How light, temperature, and measurement and growth [CO2] interactively control isoprene emission in hybrid aspen

Abstract

Plant isoprene emissions have been modelled assuming independent controls by light, temperature and atmospheric [CO2]. However, the isoprene emission rate is ultimately controlled by the pool size of its immediate substrate, dimethylallyl
diphosphate (DMADP), and isoprene synthase activity, implying that the environmental controls might interact. In addition,
acclimation to growth [CO2] can shift the share of the control by DMADP pool size and isoprene synthase activity, and thereby alter the environmental
sensitivity. Environmental controls of isoprene emission were studied in hybrid aspen (Populus tremula × Populus tremuloides) saplings acclimated either to ambient [CO2] of 380 μmol mol–1 or elevated [CO2] of 780 μmol mol–1. The data demonstrated strong interactive effects of environmental drivers and growth [CO2] on isoprene emissions. Light enhancement of isoprene emission was the greatest at intermediate temperatures and was greater
in elevated-[CO2]-grown plants, indicating greater enhancement of the DMADP supply. The optimum temperature for isoprene emission was higher
at lower light, suggesting activation of alternative DMADP sinks at higher light. In addition, [CO2] inhibition of isoprene emission was lost at a higher temperature with particularly strong effects in elevated-[CO2]-grown plants. Nevertheless, DMADP pool size was still predicted to more strongly control isoprene emission at higher temperatures
in elevated-[CO2]-grown plants. We argue that interactive environmental controls and acclimation to growth [CO2] should be incorporated in future isoprene emission models at the level of DMADP pool size.

Full text

Journal of Experimental Botany, Vol. 66, No. 3 pp. 841–851, 2015
doi:10.1093/jxb/eru443 Advance Access publication 13 November, 2014
This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)
ReseaRch PaPeR
How light, temperature, and measurement and growth [CO2]
interactively control isoprene emission in hybridaspen
ÜloNiinemets1,2,* and ZhihongSun1
1 Estonian University of Life Sciences, Kreutzwaldi 1, 51014 Tartu, Estonia
2 Estonian Academy of Sciences, Kohtu 6, 10130 Tallinn, Estonia
* To whom correspondence should be addressed. E-mail: ylo.niinemets@emu.ee
Received 11 July 2014; Revised 29 September 2014; Accepted 6 October 2014
Abstract
Plant isoprene emissions have been modelled assuming independent controls by light, temperature and atmospheric
[CO2]. However, the isoprene emission rate is ultimately controlled by the pool size of its immediate substrate, dimeth-
ylallyl diphosphate (DMADP), and isoprene synthase activity, implying that the environmental controls might interact.
In addition, acclimation to growth [CO2] can shift the share of the control by DMADP pool size and isoprene synthase
activity, and thereby alter the environmental sensitivity. Environmental controls of isoprene emission were studied
in hybrid aspen (Populus tremula × Populus tremuloides) saplings acclimated either to ambient [CO2] of 380μmol
mol–1 or elevated [CO2] of 780μmol mol–1. The data demonstrated strong interactive effects of environmental drivers
and growth [CO2] on isoprene emissions. Light enhancement of isoprene emission was the greatest at intermediate
temperatures and was greater in elevated-[CO2]-grown plants, indicating greater enhancement of the DMADP sup-
ply. The optimum temperature for isoprene emission was higher at lower light, suggesting activation of alternative
DMADP sinks at higher light. In addition, [CO2] inhibition of isoprene emission was lost at a higher temperature with
particularly strong effects in elevated-[CO2]-grown plants. Nevertheless, DMADP pool size was still predicted to more
strongly control isoprene emission at higher temperatures in elevated-[CO2]-grown plants. We argue that interactive
environmental controls and acclimation to growth [CO2] should be incorporated in future isoprene emission models
at the level of DMADP pool size.
Key words: CO2 response, dimethylallyl diphosphate, elevated [CO2], isoprene emission, light sensitivity, temperature optimum,
temperature response.
Introduction
Isoprene as a highly reactive and the most widespread volatile
molecule emitted from a series of plant species plays a major
role in air quality and climate, participating in ozone and
secondary organic aerosol generation (Claeys et al., 2004;
Fowler etal., 2009; Monson etal., 2012; Fineschi etal., 2013;
Sharkey etal., 2013). The biological role of isoprene in plants
is protection from abiotic stresses by serving as a membrane
stabilizer under heat stress (Singsaas et al., 1997; Sharkey
etal., 2001, 2008; Siwko etal., 2007), as well as a lipid-soluble
antioxidant reacting with a broad array of stress-generated
reactive oxygen species and peroxidized membrane lipids
(Loreto et al., 2001; Affek and Yakir, 2002; Vickers et al.,
2009a, b; Possell and Loreto, 2013).
Isoprene formation in chloroplasts from dimethylallyl
diphosphate (DMADP) is catalysed by isoprene synthase
(for reviews, see Li and Sharkey, 2013b; Rosenkranz and
© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology.
Abbreviations: ANOVA, analysis of variance; ANCOVA, analysis of covariance; DMADP, dimethylallyl diphosphate; GDP, geranyl diphosphate.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/), which permits
unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

