaf-level measurements
to canopy-scale fluxes of isoprene model the light and temperature dependence of isoprene emission (Geron et al., 1996).
However, these models, particularly BEIS2 (revision of the
Biogenic Emission Inventory System, see Pierce and Waldruff
1991), require many assumptions about how isoprene emission
rate is affected by micrometeorological variables such as light,
both instantaneous irradiance and irradiance during leaf development, and leaf temperature. Furthermore, it is not known
whether a big leaf model (as opposed to a layered-canopy
model) can give acceptably good fit to canopy flux measurements made by micrometeorological measurements, or how
sophisticated leaf energy balance calculations must be to obtain reliable estimates of canopy isoprene flux from leaf-level
measurements.
We have measured isoprene emission, photosynthesis, and
stomatal conductance at three canopy levels in a white oak
(Quercus alba L.) tree and a red oak (Quercus rubra L.) tree,
including the topmost canopy leaves at a height of 30 m.
Measurements were made in half and full sunlight at 30, 35
and, in some cases, 40 °C.
650
SHARKEY, SINGSAAS, VANDERVEER AND GERON
Materials and methods
Site description
The study was conducted at the Duke University Research
Forest (35°58′25″ N and 79°06′05″ W) near Chapel Hill, North
Carolina. The forest is a mature, second-growth uneven-aged
stand with the oldest trees exceeding the age of 180 years. A
40-m walkup tower facilitated measurements within, and at the
top of, the forest canopy (Geron et al. 1996).
Most measurements were made during June 23--25 and
August 20--22, 1994. June was warm and dry, whereas August
was relatively wet. In both months, there were substantial rains
within 3 days of making the measurements and so the trees
were not under water stress during the measurements, although
the trees may have experienced water stress in the weeks
leading up to the June measurements.
During both periods, measurements were made at three
canopy levels. The leaf area index (LAI) measured with an LAI
2000 (Li-Cor, Inc., Lincoln, NE) was 4.1 at the bottom of the
canopy and 3.0, 1.1 and 0.3 at canopy heights of 6, 20 and
30 m, respectively. The leaves sampled at a height of 30 m
were not quite the highest in the canopy. The tower was not
excluded from the field of view of the LAI 2000. Although the
LAI determined by the LAI-2000 differs from the true LAI, it
provides a measure of the relative shadiness of each location.
Gas exchange measurements
Photosynthesis and stomatal conductance were measured with
a prototype of the Li-Cor 6400 gas exchange measuring system. The light source was a light emitting diode with peak
irradiance at 670 nm as recommended by Tennessen et al.
(1995), who showed that red light has a negligible effect on
measurements of photosynthesis and no effect on isoprene
emission relative to xenon-arc lamp light (Tennessen et al.
1994). The air supply for the gas exchange system was drawn
through a tube held about 3 m from the tower to avoid CO2
from investigator breath. Leaf temperature was controlled by
thermoelectric modules that are part of the Li-Cor 6400 system. In addition, we used a heat gun to obtain the high temperatures required and to overcome the low power supply to
the thermoelectric modules on the Li-Cor 6400. By directing
the heat gun onto the gas exchange cuvette, we could change
the leaf temperature by 5 °C in less than 2 min.
Isoprene measurements
Isoprene emission was measured by fitting a three-way valve
to the output port of the Li-Cor 6400 cuvette. Gas flow leaving
the cuvette was directed past a portable Scentoscreen gas
chromatograph (Sentex, Ridgefield, NJ) except when required
for matching the reference and measuring gas analyzers. The
gas chromatograph had a small pump that pumped 20 ml of air
leaving the leaf chamber through a small adsorbent tube containing Carbosieve B. To inject, the argon carrier gas flow was
directed through the preconcentrator while it was heated with
nichrome wire.
An argon ionization detector was used for detection. Tritium
decay excites the argon carrier gas causing ionization of hydro-
carbons coming from the column. In the field, single point
calibrations were made by mixing liquid isoprene in a 1-dm3
flask of N2 and mixing a small amount of that gas mixture in
another 1-dm3 flask of N2 to give a 256 ppb standard. The
accuracy of this method is limited by the accuracy of the
gas-tight syringes. Over one month, the standard deviation was
less than 5% of the mean calibration factor. When the standard
was made up seven times in a row the standard deviation of the
calibration factors was 4% of the mean. Most measurements
were in the range of 50 to 300 ppb and the ambient isoprene
concentration was 1--3 ppb. We have since discovered that
making the standards in air gives a 5% lower detector response.
