Plant Science 226 (2014) 41–48
Contents lists available at ScienceDirect
Plant Science
journal homepage: www.elsevier.com/locate/plantsci
Stomatal and mesophyll conductances to CO2 in different plant
groups: Underrated factors for predicting leaf photosynthesis
responses to climate change?
Jaume Flexas a,∗,1 , Marc Carriquí a , Rafael E. Coopman b , Jorge Gago c , Jeroni Galmés a ,
Sebastià Martorell a , Fermín Morales d , Antonio Diaz-Espejo e,1
a
Research Group on Plant Biology under Mediterranean Conditions, Departament de Biologia, Universitat de les Illes Balears, Carretera de Valldemossa Km
7.5, 07122 Palma de Mallorca, Illes Balears, Spain
b
Laboratorio de Ecofisiología para la Conservación de Bosques, Instituto de Conservación, Biodiversidad y Territorio, Facultad de Ciencias Forestales y
Recursos Naturales, Universidad Austral de Chile, Casilla 567, Valdivia, Chile
c
Applied Plant and Soil Biology, Faculty of Biology, University of Vigo, 36310 Vigo, Spain
d
Estación Experimental de Aula Dei (EEAD), CSIC, Dpto. Nutrición Vegetal, Apdo. 13034, E-50080 Zaragoza, Spain
e
Irrigation and Crop Ecophysiology Group, Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS, CSIC), Avenida Reina Mercedes 10, 41012
Sevilla, Spain
a r t i c l e
i n f o
Article history:
Available online 20 June 2014
Keywords:
Climatic change
Angiosperms
Photosynthesis
Mesophyll conductance
Ferns
Diffusive limitations
a b s t r a c t
The climate change conditions predicted for the end of the current century are expected to have an impact
on the performance of plants under natural conditions. The variables which are foreseen to have a larger
effect are increased CO2 concentration and temperature. Although it is generally considered CO2 assimilation rate could be increased by the increasing levels of CO2 , it has been reported in previous studies that
acclimation to high CO2 results in reductions of physiological parameters involved in photosynthesis, like
the maximum carboxylation rate (Vc,max ), stomatal conductance (gs ) and mesophyll conductance to CO2
(gm ). On the one hand, most of the previous modeling efforts have neglected the potential role played
by the acclimation of gm to high CO2 and temperature. On the other hand, the effect of climate change
on plant clades other than angiosperms, like ferns, has received little attention, and there are no studies
evaluating the potential impact of increasing CO2 and temperature on these species.
In this study we predicted responses of several representative species among angiosperms, gymnosperms and ferns to increasing CO2 and temperature. Our results show that species with lower
photosynthetic capacity – such as some ferns and gymnosperms – would be proportionally more favored
under these foreseen environmental conditions. The main reason for this difference is the lower diffusion limitation imposed by gs and gm in plants having high capacity for photosynthesis among the
angiosperms, which reduces the positive effect of increasing CO2 . However, this apparent advantage of
low-diffusion species would be canceled if the two conductances – gs and gm – acclimate and are down
regulated to high CO2 , which is basically unknown, especially for gymnosperms and ferns. Hence, for a
better understanding of different plant responses to future climate, studies are urged in which the actual
photosynthetic response/acclimation to increased CO2 and temperature of ferns, gymnosperms and other
under-evaluated plant groups is assessed.
© 2014 Elsevier Ireland Ltd. All rights reserved.
1. Introduction
Climate change includes environmental factors that are crucial for plant photosynthesis: increased air temperature and CO2
∗ Corresponding author. Tel.: +34 971 172365; fax: +34 971 173184.
E-mail address: jaume.fl[email protected] (J. Flexas).