842 | Niinemets and Sun
Schnitzler, 2013; Sharkey et al., 2013), and the control of
isoprene synthesis under different environmental conditions
is shared between isoprene synthase and the DMADP pool
size (Rasulov et al., 2009b, 2010; Possell and Hewitt, 2011;
Sun etal., 2012b; Li and Sharkey, 2013a, b; Monson, 2013).
Changes in DMADP pool size are primarily responsible for
the hyperbolic increase of isoprene emission with increasing
quantum ux density, while the Arrhenius-type temperature
response with an optimum depends both on temperature
effects on isoprene synthase activity and DMADP pool size
(Rasulov etal., 2009b, 2010; Li et al., 2011; Monson etal.,
2012; Li and Sharkey, 2013a). In addition, increases of CO2
concentration above approximately 100–150 μmol mol–1
inhibit isoprene emission (Wilkinson etal., 2009; Possell and
Hewitt, 2011; Monson et al., 2012) due to a reduced chloro-
plastic DMADP pool size (Rasulov etal., 2009b; Wilkinson
etal., 2009; Li and Sharkey, 2013b).
Empirical isoprene emission models widely assume that
different environmental drivers operate independently (for
recent reviews, see Monson etal., 2012; Grote et al., 2013).
While empirical models have been relatively successful in sim-
ulating isoprene emission responses to temperature and light
assuming independent controls (Guenther, 1997; Guenther
etal., 1993, 2006), it is less clear whether an analogous addi-
tion of the [CO2] response (e.g. Wilkinson etal., 2009) is per-
tinent. Based on an additive [CO2] response, the models have
indicated that isoprene emissions will decline in the future
higher atmospheric [CO2] conditions (e.g. Heald etal., 2009;
Wilkinson etal., 2009). However, because the effects of envi-
ronmental drivers are mediated through the DMADP pool
size, the effects of certain environmental combinations can be
interactive rather than additive (Rasulov etal., 2009b, 2010;
Sun etal., 2012b; Li and Sharkey, 2013a, b).
Prediction of isoprene emissions in future conditions is
further complicated by acclimation of isoprene emissions
to growth [CO2] (i.e. the [CO2] at which the plant is grown).
There is evidence that growth [CO2] can modify the instanta-
neous [CO2] response of isoprene emission and the maximum
emission rate (Calfapietra etal., 2007, 2008, 2013; Wilkinson
etal., 2009; Sun etal., 2012b) due to changes in the isoprene
synthase activity and DMADP pool size (Sun etal., 2012b,
2013). Although the DMADP pool size is characteristically
reduced in plants grown under elevated [CO2] (Possell and
Hewitt, 2011; Sun et al., 2012b, 2013), isoprene synthase
activity is not always reduced and might compensate for the
reductions in DMADP pool size (Sharkey et al., 1991; Li
etal., 2009; Sun etal., 2012b). Such changes in DMADP pool
size and isoprene synthase activity in response to growth con-
ditions are important as they can alter the light and tempera-
ture responses. As we have demonstrated in a previous study
(Sun etal., 2013), higher measurement [CO2] (i.e. the [CO2] at
which the rate is measured) inhibited isoprene emission rate
at temperatures of 30–35ºC, but the [CO2] inhibition was lost
at higher temperatures, indicating enhanced DMADP availa-
bility at higher [CO2]. Such an enhancement is consistent with
the hypothesis that low DMADP availability at high [CO2]
is associated with reduced chloroplastic inorganic phosphate
levels due to imbalanced rates of starch and sucrose synthesis
consuming triose phosphates and photosynthesis providing
triose phosphates, ultimately leading to feedback inhibition
of photosynthesis (Li and Sharkey, 2013b). In fact, feedback
inhibition of photosynthetic electron transport rate and ATP
synthesis rate (Sharkey, 1985; Socias et al., 1993) can ulti-
mately be responsible for the decrease in DMADP synthesis
rate under high measurement [CO2] (Rasulov et al., 2009b).
As sucrose synthesis strongly responds to temperature (Sage
and Sharkey, 1987; Li and Sharkey, 2013b), this frees up inor-
ganic phosphate and releases the feedback inhibition, thereby
enhancing the rate of photosynthetic electron transport, and
ATP and DMADP synthesis rates. This is expected to lead
to a strong interactive effect of measurement [CO2] and tem-
perature on isoprene emissions that can further be modied
by acclimation to elevated [CO2].
In this study, we asked how the instantaneous CO2 sen-
sitivity of isoprene emission varied with other environ-
mental drivers, in particular with temperature, light, and
growth [CO2], in the strongly isoprene-emitting hybrid
aspen (Populus tremula × Populus tremuloides). We tested
the hypotheses that elevated-[CO2]-grown plants would have
modied environmental responses of isoprene emission and
that such modied environmental responses in plants accli-
mated to different [CO2] would represent interactive con-
trols on isoprene emission. In this model-based analysis, we
integrated data reported in our previous studies (Sun etal.,
2012b, 2013) as well as additional replicate measurements,
and analysed the data from the perspective of simultaneous
limitation of isoprene emission by light, temperature, and
[CO2] under different growth [CO2] regimes. The results of
this study will provide novel insights for developing models
to predict isoprene emissions in future climates.
Material and methods
Plant growth and experimental treatments
In this study, we included data for four series of replicate experiments
reported by Sun et al. (2012b, 2013) and an additional two series
of experiments conducted according to the same protocol outlined
briey here. Two-year-old saplings of hybrid aspen (Populus tremula
L. × Populus tremuloides Michx.) clone H200 (Rasulov etal., 2009a,
2011; Vahala etal., 2003, for details of the genotype) grown in whole
plant chambers were used. Four saplings were grown at a time in a
four-chamber growth/gas-exchange system (individual chamber vol-
ume 12.5 l). The [CO2] was maintained at an ambient level (average
± standard deviation) of 380 ± 10μmol mol–1 in chambers 1 and 3,
and at an elevated level of 780 ± 10 μmol mol–1 in chambers 2 and
4.The chamber air temperature was 28–30/23°C (day/night), rela-
tive humidity was 60%, and light intensity at the top of the plants
was 500–800μmol m–2 s–1 for the 12 h light period, resulting in a
moderately high daily integrated growth light of 28.1 mol m–2 d–1
(~70% of seasonal average daily integrated quantum ux density at
a completely open location in the eld) (Sun etal., 2012a, 2012b).
Measurement of temperature response curves of isoprene
emission
Isoprene emission measurements were conducted after 30–40 d of
growth under the given conditions when the plants had lled the
chambers using individual attached fully mature leaves as described
in detail by Sun etal. (2012b, 2013). After moving the plant out of
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

Interactive environmental controls on isoprene emission | 843
the chamber, the sample leaf was enclosed in a Walz GFS-3000 port-
able gas-exchange/chlorophyll uorescence system equipped with
an LED array/PAM uorimeter 3055-FL (Walz GmbH, Effeltrich,
Germany) and connected to a Fast Isoprene Sensor (FIS, Hills-
Scientic, Boulder, CO, USA). The leaf was rst stabilized at the
baseline conditions (leaf temperature of 30 °C, light intensity of
500μmol m–2 s–1, and relative air humidity of 60%). Once the steady-
state gas-exchange and isoprene emission rates had been established,
the temperature responses of photosynthesis and isoprene emission
were measured at a moderately high light intensity of 500μmol m–2
s–1 (growth light intensity) and a strong light intensity of 2000μmol
m–2 s–1 at the [CO2] of 380 and 780μmol mol–1. During the measure-
ments, the leaf temperature was increased in steps of 5°C from 30
to 50°C, and the values of the isoprene emission rate were recorded
for 8 min after the change in temperature (Sun et al., 2013). This
time corresponds to the duration of intermediate-length sunecks in
plant canopies (Pearcy, 1990) and, although arbitrary, standardiza-
tion of the time of measurement results in a common heat dose for
all plants. Such a standardization is particularly important for the
higher temperatures between 45 and 50°C that can be inhibitory
for photosynthesis (Hüve etal., 2006, 2011) and isoprene emission
(Rasulov et al., 2010, 2014a) such that steady-state photosynthe-
sis and isoprene emission rates cannot be reached at these higher
temperatures.
Normalized emission rates and fitting the temperature
responses of isoprene emission
To normalize the environmental responses of isoprene emission, we
calculated the relative light-dependent increase of isoprene emis-
sion, RL,as:
R
II
I
L
2000
=
−500
2000
,
(1)
where I2000 is the isoprene emission rate at the light intensity of
2000μmol m–2 s–1 and I500 is that at 500μmol m–2 s–1. Analogously,
the relative temperature-dependent change in isoprene emission (RT)
was calculatedas:
RII
I
T
T
=
−30
30
, (2)
where I30 is the emission rate at 30ºC and IT is that at temperature T.
The temperature response of isoprene emission rate was also t-
ted by an exponential function with a maximum (Copolovici etal.,
2005; Harley and Tenhunen, 1991):
Ie
e
cH T
ST
HT
a
d
=
+
−
−
∆
∆∆
/
()
/,
R
R
1 (3)
where T is the leaf temperature in K, R (8.314 J mol–1 K–1) is the
gas constant, c is the scaling factor, ΔHa (J mol–1) is the activation
energy, ΔHd (J mol–1) is the deactivation energy, and ΔS (J mol–1 K–1)
is the entropy term. The explained variance of temperature relation-
ships (r2) was in all cases >0.98. From this equation, the optimum
temperature for I, Topt (Niinemets etal., 1999a), is givenas:
TH
SR H
H
opt
d
d
a
=∆
∆∆
∆
+−