We assume that standards in air are a better measure and so the
values reported here have been corrected for this effect. After
these measurements were completed we discovered that the
Sentex gas chromatograph output was nonlinear. The nonlinearity was extensively characterized by preparing and measuring several dilution series in the laboratory and comparing
results with a Shimadzu gas chromatograph with photoionization detection. The Shimadzu was linear across all dilutions
from 32 to 512 ppb isoprene giving us confidence that the
dilution series were correctly prepared.
The rate of isoprene emission was calculated as the concentration of isoprene in the airstream multiplied by the flow rate,
normally 250 µmol s −1. The Li-Cor 6400 cuvette encloses 6
cm2 of leaf.
Estimating isoprene concentration inside the leaf
We estimated the concentration of isoprene in the air phase in
the leaf by equations originally developed for estimating the
CO2 concentration inside leaves.
Ii = Ia + 1.94(JI /g),
where Ii is the isoprene concentration inside the leaf, Ia is the
isoprene concentration in the air outside the leaf, 1.94 is the
square root of the ratio of molecular weights of isoprene to
water, since diffusivity is related to the kinetic energy of a
molecule, JI is the flux of isoprene from the leaf, and g is the
conductance for water vapor. To estimate CO2 concentration
inside leaves it is common practice to use the ratio of the
mutual diffusion coefficients of CO2 in air and water in air
which fortuitously is close to the square root of the ratio of
molecular weights (von Caemmerer and Farquhar 1981). Because the mutual diffusion coefficient of isoprene in air is not
known, we resorted to the simpler formulation but recognize it
may need correction in the future. Other effects that caused
errors of 1 to 3% are not accounted for in our estimation of leaf
isoprene concentration.
Dry weights
Leaf punches of approximately 1 cm2 were taken from the
leaves used for isoprene emission rate measurements in June,
dried at 60 °C for two days, and then weighed.
Xanthophyll and chlorophyll measurements
In August, samples for analysis of chlorophyll and xantho-
ISOPRENE EMISSION
phylls were taken from leaves adjacent to the leaves used to
measure isoprene emission rates. Adjacent leaves were used
because we increased leaf temperature to the damage threshold
for many leaves during the August measurements of isoprene
emission rates. Leaf samples were extracted by grinding in
acetone. The acetone extract from three grindings was combined for each sample. The extraction technique gave reproducible results despite the toughness of the oak leaves.
Xanthophyll cycle intermediates, lutein, chlorophylls, and carotenoids were quantified by HPLC on a special column (Zorbax ODS non-endcapped 4.6 × 250 mm made by Dupont and
supplied by MACMOD Analytical, Inc., Chadds Ford, PA) as
described by Tennessen et al. (1995).
Results
In June, isoprene emission rates were high (Table 1) and
increased with increasing temperature from 30 to 35 °C at
irradiances of both 1000 and 2000 µmol m −2 s −1. The energies
of activation determined over this temperature range by the
Arrhenius equation were 68 and 55 kJ mol −1 at 1000 and 2000
µmol m −2 s −1, respectively. Although isoprene emission rates
increased with increasing temperature from 30 to 35 °C, assimilation rates decreased. As a result, the proportion of photosynthate being converted to isoprene almost doubled to
nearly 8%. The decline in photosynthetic rate with increasing
temperature was correlated with reduced stomatal conductance and could be a consequence of stomatal closure. However, stomatal closure does not affect isoprene emission rate
because isoprene accumulates in the leaf until the flux is the
same before and after stomatal closure (Fall and Monson
1992). The calculated isoprene concentration inside the leaf
doubled between 30 and 35 °C. At both 30 and 35 °C, doubling
the irradiance from 1000 to 2000 µmol m −2 s −1 increased rates
of isoprene emission and photosynthesis by about 10%.
Under standard conditions of 1000 µmol m −2 s −1 and 30 °C,
isoprene emission rates were over 4 times greater at the top of
the canopy (LAI = 0.3) than near the bottom of the canopy
651
(LAI = 3.0) (Figure 1). Photosynthetic rates were 1.8 times
greater at the top of the canopy than at the bottom of the canopy
(data not shown). Leaf weight per leaf area was 52 and 133 g
m −2 at the bottom and top of the canopy, respectively. Consequently, when isoprene emission rates were expressed on a leaf
mass basis, differences between the top and the bottom of the
canopy were reduced but not eliminated.