1
These two authors contributed equally to the present work.
http://dx.doi.org/10.1016/j.plantsci.2014.06.011
0168-9452/© 2014 Elsevier Ireland Ltd. All rights reserved.
concentration. Global air temperature has, on average, increased
by 1–2 ◦ C over the past decades and it is expected to increase
not more than another 2–3 ◦ C by 2100 [1,2], CO2 concentration is
exponentially increasing, having changed from preindustrial levels of 270 ␮mol mol−1 to about 400 ␮mol mol−1 nowadays, and
is expected to reach 700 ␮mol mol−1 by the end of the century
[3]. Over the last approximately 30 years, evidence has been
obtained on the effects of increased temperature and/or increased
atmospheric CO2 on plant photosynthesis, from experiments in
42
J. Flexas et al. / Plant Science 226 (2014) 41–48
controlled growth chambers, free-air CO2 enrichment (FACE) and
other facilities [3–7]. As a result of these observations it has been
concluded that, in a predicted scenario for the end of this century:
(1) while respiration and photorespiration could increase as a consequence of rising temperature, net CO2 assimilation will largely
increase as a consequence of increased CO2 concentration; and
(2) the expected increase in photosynthesis may be attenuated by
down-regulation of photosynthetic capacity and/or acclimation of
stomatal conductance (gs ). Down-regulation of photosynthesis is
related to reduced nitrogen and Rubisco contents in leaves, and
results on average in a 10–15% reduction of the maximum velocity
of carboxylation (Vc,max ), while the maximum capacity for electron transport (Jmax ) is often much less affected [6]. However, the
extent of such down-regulation is variable, depending on plant
types, on soil nitrogen availability and plant carbon sink capacity, among other factors [6,8]. On the other hand, gs also tends to
acclimate to high CO2 , being reduced on average by 12% [9] to 20%
[6], with no differences between C3 and C4 plants, but depending
on interactions between high CO2 , ozone, nitrogen and drought,
among others [6]. Part of these decreases in gs is due to a ‘physiological’ down-regulation, i.e. the maintenance of more closed
stomata, while part is due to a ‘morphological’ acclimation, i.e. a
reduction of the number of stomata or stomatal density [10,11].
Together, increased photosynthesis and decreased stomatal conductance results in increased instantaneous water use efficiency
under high CO2 [3,4,7,12].
However, the above conclusions come essentially from studies
performed in crop species or forest trees, i.e. they are all based on
few species, mostly within the angiosperms, and a few within gymnosperms. In contrast, species from different phylogenetic clades
could potentially present different responses. For instance, it has
been highlighted recently that, in the short term, ferns do not
show stomatal conductance responses to changes in CO2 , while
conifers may respond to low but not high CO2 [11,13,14]. However,
other authors have shown CO2 -responsiveness in the stomata of
the lycophyte Selaginella uncinata [15] and the moss Physcomitrella
patens [16]. On the other hand, the fern Osmunda regalis, although
not showing a ‘physiological’ response of gs to CO2 , shows a 30%
reduction of gs when growing at 1500 ␮mol CO2 mol−1 air, which
is partly associated with reduced stomatal density [11]. Also we
have recently observed a 23% long-term decrease of gs in another
fern, N. exaltata, growing at 850 ␮mol CO2 mol−1 air (Flexas and
Morales, unpublished results). Similar long-term down-regulation
of gs under high CO2 has been observed in other species not showing
the short-term or ‘physiological’ response, like some conifers and
the primitive Gymnosperm Gingko biloba, but also in Angiosperm
species which also display the short-term response [11]. In contrast, in FACE experiments in soybean little acclimation of gs was
observed [17]. Therefore, it is likely that different species show different degrees of acclimation of gs to high atmospheric CO2 , from no
to large decreases. This would imply a significant variability among
species in the response of photosynthesis and water use efficiency
to climate change, which could result in differences in species fitness. In addition, stomatal closure with rising CO2 may not only
affect photosynthesis by increasing diffusional limitations, but it
also may result in increased leaf temperature via its effects on leaf
energy balance [18].
Besides possible variability in the response of gs to high CO2 ,
the so-called mesophyll conductance to CO2 (gm ) is an equally
important determinant of CO2 diffusion and photosynthesis, whose
potential responses to climate change conditions have been mostly
neglected up to now [19]. Many studies have shown that gm tends to
increase with increasing measuring temperature [20–26], although
acclimation may also occur [22,23,27,28]. As for high CO2 , gm has
been shown to decrease in the short term in many species [29–35],
including ferns [14], although there may be some exception [36].