ln
.
1
(4)
Equation 3 is analogous to the temperature relationship of the
Guenther et al. model (Guenther, 1997; Niinemets et al., 2010;
Monson et al., 2012; Grote et al., 2013), but we favoured it in this
study to demonstrate the mechanistic connection between the
parameters of the temperature relationship and Topt.
To characterize the initial increase of isoprene emission rate
with increasing temperature, we also calculated the average
value of Q10, the process rate at temperature T+10ºC relative
to the process rate at temperature T, for the temperature range
25–40ºC using the fitted temperature response curve parameters
(Eq.3).
The temperature response of isoprene emission is a mixed
response that is driven by temperature effects on the DMADP pool
size (CDMADP, nmol m–2) and on the isoprene synthase rate constant
(k, s–1):
I kC=DMADP.
(5)
Implicit in Eq. 5 is that the Km value for DMADP of isoprene
synthase is large relative to the concentrations of DMADP char-
acteristically observed in chloroplasts (Rasulov et al., 2009a, b,
2010) such that the rate constant, k, does not depend on substrate
concentration over the given DMADP range. The sources of vari-
ation due to changes in k and CDMADP can be separated using the
response coefcient analysis (Poorter and Nagel, 2000) that pro-
vides the fractions of variance in I due to both of its components
(Appendix 1). Using available information on k and CDMADP at
30 °C (Sun et al., 2012b), the response coefcients were calcu-
lated as described in Appendix 1.As this simplied analysis does
not consider possible modications in the temperature depend-
ence of k by measurement [CO2], and measurement and growth
[CO2] interaction (Appendix 1), the response coefcients were only
employed to gain insight into the changes in the light sensitivity of
isoprene emission.
Data analyses
In the following, the growth [CO2] treatments (380 vs 780μmol
mol–1) are denoted as ‘ambient’ and ‘elevated’, and the measure-
ment [CO2] (380 vs 780μmol mol–1) as 380 and 780. Thus, in this
analysis, we had four combinations of growth and measurement
[CO2]: ambient (380), ambient (780), elevated (380), and elevated
(780), and two additional combinations of the measurement light
intensity: a moderately high light intensity of 500 μmol m–2 s–1
and strong light intensity of 2000 μmol m–2 s–1. The effects of
combinations of [CO2] treatment and measurement [CO2] at dif-
ferent light intensities and temperatures were analysed by analy-
sis of variance (ANOVA) followed by Tukey’s test (growth [CO2]
treatments involving independent samples) and by paired-samples
t-tests (paired comparisons between different light and measure-
ment [CO2]). Correlative relationships among leaf traits were ana-
lysed by linear regressions. To compare the statistical relationships
among [CO2] treatments at different light intensities and measure-
ment [CO2], analysis of covariace (ANCOVA) was used. The sepa-
rate slope ANCOVA model with the interaction term (treatment
with covariate) was tted rst, followed by the common-slope
model (without the interaction term) when the interaction term
was statistically not signicant. For all analyses, we used SPSS 17.0
(IBM SPSS Statistics), and all statistical tests were considered sig-
nicant at P<0.05.
Results
Dependencies of isoprene emission rate on
temperature
Increases in temperature enhanced the isoprene emission rate
(I) up to 45–50ºC (Fig.1) with the optimum temperature of
isoprene emission (Eq. 4)varying from 43 to 49ºC across all
the data (Table1). Although the temperature responses were
similar under the two light intensities of 500 and 2000μmol
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

844 | Niinemets and Sun
m–2 s–1 (Fig. 1a, b), the light-dependent enhancement of I
decreased with increasing temperature (Fig. 2). The light-
dependent increase of isoprene emission rate (Eq. 1) did
not depend on measurement [CO2] in ambient-[CO2]-grown
plants, but in elevated-[CO2]-grown plants, the increase was
greater at the higher measurement [CO2] of 780μmol mol–1
than at 380μmol mol–1 (Fig.2).
Temperature response curve characteristics
of isoprene emission in relation to growth and
measurement [CO2] and light intensity
The optimum temperature (Topt) for isoprene emission did
not depend on the measurement [CO2] for ambient-[CO2]-
grown plants, but Topt was greater at the measurement [CO2]
of 780 μmol mol–1 than at 380 μmol mol–1 in elevated-
[CO2]-grown plants (Table1). Topt was greater at a moder-
ate light intensity of 500 μmol m–2 s–1 in all cases, except
for the measurements at 380 μmol mol–1 in ambient-CO2-
grown plants (Table1). Overall, the average Q10 values for
the temperature range of 25–40ºC (Table1) were greater for
elevated-[CO2]-grown plants (Table 1). In ambient-[CO2]-
grown plants measured at 780 μmol mol–1, Q10 was greater
at the higher light value, while the opposite was true for
elevated-[CO2]-grown plants measured at 380 μmol mol–1
(Table1).
To gain insight into the sources of variation in Topt, we
also analysed the correlations of Topt with temperature
response curve parameters (Eq. 3)and with traits charac-
terizing the temperature sensitivity of emissions to lower
and higher temperatures (Q10 and RT, Eq. 2). As isoprene
synthase itself has a very high optimum temperature of
around 50ºC (Monson etal., 1992; Lehning et al., 1999;
Rasulov etal., 2010), lower Topt values than those for iso-
prene synthase suggest limitation of isoprene synthesis by
the DMADP pool size (Rasulov etal., 2010). Accordingly,
Fig.1. Temperature responses of isoprene emission rate in hybrid aspen leaves grown under ambient (380μmol mol–1) and elevated (780μmol mol–1)
CO2 concentrations (reanalysis of the data of Sun etal., 2013). Isoprene emission rate was measured both at ambient and elevated [CO2] and at a
moderately high light intensity of 500μmol m–2 s–1 (a) and a strong light intensity of 2000μmol m–2 s–1 (b). A(380) and E(380) denote plants grown under
ambient [CO2] of 380μmol mol–1 and elevated [CO2] of 780μmol mol–1, and both measured at [CO2] of 380μmol mol–1, while A(780) and E(780) denote
plants grown under ambient [CO2] of 380μmol mol–1 and elevated [CO2] of 780μmol mol–1, and both measured at [CO2] of 780μmol mol–1. Reported
data are averages ± standard error (SE) of 8–10 replicate leaves for each combination of environmental drivers. The insets demonstrate the curves fitted
to the data (Eq. 3).
Table1. Average (± SE) temperature optimum of isoprene emission rate (Topt, Eq. 3)and average Q10 values in hybrid aspen leaves
grown under different [CO2] and measured under different [CO2] and light intensities
[CO2] (μmol mol–1) Light intensity (μmol m–2 s–1)
Growth Measurement 500 2000 500 2000
Topt (°C) Q10
380 (ambient) 380 45.87 ± 0.46aA 46.19 ± 0.32bA 3.92 ± 0.19aA 3.77 ± 0.05aA
380 (ambient) 780 46.80 ± 0.46abA 45.15 ± 0.39abB 3.83 ± 0.06aA 4.51 ± 0.18aB
780 (elevated) 380 45.27 ± 0.26aA 44.50 ± 0.38aB 5.18 ± 0.35bA 4.18 ± 0.18aB
780 (elevated) 780 47.58 ± 0.30bA 46.13 ± 0.22bB 4.57 ± 0.40abA 4.38 ± 0.28aA
Q10 is given as the process rate at temperature T+10 relative to the process rate at temperature T. It was calculated from the fitted emission vs.
leaf temperature relationships as an average for the temperature range of 25–40ºC. Means with the same lowercase letter are not significantly
different (P>0.05) among growth and measurement [CO2] combinations (one-way ANOVA followed by Tukey’s test), while means with the same
uppercase letter are not significantly different among different measurement light intensities (paired-samples t-test).
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