The rate of isoprene emission in August was less than that
observed in June (Table 2), although the activation energies at
2000 and 1000 µmol m −2 s −1 were similar to those determined
in June at 55 and 80 kJ mol −1, respectively. In August, isoprene
emission rate increased with increasing temperature until the
temperature was above 40 °C. As in June, photosynthetic rate
and stomatal conductance fell with increasing temperature so
that, at 40 °C and in full sunlight, isoprene accounted for
13.7% of the carbon fixed in photosynthesis and the isoprene
concentration was 2.6 ppm inside the leaf. Biosynthesis of
isoprene from photosynthetic precursors likely results in four
carbons being lost as CO2, and so in addition to the 13.7% of
photosynthesis lost as isoprene, another 11% of potential photosynthate is lost as a by-product of isoprene synthesis giving
a total carbon cost of about 25% of photosynthesis.
The gradient in leaf-area based isoprene emission capacity
from the top to the bottom of the canopy was about 3-fold at
30 °C and 5-fold at 35 °C (Table 2). Both assimilation capacity
and leaf chlorophyll concentration were highest in the middle
of the canopy (Table 3), whereas the chlorophyll a/b ratio was
highest at the top of the canopy and declined with depth in the
canopy paralleling the decrease in irradiance from the top to
the bottom of the canopy.
Contents of the xanthophyll cycle intermediates, violaxanthin (V), antheraxanthin (A), and zeaxanthin (Z), were high in
leaves at the top of the canopy and declined with depth in the
canopy. In addition, the epoxidation state ((A+0.5V)/(Z + V +
A)) (Demmig-Adams and Adams 1992) was lowest at the top
of the canopy and increased down the canopy. Lutein, which
does not participate in the light-protecting xanthophyll cycle,
Table 1. Isoprene emission rate and other gas exchange parameters
measured in white oak (Quercus alba) leaves at the top of the canopy
in June 1994. Values are given as means ± SE (n = 3 measurements on
each of three leaves). Units are: isoprene emission rate, nmol m −2 s −1;
photosynthetic CO2 assimilation rate, µmol m −2 s −1; I/A is the ratio of
carbon atoms emitted as isoprene to carbon atoms assimilated in
photosynthesis expressed as %; conductance to water vapor exchange,
mol m−2 s−1, and isoprene concentration, nmol mol−1 air inside the leaf.
Measurements were made at either 1000 or 2000 µmol m −2 s −1 and at
30 or 35 °C.
Light/temp
1000/30
Isoprene
87 ± 9
Assimilation 11.2 ± 0.4
I/A
3.9 ± 0.4
Conductance 0.23 ± 0.01
Isoprene conc. 759 ± 80
1000/35
2000/30
135 ± 8
9.1 ± 0.3
7.4 ± 0.2
0.19 ± 0.00
1363 ± 56
108 ± 8.3
12.9 ± 0.7
4.2 ± 0.1
0.28 ± 0.02
746 ± 17
2000/35
155 ± 12
10.0 ± 0.2
7.8 ± 0.5
0.21 ± 0.01
1471 ± 69
Figure 1. Isoprene emission rate from canopy leaves of white oak,
Q. alba. Leaf temperature was 35 °C and irradiance was 2000 µmol
m −2 s −1 . Error bars represent standard errors of the mean, n = 3
measurements on each of three to five leaves.
652
SHARKEY, SINGSAAS, VANDERVEER AND GERON
Table 2. Isoprene emission rate and other gas exchange parameters measured in white oak (Quercus alba) leaves in August 1994. Values given are
means ± SE (n = 1 measurement on each of five leaves). Units are given in Table 1. Measurements were made atthe top (LAI = 0.3), middle (LAI
= 1) and near the bottom (LAI = 3) of the tree canopy. Measurements were made at either 1000 or2000 µmol m −2 s −1 and at 30 or 35 °C.
Light/temperature
1000/30
1000/35
2000/30
2000/35
Top of the canopy
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
65 ± 3
8.3 ± 0.6
4.0 ± 0.4
0.18 ± 0.03
703 ± 29
107 ± 5
7.1 ± 0.8
8.2 ± 1.5
0.15 ± 0.02
1379 ± 71
76 ± 5
8.6 ± 0.8
4.6 ± 0.6
0.16 ± 0.03
916 ± 62
126 ± 7
7.9 ± 0.9
8.5 ± 1.2
0.16 ± 0.03
1536 ± 91
Middle of the canopy
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
47 ± 2
10.3 ± 0.5
2.3 ± 0.08
0.33 ± 0.05
273 ± 12
69 ± 2
9.2 ± 0.2
3.7 ± 0.2
0.28 ± 0.01
479 ± 16
53 ± 3
11.4 ± 0.5
2.3 ± 0.1
0.35 ± 0.04
294 ± 14
81 ± 3
9.7 ± 0.5
4.2 ± 0.2
0.26 ± 0.03
603 ± 17
Bottom of the canopy
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
19 ± 0.8
3.8 ± 0.5
2.7 ± 0.3
0.10 ± 0.01
372 ± 15
22 ± 2
3.7 ± 0.5
3.3 ± 0.1
0.10 ± 0.01
435 ± 48
21 0.2
4.9 ± 0.5
2.2 ± 0.2
0.11 ± 0.01
367 ± 3
27 ± 0.9
4.0 ± 0.4
3.4 ± 0.3
0.09 ± 0.01
576 ± 20
2000/40
180 ± 10
6.7 ± 0.6
13.7 ± 1.0
0.13 ± 0.01
2681 ± 155
Table 3. Light environment and chlorophyll and xanthophyll contents of Q. alba leaves at three heights in the canopy of a single tree. Values are
means and standard errors, n = 3.