In contrast, long-term acclimation of gm to growth at high CO2 has
been evaluated only in a few species [17,34,37,38]. These studies
have also revealed large variability among species, from no acclimation in aspen [37] or soybean [17] to more than 50% decreases in
cucumber [37], Platanus orientalis [34] or grapevine [38]. In sweetgum, an increase of gm was reported at high CO2 [37]. Recently
we have observed an 11% reduction in the fern Nephrolepsis exaltata (Flexas and Morales, unpublished results). Therefore, it appears
that not only the response of gs to CO2 may be more variable than
often considered, but also how gm will be affected with growth of
plants at high CO2 is uncertain, and the coordination of the two
conductances may exert a strong constraint on the degree of high
temperature and CO2 -induced increase of photosynthesis. Moreover, it has recently been shown that at least part of the observed
apparent decreases of gm at high CO2 can be a mathematical artifact
[39,40] which, overall, increases the uncertainty as to what will be
the ‘true’ response of gm to climate change.
Therefore, studies are needed with non-model species belonging to different phylogenetic plant groups under climate-simulated
conditions, in order to gain insights on possible differences among
species in response to climate change. Meanwhile, the aims of the
present study are: (1) to analyze how decreases of gs and/or gm in
response to climate change could dampen the potential increases
of photosynthesis under high CO2 ; and (2) to compare the potential response of some previously photosynthetically-characterized
species having a range in CO2 conductance values, i.e. ferns, gymnosperms and angiosperms (including crops and evergreen and
deciduous trees).
2. Materials and methods
2.1. Simulation of photosynthesis under current conditions
According to the biochemical model of leaf photosynthesis by
Farquhar et al. [41], the net assimilation rate (AN ) is determined by
the minimum of the RuBP-saturated (Ac ) and RuBP-limited (Aj ) CO2
assimilation rates:
AN = min (Ac , Aj ),
− ∗)
(1)
Ac =
Vc,max (Cc
− Rd ,
Cc + Kc (1 + Oi /Ko )
(2)
Aj =
(Cc − ∗ ) J/4
− Rd
Cc + 2 ∗
(3)
where Vc,max is the maximal carboxylation rate, Cc is the CO2 concentration at the site of carboxylation in the chloroplast stroma, Kc
is the Michaelis–Menten constant for CO2 and Ko is that for O2 , * is
the CO2 compensation point in the absence of mitochondrial respiration, J is the CO2 -saturated electron transport rate of the thylakoid
reactions which ultimately supply the necessary energy in the form
of ATP and NADPH for the regeneration of RuBP, and Rd is the mitochondrial respiration rate in the light. We use Aj under saturating
light conditions (1600 ␮mol m−2 s−1 ) and therefore the value of
maximum J (Jmax ) was used in all our simulations. Values for * ,
Kc , Ko at 25 ◦ C were obtained from Bernacchi et al. [20]. These values, which depend on the characteristics of Rubisco, were assumed
to be identical among the species simulated in this study according
to our recent results showing that at least the specificity factor is
similar in ferns as in angiosperms [14]. The values for Vc,max and Jmax
at 20–30 ◦ C were obtained directly from published studies and are
summarized in Supplemental Table 1. Since Rd was not reported in
all studies, it was assumed in all cases to be 0.5 ␮mol CO2 m−2 s−1 .
While it could be more correct to retrieve a different Rd for each
species based on, e.g. previously published AN –Rd or Vc,max –Rd relationships, a previous sensitivity analysis of the model revealed that
J. Flexas et al. / Plant Science 226 (2014) 41–48
43
Table 1
Simulations of CO2 assimilation rate (AN ) performed with equation (1) considering the photosynthetic and diffusional parameters reported for each species in Table S1. AN
control simulates AN under current conditions, Scenario 0: only air temperature is increased by 4 ◦ C; Scenario 1: scenario 0 plus an increase of ambient CO2 to 700 ␮mol mol−1 ,
Scenario 2: scenario 1 plus a decrease in gs by 20%; Scenario 3: scenario 1 plus a decrease in gm by 35%; Scenario 4: scenario 1 plus gs and gm acclimation; Scenario 5: scenario
4 plus reduction of Vcmax and Jmax by 15% and 5% respectively. Simulations of scenarios are expressed both as absolute rates and as % of increase of AN with respect to control
rates.