Interactive environmental controls on isoprene emission | 845
variation in Topt at a given measurement [CO2] and light
level should reect differences in the heat-dependent decay
of the DMADP pool size. Topt was positively correlated
with a relative increase of isoprene emission rate at 50°C
(RT, Eq. 2; Fig. 3). In this relationship, the interaction
terms, RT×(growth [CO2]) (P>0.1), RT×(light intensity)
(P>0.6) and RT×(measurement [CO2]) (P>0.7) were not
statistically signicant. According to the common-slope
ANCOVA model, both light intensity (P<0.03, Table 1),
and growth [CO2] (P<0.05, Fig.3) were statistically signi-
cant factors, implying that Topt was lower at a given RT both
at higher measurement light and in elevated-[CO2]-grown
plants (Fig.3), suggesting a greater control by the DMADP
poolsize.
In contrast to these correlations, Topt was not correlated
with the average Q10 for the temperature range 25–40 °C
(Table 1, r2=0.07, P>0.07 for all data pooled), and the cor-
relations were much weaker for RT values calculated for tem-
peratures of 45°C (r2=0.20, P<0.05 for ambient-[CO2]-grown
plants and r2=0.10, P>0.1 for elevated-[CO2]-grown plants)
and 40°C (r2=0.01 for ambient-CO2-grown and r2=0.02 for
elevated-[CO2]-grown plants, P>0.8 for both). In addition,
differences in Topt were mainly associated with differences in
the deactivation energy (ΔHd, Eq. 3). Thus, the magnitude of
the initial increase of isoprene emission at lower temperatures
and the onset of the emission decrease at higher temperatures
were essentially independent.
Sources of variation in isoprene emission rate due
to the isoprene synthase rate constant and DMADP
poolsize
The isoprene emission rate through the temperature range
30–50°C increased both with the predicted isoprene synthase
rate constant (k) and with the DMADP pool size (CDMADP,
Fig.4). According to separate slope ANCOVA analyses, the
slopes of I versus k (P>0.5) and I versus CDMADP were not
signicantly different among elevated- and ambient-[CO2]-
grown plants. However, elevated-[CO2]-grown plants had a
lower isoprene emission rate at a given k and higher isoprene
emission rate at a given DMADP pool size (P<0.001 for com-
mon-slope ANCOVA analyses).
The light sensitivity of isoprene emission was positively
correlated with the DMADP response coefcient across all
the data, while the correlation was negative for the response
coefcient for k (r2=0.42, P<0.001 for both).
Fig.2. Relative increase of isoprene emission rate by increasing light level (Eq. 1)at different temperatures in hybrid aspen leaves grown under ambient
[CO2] of 380μmol mol–1 and elevated [CO2] of 780μmol mol–1 and measured at two different CO2 concentrations (treatments as in Fig.1). The increase
of isoprene emission rate by increasing light was calculated as (I2000 – I500)/I500 where I2000 is the isoprene emission rate at the light intensity of 2000μmol
m–2 s–1 and I500 is that at the light intensity of 500μmol m–2 s–1. Data are averages (+SE) of 8–10 replicate leaves. Different letters indicate significant
differences among growth and measurement [CO2] combinations (P<0.05).
Fig.3. Relationships of the optimum temperature for isoprene emission
(Eq. 4)with the relative increase of isoprene emission rate (I) with
increasing temperature from 30 to 50°C (Eq. 2)in hybrid aspen leaves
grown under two different CO2 concentrations (ambient vs elevated) and
measured at different ambient CO2 concentrations of 380 and 780μmol
mol–1, and at different light intensities of 500 and 2000μmol m–2 s–1
(symbols for different light intensities not shown separately). Separate
regression lines were fitted to the data from different growth CO2
treatments (P<0.05 for the growth CO2 effect according to a common-
slope ANCOVA model). Table1 shows a comparison of average Topt
values at different growth and measurement [CO2] conditions.
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

846 | Niinemets and Sun
Discussion
Interactive light and temperature dependencies of
isoprene emission
Our study highlights a complex interplay between different
environmental drivers and growth [CO2] treatments on leaf
isoprene emission, identifying three novel features of how
isoprene emissions respond to light and temperature in plants
grown at different [CO2]:
1. The optimum temperature and the initial rate of increase
with temperature (Q10) for isoprene emission varied in
dependence on light intensity and growth [CO2] (Fig. 1,
Table1).
2. The light sensitivity of isoprene emission, dened as the
change of isoprene emission rate with increasing light
level, decreased with increasing temperature (Fig.2).
3. The light sensitivity was greater in elevated-[CO2]-grown
plants, especially when assessed at higher [CO2] (Fig.2).
We argue that these interactive effects reect changes in the
share of control of emission rates by the DMADP pool size
and isoprene synthase activity. There is evidence that both
instantaneous light and [CO2] dependencies of isoprene emis-
sion are driven primarily by light- and [CO2]-driven changes
in the DMADP pool size (Rasulov etal., 2009a, b; Li etal.,
2011; Possell and Hewitt, 2011; Li and Sharkey, 2013a), while
the temperature dependence is a mixed response, driven both
by temperature-dependent changes in DMADP pool size
and isoprene synthase activity (Rasulov etal., 2010, 2011; Li
etal., 2011; Li and Sharkey, 2013a). As we demonstrated in
our previous study (Sun etal., 2012b) and conrmed by the
ux control analysis (Fig. 4), elevated-[CO2]-grown plants
had greater isoprene synthase activity but a lower DMADP
pool size (Sun etal., 2012b).
In the following, we address the facets of the isoprene
emission response to these complex multifactorial environ-
mental interactions and acclimation responses based on the
immediate effects of environmental conditions on the rate
of DMADP synthesis as well as growth-[CO2]-dependent
changes in overall DMADP pool size and isoprene synthase
activity. We emphasize that the responses highlighted here
reect changes in the shape of the response curves and the
way the controls operate, interactively versus additively. These
modications are driven primarily by the relative share of the
control by DMADP pool size and isoprene synthase activ-
ity. In addition to these modications, environmental accli-
mation, e.g. such as acclimation to different growth [CO2] or
growth temperatures, also affects the overall emission rate
by altering the absolute values of isoprene synthase activity
and DMADP pool size, for example through leaf structural
modications such as enhanced stacking of mesophyll cells
per unit leaf area as manifested in increased leaf thickness
(Sun etal., 2012b; Rasulov etal., 2014a).
Modification of temperature responses of isoprene
emission by light and [CO2]
Variations in the optimum temperatures of isoprene emission,
Topt, between approximately 40 and 48°C have been observed
in several studies (e.g. Singsaas and Sharkey, 1998; Niinemets
et al., 1999b; Singsaas et al., 1999; Rasulov et al., 2010).
However, these modications have been difcult to explain
and reproduce by models, and a constant optimum tempera-
ture of 41°C has commonly been used in models of isoprene
emission (Guenther etal., 1993; Guenther, 1997; see Niinemets
etal., 2010, for a review). In recent modelling efforts, optimum
temperature has been linked to the past weather conditions
(Guenther etal., 2006; Guenther et al., 2012), assuming that
Topt increases as leaves acclimate to hotter temperatures, but
Fig.4. Correlations of isoprene emission rate measured under different combinations of environmental drivers [growth and measurement [CO2]
(μmol mol–1) and quantum flux density (μmol m–2 s–1)] with predicted isoprene synthase rate constant (k, a) and isoprene substrate DMADP pool size
(CDMADP, b). The isoprene emission rate is given as kCDMADP (Eq. 5), whereas the components of k and CDMADP were resolved by a modelling analysis
as explained in Materials and methods. Experimental treatments were as in Fig.1. Different data points within the given data series correspond to
the average values at each temperature (the same data as in Fig.1). Data were fitted by linear regressions separately for elevated-[CO2]-grown plants
[r2=0.78 for (a) and r2=0.84 for (b)] and ambient-[CO2]-grown plants [r2=0.88 for (a) and r2=0.86 for (b)] (P<0.001 for all regressions).
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