LAI
Total chlorophyll, µmol m −2
Chl a/b
Xanthophyll cycle intermediates, µmol m −2
Epoxidation, %
Lutein, µmol m −2
β-Carotene, µmol m −2
Top
Middle
Bottom
0.3
454 ± 18
3.15 ± 0.05
16.8 ± 0.8
31 ± 1
33 ± 2
49 ± 8
1.0
477 ± 16
3.04 ± 0.01
4.8 ± 0.5
62 ± 10
32 ± 1
58 ± 1
3.0
383 ± 6
2.63 ± 0.03
1.02 ± 0.3
100 ± 0
30 ± 1
45 ± 2
and β-carotene contents did not vary through the canopy,
indicating that the differences in chl a/b ratio, contents of
xanthophyll cycle intermediates, and epoxidation state were
specific responses, presumably to light environment, and not
simply a reflection of the general health or specific leaf area of
the leaves.
In August, we measured isoprene emission and gas exchange on leaves of several branches of red oak (Quercus
rubra) that were cut from the top of a tree near the tower. The
rates of isoprene emission were less than for white oak when
measured at a PPFD of 1000 µmol m −2 s −1 but the difference
was less obvious at a PPFD of 2000 µmol m −2 s −1. Assimilation rate of red oak was also less than that of white oak at a
PPFD of 1000 mol µmol m −2 s −1 but not at a PPFD of 2000
µmol m −2 s −1, an effect that could be explained by lower
stomatal conductance at low irradiances in red oak compared
with white oak. Because stomata were more closed at low
irradiances in red oak than in white oak, the concentration of
isoprene was nearly constant across treatments despite the
large change in isoprene emission rate. In red oak, both stomatal conductance and photosynthetic rate increased when the
temperature was increased from 35 to 40 °C, whereas in white
oak both stomatal conductance and photosynthetic rate declined over this temperature range (Table 4).
We tested whether isoprene emission rate increased when
the temperature was above 40 °C in one white oak leaf and
three red oak leaves. In all cases, isoprene emission rate increased as temperature increased to 42 °C. In one red oak leaf,
isoprene emission rate increased up to 44 °C. For two red oak
leaves and the white oak leaf the temperature was held at 42 °C
for 5 to 20 min and isoprene emission rate fell over this time.
Discussion
Isoprene emission rates were much higher from leaves at the
top of the canopy of a white oak tree than from leaves lower in
ISOPRENE EMISSION
653
Table 4. Isoprene emission rate and other gas exchange parameters measured in red oak (Quercus rubra) leaves in August 1994. Values given are
means ± SE (n = 3). Units are given in Table 1. Measurements were made on leaves attached to a branch cut from the top of a 30-m tree and placed
in water. Measurements were made at either 1000 or 2000 µmol m −2 s −1 and at 30 or 35 °C.
Light/temperature
1000/30
1000/35
2000/30
2000/35
Isoprene
Assimilation
I/A
Conductance
Isoprene conc.
43 ± 7
3.0 ± 0.4
7.8 ± 2.0
0.07 ± 0.02
1385 ± 511
102 ± 5
5.2 ± 0.6
8.9 ± 1.3
0.15 ± 0.02
1398 ± 119
66 ± 6
5.3 ± 0.9
5.7 ± 1.4
0.11 ± 0.03
1325 ± 485
100 ± 6
6.0 ± 0.5
8.5 ± 1.1
0.17 ± 0.02
1213 ± 175
the canopy. Sun leaves are known to have a greater capacity for
isoprene emission than shade leaves (here capacity means the
rate of emission under standard conditions, most often at a
PPFD of 1000 µmol m −2 s −1 and 30 °C) (Sharkey et al. 1991,
Harley et al. 1994). Leaves at the tops of trees will also have
the lowest water potential. The white oak tree was about 30 m
tall and so the gravity component of the water potential is
about 0.3 MPa; however, resistance to water flow would reduce the water potential below −0.3 MPa in the top of the tree.