Species
Tleaf
◦
C
AN control
␮mol m−2 s−1
Scenario 0
␮mol m−2 s−1 (%)
Scenario 1
␮mol m−2 s−1 (%)
Scenario 2
␮mol m−2 s−1 (%)
Scenario 3
␮mol m−2 s−1 (%)
Scenario 4
␮mol m−2 s−1 (%)
Scenario 5
␮mol m−2 s−1 (%)
Angiosperms
Arabidopsis thaliana
25
12.6
Carya cathayensis
28
10.6
Cistus albidus
28
21.6
Eucalytus globulus
25
11.3
Fagus sylvatica
25.8
Junglans nigraxregia
25
14.0
Nicotiana tabacum
25
16.2
Olea europaea
25
19.8
Oryza sativa
30
18.0
Quercus canariensis
20
12.4
Quercus ilex
28
10.9
Quercus ilex
25
10.4
Vitis vinifera
25
14.2
12.7
(0.5)
10.0
(-4.7)
22.0
(1.8)
11.5
(2.1)
8.9
(-3.8)
15.0
(7.4)
16.5
(1.8)
20.8
(5.1)
17.0
(-5.3)
12.0
(-2.5)
10.0
(-7.4)
10.0
(-3.5)
14.3
(0.7)
18.4
(46.0)
18.0
(70.4)
31.2
(44.6)
16.3
(44.9)
14.9
(60.5)
20.1
(44.1)
25.5
(57.4)
28.5
(44.0)
27.9
(55.1)
21.1
(70.4)
17.3
(70.4)
17.7
(71.0)
23.4
(64.8)
17.9
(42.0)
16.6
(57.6)
30.2
(40.3)
15.9
(41.5)
14.0
(50.3)
19.7
(40.9)
23.8
(46.9)
27.7
(39.9)
21.1
(17.3)
18.8
(52.3)
15.8
(45.6)
16.1
(55.2)
21.4
(50.7)
17.8
(41.7)
16.4
(55.4)
29.8
(38.0)
15.8
(40.7)
13.8
(49.0)
19.5
(39.5)
24.1
(48.7)
27.7
(39.9)
26.5
(47.4)
19.7
(59.6)
15.1
(39.1)
16.3
(57.1)
21.6
(52.1)
17.3
(37.4)
15.2
(44.4)
28.8
(33.5)
15.4
(37.2)
12.9
(39.3)
19.0
(36.2)
22.5
(38.8)
27
(36.4)
25.6
(42.7)
17.7
(43.3)
13.9
(27.8)
14.7
(41.6)
19.8
(39.4)
16.7
(32.7)
14.1
(34.2)
28.1
(30.3)
14.9
(32.1)
12.47
(34.0)
18.3
(31.5)
22
(35.8)
26
(31.3)
23.9
(32.9)
16.7
(35.3)
13.7
(26.5)
14.0
(35.4)
19.5
(37.3)
Gymnosperms
Abies pinsapo
25
12.2
Pinus densiflora telmateia
25.8
13.8
Pinus radiata exaltata
20
8.2
12.0
(-1.0)
15.0
(8.9)
9.3
(13.6)
20.9
(71.4)
19.2
(39.5)
11.9
(45.4)
18.4
(51.3)
18.9
(37.6)
11.7
(42.6)
19.1
(57.1)
18.8
(36.7)
11.7
(42.6)
17.0
(39.6)
18.5
(34.7)
11.5
(39.7)
16.9
(38.4)
17.8
(29.4)
11.0
(33.9)
Pteridophytes
Adiantum capillus-veneris
25
5.1
Asplenium scolopendrium
25
3.2
Blechnum gibbum
25
4.2
Equisetum telmateia
25
11.4
Osmunda regalis
25
6.8
Pteridium aquilinum
25
8.9
5.1
(0)
3.2
(0)
4.3
(2.3)
12.2
(7.0)
7.1
(4.4)
8.9
(0)
8.9
(74.5)
5.6
(75)
7
(66.6)
16.8
(47.3)
11
(61.7)
15.1
(69.6)
8.2
(60.7)
5
(56.2)
6.4
(52.3)
16.2
(42.1)
10.3
(51.4)
13.8
(55.0)
7.3
(43.1)
5
(56.2)
6.5
(54.7)
16.4
(43.8)
10.2
(50)
13.3
(49.4)
6.8
(33.3)
4.5
(40.6)
5.9
(40.4)
15.8
(38.6)
9.6
(41.1)
12.2
(37.0)
6.7
(31.3)
4.4
(37.5)
5.8
(37.5)
15.3
(31.9)
9.3
(35.2)
12.1
(35.9)
9.3
changing this parameter within a physiological range does not significantly affect the obtained results.