Interactive environmental controls on isoprene emission | 847
empirical and mechanistic support for such a relationship is
scarce. Our study provides important evidence that Topt can
vary in dependence on measurement light intensity and meas-
urement and growth [CO2] (Table 1, Figs 1 and 3). In addi-
tion, although the steady-state Topt for isoprene emission can
be relatively low, the role of isoprene in improving heat tol-
erance has mainly been associated with enhanced resistance
of short-term increases in leaf temperature such as observed
during light ecks (Behnke etal., 2007, 2013; Way etal., 2011;
Monson etal., 2013). We argue that it is the transient Topt as
estimated in our study that characterizes the leaf capacity to
cope with such transient increases in leaf temperature.
What could be the mechanism for light- and [CO2]-dependent
changes in Topt? As discussed above, the temperature optimum
for isoprene synthase is characteristically signicantly higher
than that for the DMADP pool size, suggesting that variation
in Topt with varying measurement and growth [CO2] and light
level should be driven primarily by changes in the DMADP
pool size. This reasoning is supported by the increase in Topt
with the temperature sensitivity of isoprene emission, (I50 –
I30)/I30 (Fig. 3). The temperature sensitivity, (I50 – I30)/I30,
itself depends both on temperature effects on isoprene syn-
thase activity and DMADP pool size, but provided Topt is less
than the optimum for isoprene synthase activity, the way this
characteristic is correlated with Topt depends on the extent to
which isoprene emission is controlled by the DMADP pool
size at higher temperatures. Thus, a greater Topt at a given
value of (I50 – I30)/I30 in ambient-[CO2]-grown plants (Fig.3) is
in agreement with their greater DMADP pool size at the given
isoprene synthase activity (Sun etal., 2012b).
Leaves grown and measured at the higher [CO2] of
780μmol mol–1 had both a greater Topt (Fig.1, Table1) and
(I50 – I30)/I30 (Fig.3). In fact, as much of the carbon released
in heat-stressed leaves is derived from ‘old’ carbon sources,
in particular from starch hydrolysis (Sharkey and Yeh,
2001; Fortunati etal., 2008), this strong enhancement might
reect more readily available alternative carbon sources for
DMADP formation in elevated-[CO2]-grown plants consist-
ent with their greater starch and soluble sugar content (Sun
et al., 2012b, 2013). However, we cannot currently rule out
improved heat resistance of isoprene synthase in elevated-
[CO2]-grown plants. Although isoprene synthase is opera-
tionally a soluble enzyme, it is strongly pH dependent (for
reviews, see Rajabi Memari et al., 2013; Rosenkranz and
Schnitzler, 2013). Increased chloroplast membrane leakiness
at high temperatures (Schrader etal., 2004; Wise etal., 2004)
is expected to reduce stromal pH, and thus isoprene synthase
might increasingly operate outside its optimum pH range.
As growth under elevated [CO2] results in more heat-stable
membranes in hybrid aspen (Sun et al., 2013), the onset of
the reduction in isoprene synthase activity due to chloroplast
membrane leakiness might have shifted to higher temperatures
in elevated-[CO2]-grown plants. In fact, the response coef-
cient analysis based on constant isoprene synthase character-
istics (Appendix 1)suggested that isoprene synthase limited
the ux at higher temperatures less in elevated-[CO2]-grown
plants than in ambient-[CO2]-grown plants (data not shown).
We argue that additional studies are needed that explicitly
characterize the isoprene synthase temperature dependencies
in plants grown under different [CO2] conditions.
The explanation based on DMADP control of Topt also
does not explain why Topt was greater at a lower light intensity
across the treatments and at a given (I50 – I30)/I30 (Table1).
Stronger activation of alternative sinks for DMADP under
high light and temperature such as for the synthesis of photo-
protective carotenoids, in particular, xanthophyll cycle carot-
enoids (Havaux and Tardy, 1996; Havaux and Niyogi, 1999),
could provide a possible explanation. Xanthophylls (oxygen-
ated carotenoids) and non-oxygenated carotenoids and toco-
pherols (vitamin E) play an important role in maintaining
the integrity of the photosynthetic membranes under oxi-
dative stress that typically occurs both under heat and high
light (Singsaas etal., 1997; Vickers etal., 2009a; Loreto and
Schnitzler, 2010; Velikova et al., 2011). Recent data demon-
strate that chloroplastic synthesis of higher-molecular-mass
isoprenoids can operate at rates high enough to compete for
DMADP at the level of geranyl diphosphate (GDP) synthe-
sis (Ghirardo etal., 2014; Rasulov etal., 2014b). In fact, due
to a lower Km for DMADP of GDP synthases than that for
isoprene synthase (reviewed by Rajabi Memari etal., 2013),
activation of higher isoprenoid synthases and a concomitant
reduction in the DMADP pool can have signicant effects on
isoprene synthesis, while larger isoprenoid synthesis still pro-
ceeds with a maximum rate. Of course, none of these expla-
nations rules out the effect of heat stress per se, in particular
under high light, on the observed patterns.
Differences in average Q10 values among the measurement
light intensities for ambient-[CO2]-grown plants measured at
780μmol mol–1 and for elevated-[CO2]-grown plants measured
at 380μmol mol–1 (Table1) further highlight the fact that light
and temperature controls can interact at moderately high tem-
peratures as well. In the case of ambient-[CO2]-grown plants,
enhanced Q10 at higher measurement light (Table1) is indicative
of enhancement of the DMADP pool size by increased light
level, reducing the imbalance between isoprene synthase activ-
ity and DMADP pool size (see also the discussion below for
light sensitivity). In contrast, lower Q10 in elevated-[CO2]-grown
plants at higher light similarly to lower Topt (Table1) suggests
that the activation of alternative DMADP sinks at higher light
can already occur at moderately high temperatures. Clearly,
these data suggest that the interactive effects of [CO2] and light
on the temperature response of isoprene emission vary for high
(characterized by Topt) and moderate (characterized by average
Q10 value for the temperature range 25–40°C) leaf temperatures.
Altered light sensitivity of isoprene emission under
different temperatures
The enhanced light sensitivity of isoprene emission in ele-
vated-[CO2]-grown plants is in agreement with experimental
observations on their lower DMADP pool size and greater
isoprene synthase activity. Given the smaller DMADP pool
size, which strongly curbs isoprene emission, any increase in
DMADP pool size at higher light readily results in a higher
isoprene synthesis rate (Fig. 2). This response was particu-
larly strong at a higher measurement [CO2] (Fig.2), possibly
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