Water stress is known to increase the capacity for isoprene
emission in kudzu (Sharkey and Loreto 1993). Finally, high
temperature can increase the capacity for isoprene emission
(Sharkey and Loreto 1993) and high leaf temperature is more
likely at the top of the tree than elsewhere because of both the
higher radiation load and the reduced water supply relative to
other parts of the tree. All of these factors may have contributed
to the high isoprene emission capacity at the top of the white
oak tree.
Our results support the thermal protection hypothesis put
forward by Sharkey and Singsaas (1995) to account for plant
isoprene emission. The estimates of isoprene concentration
inside the leaves confirmed that the concentration of isoprene
required to provide thermal protection occurs in natural conditions. Leaves subjected to the highest temperatures had the
greatest capacity for isoprene emission. The distribution of
isoprene emission capacity through the canopy was similar to
the distribution of xanthophyll cycle intermediates. Xanthophylls are believed to protect leaves against excess radiation
damage (Demmig-Adams and Adams 1992), whereas isoprene
is believed to protect leaves against excess heat damage
(Sharkey and Singsaas 1995). The contents of lutein, a xanthophyll that does not participate in the light-protecting xanthophyll cycle, and β-carotene were constant throughout the
canopy.
The increase in isoprene emission rate with increasing temperature continued to higher temperatures in the field than had
been assumed based on laboratory studies (Guenther et al.
1993). For example, Guenther et al. (1991) concluded that
39 °C was the peak temperature for isoprene emission. However, although isoprene emission becomes unstable above
40 °C, we observed very high rates for short periods at above
40 °C. Such high temperatures are likely on hot days with little
wind, the same conditions that lead to ozone pollution episodes. Because leaf temperature tends to be variable, even on
still days, it is possible that the transient very high rate of
2000/40
165 ± 21
6.9 ± 0.1
11.9 ± 1.4
0.21 ± 0.01
1534 ± 196
isoprene emission seen immediately after raising the temperature above 40 °C is relevant to modeling isoprene emission for
isoprene emission inventories. If so, it may be useful to use the
Arrhenius equation to describe the temperature response of
isoprene emission because only two parameters would be
required, the basal emission rate and the physiologically relevant activation energy. The current temperature response models require basal emission rate plus three empirical coefficients
(Guenther et al. 1991).
Red oak leaves exhibited a higher temperature optimum for
photosynthesis than white oak leaves. Unfortunately we were
restricted to using cut branches of red oak because the tree was
between 5 and 10 m from the tower. Cutting branches may
affect isoprene emission (Loreto and Sharkey 1993) and the
degree to which it did so in this study is unknown. In white oak,
the stomata closed at high temperatures, whereas in red oak
photosynthetic rate and stomatal conductance increased with
increasing temperatures between 35 and 40 °C. This difference
in stomatal behavior would keep red oak leaves cooler than
white oak leaves under natural conditions and could explain
why the red oak leaves had a slightly lower capacity for
isoprene emission than the white oak leaves.
We made measurements in June and August expecting to
find a developmental increase in isoprene emission (Grinspoon
et al. 1991, Kuzma and Fall 1993, F. Loreto and T.D. Sharkey
unpublished data with red oak); however, isoprene emission
rates were higher in June than in August, indicating that oak
leaves are fully competent for isoprene emission early in the
season.
We conclude that most isoprene will be lost from directly
sunlit leaves. Isoprene emission is temperature and light dependent and not completely saturated even at a PPFD of 1000
µmol m −2 s −1. Sunlit leaves are likely to be warmer than shaded
leaves. In addition, leaves that are often in the sun have as
much as a 5-fold greater capacity for isoprene emission than
leaves lower in the canopy that are only rarely sunlit.
Note added in proof
We now estimate the ratio of diffusivities of water to isoprene to be
2.83 based on an analysis given by Lyman, W.J., W.F. Reehl and D.H.
Rosenblatt (1990) In Estimation Methods. Amer. Chem. Soc. Washington, DC, pp 17-9--17-17.
654
SHARKEY, SINGSAAS, VANDERVEER AND GERON
Acknowledgments
We thank Professor John Norman for discussions on leaf micrometereology and on the use of the LAI 2000. Research supported by U.S.
Environmental Protection Agency cooperative research agreement CR
823791-01-1 and U.S. National Science Foundation grant IBN9317900.
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