The use of Eqs. (1)–(3) requires the estimation of Cc , which is a
function of ambient CO2 (Ca ), AN , gs and gm . The values of gs and gm
were also directly obtained from the same published data sets as
Vcmax and Jmax (Supplemental Table 1).
Then, we ended up with two unknowns, AN and Cc , and two Eqs.
(2) or (3) for AN and Fick’ law for Cc :
Cc = Ca −
AN
AN
−
gs
gm
(4)
The resultant quadratic equations for Rubisco-limited photosynthesis Eq. (2) and for electron transport limited photosynthesis
Eq. (3) were solved according to von Caemmerer and Evans [42]
and Ethier and Livingston [43].
2.2. Simulation of photosynthesis under future conditions
First, a theoretical simulation of the response of AN to progressively increasing atmospheric CO2 and temperature in two
hypothetical species with high and low conductances (gs and gm )
and with or without acclimation of such conductances was performed (see Section 3, Figs. 1 and 2). To do this we considered
that Vcmax and Jmax were unchanged at different CO2 concentrations, while for the response of gs and gm we extracted published
responses averaged for several species from Franks et al. [7] and
Flexas et al. [23], respectively (Supplemental Fig. 1). The responses
of Vc,max , Jmax and gm to temperature were from Bernacchi et al. [20].
Subsequently, the response of photosynthesis to increasing
ambient CO2 up to 700 ␮mol mol−1 and temperature to +4 ◦ C
above the original measuring temperature for each species was
also calculated for some selected species in which photosynthetic
characteristics were previously determined at ambient CO2 (Supplemental Tables 1 and 2). Basically, we selected all the studies
44
J. Flexas et al. / Plant Science 226 (2014) 41–48
Fig. 1. Relationship between calculated and reported CO2 assimilation rate (AN ) for
all the angiosperm, gymnosperm and fern species of this study. The photosynthetic
and diffusional parameters reported for each species and used in the simulations
are summarized in Table S1.
and species available in the literature for which gs , gm , Vc,max and
Jmax were characterized, with some restrictions: (1) data obtained
using the so-called curve-fitting methods were excluded, as these
methods are somewhat uncertain due to over-parameterization,
for which a similar AN can be obtained by varying gm , Vc,max or
both; (2) only studies with a reliable gm value obtained using either
isotope- or fluorescence-based methods were then chosen; and (3)
only studies in which Vc,max and Jmax were analyzed on a Cc – not Ci –
basis were selected. The values chosen were in all cases for control
plants in the absence of any stress, for which the obtained values
can be considered representative of the optimum for each species,
although we acknowledge that these variables change dynamically
with plant age, growing conditions, etc. The simulation for the chosen species was repeated assuming different scenarios depending
on differences in the possible acclimation of the four most influencing parameters – gs , gm , Vcmax and Jmax – as follows (see Section
3, Figs. 1 and 2):
Scenario 0: only temperature, not CO2 , is increased to +4 ◦ C above
the original measuring temperature
Scenario 1: both temperature and CO2 are increased, the latter to
700 ␮mol mol−1 , but there is no acclimation of Vc,max , Jmax , gs or gm
Scenario 2: like scenario 1 but with an acclimation of gs only
Scenario 3: like scenario 1 but with an acclimation of gm only
Scenario 4: like scenario 1 but with an acclimation of both gs and
gm
Scenario 5: like scenario 1 but with an acclimation of all parameters, gs , gm , Vc,max and Jmax
When a given parameter was considered acclimated in a given
scenario, the assumed percentages of change were a reduction
of 20% for gs , 15% for Vcmax and 5% for Jmax These percentages
were obtained as means for different species from experiments
performed under conditions simulating these foreseen climatic
variables, mostly from FACE experiments, and summarized in
Ainsworth and Long [6]. As for gm , its reduction was taken as
35%, which is the average of previously reported values for several species (summarized in Supplementary Table 3). We have
to highlight that this assumption is merely hypothetical, as it is
not possible with current analytical tools to discern whether the
reported decreases of gm at high CO2 are totally or partially real
or mere mathematical artifacts due to the equations used [39,40].