848 | Niinemets and Sun
indicating a lower initial DMADP pool size and stronger
control of the emission ux by DMADP under such con-
ditions, as discussed above. Although the Km value of iso-
prene synthase for DMADP is large (Rasulov etal., 2009a,
2011, 2014b), a larger pool of DMADP relative to isoprene
synthase activity can result in an increasingly non-linear
Michaelis–Menten-type hyperbolic response (Rasulov etal.,
2009a, 2014b), reducing the increase of isoprene emission for
a given increase of DMADP poolsize.
Although the light enhancement of isoprene emission
became weaker with increasing temperature, the stronger
light enhancement in elevated-[CO2]-grown plants under high
measurement [CO2] was maintained over the entire tempera-
ture range. We suggest that these patterns result from multiple
mechanisms operating at different parts of the temperature
response of light sensitivity. First, the increase in tempera-
ture is initially associated with enhanced DMADP synthe-
sis rate (Rasulov et al., 2010; Li et al., 2011). This reduces
the DMADP limitation of isoprene emission at lower light
at higher temperature. Secondly, increases in temperature
enhance isoprene synthase activity, making isoprene synthase
less sensitive to the DMADP pool size (Rasulov etal., 2010).
Given these modications, it is still puzzling why the light sen-
sitivity of isoprene emission remained greater in elevated-[CO2]-
grown leaves under high measurement [CO2] and high temperature
(Fig.2). This response might initially seem counterintuitive as it
suggests a more enhanced DMADP pool size in elevated-[CO2]-
grown leaves under high measurement [CO2]. However, heat-
depressed quantum yield of photosynthesis and photosynthetic
electron transport as observed by Sun et al. (2012b), especially
under high light, can be responsible for curtailed enhancement
of DMADP for isoprene synthesis in the case of ambient-[CO2]-
grown plants. This, combined with the lower contribution of alter-
ative carbon sources as (see Sun et al., 2013 for a discussion) can
be responsible for enhanced light sensitivity of isoprene emission,
similarly to enhanced temperature stability (Table1).
Conclusions
Our study highlights a number of important differences
among temperature responses under different growth [CO2]
treatments and under different measurement [CO2] and light
intensities that collectively suggest that the effects of environ-
mental drivers interactively affect isoprene emission at the
level of the DMADP pool size. Thus, future models should
focus on predicting integrated environmental controls on
DMADP pool size rather than considering each environmen-
tal driver independently of others. Several semi-mechanistic
models have recently been put forward that link isoprene
emissions to photosynthetic electron ow and isoprene syn-
thase activity (Niinemets et al., 1999b; Arneth et al., 2007;
Grote et al., 2014; Morfopoulos etal., 2014). These models
do not yet have the capacity to predict changes in DMADP
pool size, and thus application of these models depends criti-
cally on our ability to predict the environmental controls on
photosynthetic electron transport and partitioning of the
electron ow between different electron-consuming sinks.
Nevertheless, recent semi-mechanistic models do a good job
in phenomenologically capturing several of the interactive
environmental responses (Grote et al., 2014; Morfopoulos
etal., 2014).
The study further highlights the important interactive
effects of acclimation to growth [CO2] on isoprene light
and temperature responses. Consideration of such effects
in models again requires understanding of growth [CO2]
effects on isoprene synthase activity, changes in DMADP
partitioning between isoprene synthesis and larger molecu-
lar mass isoprenoids, and possible modications in isoprene
synthase temperature responses. Process-based simulation
of the competition for DMADP by isoprene synthase and
geranyl diphosphate synthase might be particularly difcult,
although linking GDP synthesis to carotenoid turnover rate
as driven by photo-inhibition and heat stresses (Ramel etal.,
2012; Havaux, 2013) can be a promising way to link iso-
prene emissions to stress and long-term environmental con-
ditions. Nevertheless, there appears to be a large variation
among species in their acclimation capacity to growth [CO2]
(Wilkinson et al., 2009; Sun etal., 2012b). We suggest that
more experimental work with different model species grown
under different [CO2] regimes is needed to gain insight into
the factors controlling the partitioning of DMADP among
isoprene and other competing pathways. Such an under-
standing is crucial for realistic parameterization of interac-
tive environmental control of isoprene emission under global
change.
Acknowledgements
Financial support for this study was provided by the Estonian Ministry of
Science and Education (institutional grant IUT-8-3), the Estonian Science
Foundation (9253), the European Commission through the European
Regional Fund (the Center of Excellence in Environmental Adaptation),
the European Social Fund (Doctoral Studies and Internationalization
Programme DoRa), and the European Research Council (advanced grant
322603, SIP-VOL+).
Appendix 1.Response coefficients for modelling
temperature responses
Assuming that the temperature response of the isoprene
synthase rate constant (k, s−1) is the inherent property of
isoprene synthase, we used the shape of the temperature
relationship of k analogous to Eq. 3 previously estimated for
hybrid aspen isoprene synthase (Rasulov et al., 2010) and
scaled it to the measurements of k observed at different com-
binations of growth and measurement [CO2] at 30°C (data
of Sun etal., 2012b). After scaling, k was predicted for each
individual leaf through the entire temperature response, and
the modelled DMADP pool size (CDMADP, nmol m−2) was
calculated as I/k.
The sources of variation in isoprene emission rate from the
rate I1 to the rate I2 in response to environmental variation
can be partitioned among k and CDMADP using the response
coefcient analysis (Poorter and Nagel, 2000). The relative
change of isoprene emission I1/I2 is given as:
I
I
kC
kC
1
2
11
22
=
DMADP,
DMADP,
.
(6)
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

Interactive environmental controls on isoprene emission | 849
Natural logarithmic transformation of both sides of the
equation gives:
ln ln ln ln
ln ln
,
II kk
CC
12 12
12
−
()
=−
()
+−
()
DMADP, DMADP,
(7)
and dividing by (lnI1 – lnI2) yields:
112
12
12
12
=−
()
−
()
+−
()
−
(
ln ln
ln ln
ln ln
ln ln
kk
II
CC
II
DMADP, DMADP,
))
,
(8)
where the rst part of the equation provides the fraction of
variance in I that is due to the variation in isoprene synthase
rate constant and the second part provides the fraction of
variance that is due to the variation in DMADP pool size.
Thus, the response coefcients for k and DMADP pool size
characterize the sensitivity of isoprene emission to variations
in these drivers at the given isoprene synthesis rate (mathe-
matically, the response coefcient for k can also be dened
as
dk
k
dI
I
). The concept of response coefcient is analogous
to ux control coefcients in metabolic ux control analy-
sis (Woodrow and Mott, 1993; Stitt and Schulze, 1994). The
response coefcients were calculated through the tempera-
ture dependence of isoprene emission relative to the values at
30°C (k2, CDMADP,2, and I2).
References
Affek HP, Yakir D. 2002. Protection by isoprene against singlet oxygen in
leaves. Plant Physiology 129, 269–277.
Arneth A, Niinemets Ü, Pressley S, etal. 2007. Process-based
estimates of terrestrial ecosystem isoprene emissions: incorporating the
effects of a direct CO2-isoprene interaction. Atmospheric Chemistry and
Physics 7, 31–53.
Behnke K, Ehlting B, Teuber M, Bauerfeind M, Louis S, Hänsch R,
Polle A, Bohlmann J, Schnitzler J-P. 2007. Transgenic, non-isoprene
emitting poplars don’t like it hot. The Plant Journal 51, 485–499.
Behnke K, Ghirardo A, Janz D, etal. 2013. Isoprene function in
two contrasting poplars under salt and sunflecks. Tree Physiology 33,
562–578.
Calfapietra C, Pallozzi E, Lusini I, Velikova V. 2013. Modification
of BVOC emissions by changes in atmospheric [CO2] and air pollution.
In: Niinemets Ü, Monson RK, eds. Biology, controls and models of tree
volatile organic compound emissions. Tree Physiology, 5.Berlin: Springer,
253–284.
Calfapietra C, Scarascia Mugnozza G, Karnosky DF, Loreto F,
Sharkey TD. 2008. Isoprene emission rates under elevated CO2 and O3
in two field-grown aspen clones differing in their sensitivity to O3. New
Phytologist 179, 55–61.
Calfapietra C, Wiberley AE, Falbel TG, Linskey AR, Scarascia
Mugnozza G, Karnosky DF, Loreto F, Sharkey TD. 2007. Isoprene
synthase expression and protein levels are reduced under elevated O3 but
not under elevated CO2 (FACE) in field-grown aspen trees. Plant, Cell &
Environment 30, 654–661.
Claeys M, Graham B, Vas G, etal. 2004. Formation of secondary
organic aerosols through photooxidation of isoprene. Science 303,
1173–1176.
Copolovici LO, Filella I, Llusià J, Niinemets Ü, Peñuelas J. 2005. The
capacity for thermal protection of photosynthetic electron transport varies
for different monoterpenes in Quercus ilex. Plant Physiology 139, 485–496.
Fineschi S, Loreto F, Staudt M, Peñuelas J. 2013. Diversification
of volatile isoprenoid emissions from trees: evolutionary and ecological
perspectives In: Niinemets Ü, Monson RK, eds. Biology, controls and
models of tree volatile organic compound emissions. Tree Physiology,
5.Berlin: Springer, 1–20.
Fortunati А, Barta C, Brilli F, Centritto M, Zimmer I, Schnitzler J-P,
Loreto F. 2008. Isoprene emission is not temperature-dependent during
and after severe drought-stress: a physiological and biochemical analysis.
The Plant Journal 55, 687–697.
Fowler D, Pilegaard K, Sutton MA, etal. 2009. Atmospheric
composition change: ecosystems–atmosphere interactions. Atmospheric
Environment 43, 5193–5267.
Ghirardo A, Wright LP, Bi Z, Rosenkranz M, Pulido P, Rodríguez-
Concepción M, Niinemets Ü, Brüggemann N, Gershenzon J,
Schnitzler J-P. 2014. Metabolic flux analysis of plastidic isoprenoid
biosynthesis in poplar leaves emitting and nonemitting isoprene. Plant
Physiology 165, 37–51.
Grote R, Monson RK, Niinemets Ü. 2013. Leaf-level models of
constitutive and stress-driven volatile organic compound emissions. In:
Niinemets Ü, Monson RK, eds. Biology, controls and models of tree volatile
organic compound emissions. Tree Physiology, 5.Berlin: Springer, 315–355.
Grote R, Morfopoulos C, Niinemets Ü, Sun Z, Keenan TF, Pacifico
F, Butler T. 2014. A fully integrated isoprenoid emission model coupling
emissions to photosynthetic characteristics. Plant, Cell & Environment 37,
1965–1980.
Guenther A. 1997. Seasonal and spatial variations in natural volatile
organic compound emissions. Ecological Applications 7, 34–45.
Guenther A, Karl T, Harley P, Wiedinmyer C, Palmer PI, Geron C.
2006. Estimates of global terrestrial isoprene emissions using MEGAN
(Model of Emissions of Gases and Aerosols from Nature). Atmospheric
Chemistry and Physics 6, 3181–3210.
Guenther AB, Jiang X, Heald CL, Sakulyanontvittaya T, Duhl
T, Emmons LK, Wang X. 2012. The Model of Emissions of Gases
and Aerosols from Nature version 2.1 (MEGAN2.1): an extended and
updated framework for modeling biogenic emissions. Geoscientific Model
Development 5, 1471–1492.
Guenther AB, Zimmerman PR, Harley PC, Monson RK, Fall R. 1993.
Isoprene and monoterpene emission rate variability: model evaluations and
sensitivity analyses. Journal of Geophysical Research 98, 12609–12617.
Harley PC, Tenhunen JD. 1991. Modeling the photosynthetic response
of C3 leaves to environmental factors. In: Boote KJ, ed. Modeling crop
photosynthesis—from biochemistry to canopy. CSSA Special Publication,
No. 19. Madison: Agronomy and Crop Science Society of America, 17–39.
Havaux M. 2013. Carotenoid oxidation products as stress signals in
plants. The Plant Journal 79, 597–606.
Havaux M, Niyogi KK. 1999. The violaxanthin cycle protects plants from
photooxidative damage by more than one mechanism. Proceedings of the
National Academy of Sciences, U S A 96, 8762–8767.
Havaux M, Tardy F. 1996. Temperature-dependent adjustment of
the thermal stability of photosystem II in vivo: possible involvement of
xanthophyll-cycle pigments. Planta 198, 324–333.
Heald CL, Wilkinson MJ, Monson RK, Alo CA, Wang G, Guenther
A. 2009. Response of isoprene emission to ambient CO2 changes and
implications for global budgets. Global Change Biology 15, 1127–1140.
Hüve K, Bichele I, Rasulov B, Niinemets Ü. 2011. When it is too hot
for photosynthesis: heat-induced instability of photosynthesis in relation to
respiratory burst, cell permeability changes and H2O2 formation. Plant, Cell
& Environment 34, 113–126.
Hüve K, Bichele I, Tobias M, Niinemets Ü. 2006. Heat sensitivity of
photosynthetic electron transport varies during the day due to changes in
sugars and osmotic potential. Plant, Cell & Environment 29, 212–228.
Lehning A, Zimmer I, Steinbrecher R, Brüggemann N, Schnitzler JP.
1999. Isoprene synthase activity and its relation to isoprene emission in
Quercus robur L.leaves. Plant, Cell & Environment 22, 495–504.
Li D, Chen Y, Shi Y, He X, Chen X. 2009. Impact of elevated CO2 and
O3 concentrations on biogenic volatile organic compounds emissions from
Ginkgo biloba. Bulletin of Environmental Contamination and Toxicology 82,
473–477.
Li Z, Ratliff EA, Sharkey TD. 2011. Effect of temperature on postillumination
isoprene emission in oak and poplar. Plant Physiology 155, 1037–1046.
Li Z, Sharkey TD. 2013a. Metabolic profiling of the methylerythritol
phosphate pathway reveals the source of post-illumination isoprene burst
from leaves. Plant, Cell & Environment 36, 429–437.
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

850 | Niinemets and Sun
Li Z, Sharkey TD. 2013b. Molecular and pathway controls on biogenic
volatile organic compound emissions. In: Niinemets Ü, Monson RK, eds.
Biology, controls and models of tree volatile organic compound emissions.
Tree Physiology, 5.Berlin: Springer, 119–151.
Loreto F, Mannozzi M, Maris C, Nascetti P, Ferranti F, Pasqualini S.
2001. Ozone quenching properties of isoprene and its antioxidant role in
leaves. Plant Physiology 126, 993–1000.
Loreto F, Schnitzler J-P. 2010. Abiotic stresses and induced BVOCs.
Trends in Plant Science 15, 154–166.
Monson RK. 2013. Metabolic and gene expression controls on the
production of biogenic volatile organic compounds. In: Niinemets Ü,
Monson RK, eds. Biology, controls and models of tree volatile organic
compound emissions. Tree Physiology, 5.Berlin: Springer, 153–179.
Monson RK, Grote R, Niinemets Ü, Schnitzler J-P. 2012. Tansley
review. Modeling the isoprene emission rate from leaves. New Phytologist
195, 541–559.
Monson RK, Jaeger CH, Adams WW, III, Driggers EM, Silver
GM, Fall R. 1992. Relationships among isoprene emission rate,
photosynthesis, and isoprene synthase activity as influenced by
temperature. Plant Physiology 98, 1175–1180.
Monson RK, Jones RT, Rosenstiel TN, Schnitzler J-P. 2013. Why only
some plants emit isoprene. Plant, Cell & Environment 36, 503–516.
Morfopoulos C, Sperlich D, Peñuelas J, Filella I, Llusià J, Medlyn
BE, Niinemets Ü, Possell M, Sun Z, Prentice IC. 2014. A model of
plant isoprene emission based on available reducing power captures
responses to atmospheric CO2. New Phytologist 203, 125–139.
Niinemets Ü, Monson RK, Arneth A, Ciccioli P, Kesselmeier J, Kuhn
U, Noe SM, Peñuelas J, Staudt M. 2010. The leaf-level emission factor
of volatile isoprenoids: caveats, model algorithms, response shapes and
scaling. Biogeosciences 7, 1809–1832.
Niinemets Ü, Oja V, Kull O. 1999a. Shape of leaf photosynthetic electron
transport versus temperature response curve is not constant along canopy
light gradients in temperate deciduous trees. Plant, Cell & Environment 22,
1497–1514.
Niinemets Ü, Tenhunen JD, Harley PC, Steinbrecher R. 1999b. A
model of isoprene emission based on energetic requirements for isoprene
synthesis and leaf photosynthetic properties for Liquidambar and Quercus.
Plant, Cell & Environment 22, 1319–1336.
Pearcy RW. 1990. Sunflecks and photosynthesis in plant canopies.
Annual Review of Plant Physiology and Plant Molecular Biology 41,
421–453.
Poorter H, Nagel O. 2000. The role of biomass allocation in the
growth response of plants to different levels of light, CO2, nutrients and
water: a quantitative review. Australian Journal of Plant Physiology 27,
595–607.
Possell M, Hewitt CN. 2011. Isoprene emissions from plants are
mediated by atmospheric CO2 concentrations. Global Change Biology 17,
1595–1610.
Possell M, Loreto F. 2013. The role of volatile organic compounds in
plant resistance to abiotic stresses: responses and mechanisms. In:
Niinemets Ü, Monson RK, eds. Biology, controls and models of tree
volatile organic compound emissions. Tree Physiology, 5.Berlin: Springer,
209–235.
Rajabi Memari H, Pazouki L, Niinemets Ü. 2013. The
biochemistry and molecular biology of volatile messengers in trees.
In: Niinemets Ü, Monson RK, eds. Biology, controls and models of
tree volatile organic compound emissions. Tree Physiology, 5.Berlin:
Springer, 47–93.
Ramel F, Birtic S, Ginies C, Soubigou-Taconnat L, Triantaphylides
C, Havaux M. 2012. Carotenoid oxidation products are stress signals that
mediate gene responses to singlet oxygen in plants. Proceedings of the
National Academy of Sciences, U S A 109, 5535–5540.
Rasulov B, Bichele I, Hüve K, Vislap V, Niinemets Ü. 2014a.
Acclimation of isoprene emission and photosynthesis to growth
temperature in hybrid aspen: resolving structural and physiological
controls. Plant, Cell & Environment doi: 10.1111/pce.12435 [Epub ahead
of print].
Rasulov B, Bichele I, Laisk A, Niinemets Ü. 2014b. Competition
between isoprene emission and pigment synthesis during leaf
development in aspen. Plant, Cell & Environment 37, 724–741.
Rasulov B, Copolovici L, Laisk A, Niinemets Ü. 2009a.
Postillumination isoprene emission: in vivo measurements of
dimethylallyldiphosphate pool size and isoprene synthase kinetics in aspen
leaves. Plant Physiology 149, 1609–1618.
Rasulov B, Hüve K, Bichele I, Laisk A, Niinemets Ü. 2010.
Temperature response of isoprene emission in vivo reflects a combined
effect of substrate limitations and isoprene synthase activity: a kinetic
analysis. Plant Physiology 154, 1558–1570.
Rasulov B, Hüve K, Laisk A, Niinemets Ü. 2011. Induction of a longer-
term component of isoprene release in darkened aspen leaves: origin and
regulation under different environmental conditions. Plant Physiology 156,
816–831.
Rasulov B, Hüve K, Välbe M, Laisk A, Niinemets Ü. 2009b. Evidence
that light, carbon dioxide and oxygen dependencies of leaf isoprene
emission are driven by energy status in hybrid aspen. Plant Physiology
151, 448–460.
Rosenkranz M, Schnitzler J-P. 2013. Genetic engineering of BVOC
emissions from trees. In: Niinemets Ü, Monson RK, eds. Biology, controls
and models of tree volatile organic compound emissions. Tree Physiology,
5.Berlin: Springer, 95–118.
Sage RF, Sharkey TD. 1987. The effect of temperature on the occurence
of O2 and CO2 insensitive photosynthesis in field grown plants. Plant
Physiology 84, 658–664.
Schrader SM, Wise RR, Wacholtz WF, Ort DR, Sharkey TD. 2004.
Thylakoid membrane responses to moderately high leaf temperature in
Pima cotton. Plant, Cell & Environment 27, 725–735.
Sharkey TD, Chen XY, Yeh S. 2001. Isoprene increases thermotolerance
of fosmidomycin-fed leaves. Plant Physiology 125, 2001–2006.
Sharkey TD, Gray DW, Pell HK, Breneman SR, Topper L. 2013.
Isoprene synthase genes form a monophyletic clade of acyclic terpene
synthases in the Tps-b terpene synthase family. Evolution 67, 1026–1040.
Sharkey TD, Loreto F, Delwiche CF. 1991. High carbon dioxide and
sun/shade effects on isoprene emission from oak and aspen tree leaves.
Plant, Cell & Environment 14, 333–338.
Sharkey TD, Wiberley AE, Donohue AR. 2008. Isoprene emission from
plants: why and how. Annals of Botany 101, 5–18.
Sharkey TD, Yeh SS. 2001. Isoprene emission from plants. Annual
Review of Plant Physiology and Plant Molecular Biology 52, 407–436.
Sharkey TD. 1985. Photosynthesis in intact leaves of C3 plants: physics,
physiology and rate limitations. Botanical Review 51, 53–105.
Singsaas EL, Laporte MM, Shi J-Z, Monson RK, Bowling DR,
Johnson K, Lerdau M, Jasentuliytana A, Sharkey TD. 1999. Kinetics
of leaf temperature fluctuation affect isoprene emission from red oak
(Quercus rubra) leaves. Tree Physiology 19, 917–924.
Singsaas EL, Lerdau M, Winter K, Sharkey TD. 1997. Isoprene
increases thermotolerance of isoprene-emitting species. Plant Physiology
115, 1413–1420.
Singsaas EL, Sharkey TD. 1998. The regulation of isoprene emission
responses to rapid leaf temperature fluctuations. Plant, Cell & Environment
21, 1181–1188.
Siwko ME, Marrink SJ, de Vries AH, Kozubek A, Uiterkamp AJMS,
Mark AE. 2007. Does isoprene protect plant membranes from thermal
shock? Amolecular dynamics study. Biochimica et Biophysica Acta—
Biomembranes 1768, 198–206.
Socias FX, Medrano H, Sharkey TD. 1993. Feedback limitation of
photosynthesis of Phaseolus vulgaris L.grown in elevated CO2. Plant, Cell
& Environment 16, 81–86.
Stitt M, Schulze D. 1994. Does Rubisco control the rate of
photosynthesis and plant growth? An exercise in molecular ecophysiology.
Plant, Cell & Environment 17, 465–487.
Sun Z, Copolovici L, Niinemets Ü. 2012a. Can the capacity for isoprene
emissions acclimate to environmental modifications during autumn
senescence in temperate deciduous tree species Populus tremula?
Journal of Plant Research 125, 263–274.
Sun Z, Hüve K, Vislap V, Niinemets Ü. 2013. Elevated [CO2] magnifies
isoprene emissions under heat and improves thermal resistance in hybrid
aspen. Journal of Experimental Botany 64, 5509–5523.
Sun Z, Niinemets Ü, Hüve K, Noe SM, Rasulov B, Copolovici L,
Vislap V. 2012b. Enhanced isoprene emission capacity and altered
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023

Interactive environmental controls on isoprene emission | 851
light responsiveness in aspen grown under elevated atmospheric CO2
concentration. Global Change Biology 18, 3423–3440.
Vahala J, Keinänen M, Schützendübel A, Polle A, Kangasjärvi J.
2003. Differential effects of elevated ozone on two hybrid aspen genotypes
predisposed to chronic ozone fumigation. Role of ethylene and salicylic
acid. Plant Physiology 132, 196–205.
Velikova V, Várkonyi Z, Szabó M, etal. 2011. Increased thermostability
of thylakoid membranes in isoprene-emitting leaves probed with three
biophysical techniques. Plant Physiology 157, 905–916.
Vickers CE, Gershenzon J, Lerdau MT, Loreto F. 2009a. A unified
mechanism of action for volatile isoprenoids in plant abiotic stress. Nature
Chemical Biology 5, 283–291.
Vickers CE, Possell M, Cojocariu CI, Velikova VB, Laothawornkitkul
J, Ryan A, Mullineaux PM, Hewitt CN. 2009b. Isoprene synthesis
protects transgenic tobacco plants from oxidative stress. Plant, Cell &
Environment 32, 520–531.
Way DA, Schnitzler J-P, Monson RK, Jackson RB. 2011. Enhanced
isoprene-related tolerance of heat- and light-stressed photosynthesis at
low, but not high, CO2 concentrations. Oecologia 166, 273–282.
Wilkinson MJ, Monson RK, Trahan N, Lee S, Brown E, Jackson
RB, Polley HW, Fay PA, Fall R. 2009. Leaf isoprene emission rate as a
function of atmospheric CO2 concentration. Global Change Biology 15,
1189–1200.
Wise RR, Olson AJ, Schrader SM, Sharkey TD. 2004. Electron
transport is the functional limitation of photosynthesis in field-grown Pima
cotton plants at high temperature. Plant, Cell & Environment 27, 717–724.
Woodrow IE, Mott KA. 1993. Modelling C3 photosynthesis: a sensitivity
analysis of the photosynthetic carbon-reduction cycle. Planta 191, 421–432.
Downloaded from https://academic.oup.com/jxb/article/66/3/841/479621 by guest on 21 March 2023