The response to temperature of Rubisco kinetic parameters of the
species included in the present study was assumed to present the
same temperature dependency as that for Rubisco from Nicotiana
Fig. 2. Simulations used to evaluate the effect of increasing CO2 on the net CO2
assimilation rate (AN ) considering changes in diffusional parameters – stomatal conductance (gs ) or CO2 mesophyll conductance (gm ) – and photosynthetic capacity
– defined by the maximum carboxylation rate (Vc,max ) and the maximum potential electron transport rate (Jmax ). (A) Vc,max and Jmax = 150 ␮mol m−2 s−1 , gs and
gm = 0.5 mol m−2 s−1 ; (B) gs and gm were reduced to 0.1 mol m−2 s−1 ; (C) as in (B)
plus a reduction in Vc,max and Jmax to 50 ␮mol m−2 s−1 . The values for temperature in
this theoretical cases was assumed to be 25 ◦ C. The short-term responses of gs and gm
shown in Supplemental Fig. 2 were used for this simulation. Horizontal arrows indicate the direction of change due to differences in ambient CO2 , and vertical arrows
the direction of change due to increasing diffusional limitations.
tabacum, and that the equations hold for the small range of temperatures from 25 ◦ C to 29 ◦ C considered in the present study [20].
3. Results and discussion
Photosynthesis models are often applied assuming identical
Rubisco kinetic properties among different plant species, being
nowadays the most used as described by Bernacchi et al. [20]. Here
we have used the same kinetics, and also we have fixed a constant value for leaf respiration in the light. It may be argued that
this is not correct when trying to compare the response of different species showing large differences in photosynthetic capacity,
J. Flexas et al. / Plant Science 226 (2014) 41–48
and indeed we show in a companion paper [44] that differences
in Rubisco kinetics may be important in determining the actual
rates of photosynthesis and their response to climate change. On
the other hand, although inter-specific differences in Rubisco kinetics have been described, among C3 plants these are relatively
minor (although significant and perhaps important) [45–48], and
ferns present similar Rubiscos to other C3 species [14]. In order
to validate whether reasonable results are obtained with the two
main assumptions – i.e. that all species share Rubisco kinetics
as those in [20], and all show a respiration rate in the light of
0.5 ␮mol m−2 s−1 – we ran the calculation for the normal conditions
at which the plants were measured (i.e. typically saturating light,
20–30 ◦ C, 400 ␮mol CO2 mol−1 air) and compared the obtained calculated net CO2 assimilation (AN ) with the actual AN measured
in the original studies (Fig. 1). For most of the species the calculated AN was close to the measured value, although for some there
was a large disagreement. As a selection criteria, we excluded all
the species for which the difference between calculated and measured AN was larger than 20% (Supplemental Table 2). Applying this
selection criterion, 21 species (12 angiosperms – one of them, Quercus ilex, from two different sources – 3 gymnosperms and 6 ferns)
were kept in the study, while another 9 species (6 angiosperms, 2
gymnosperms and 1 fern) were discarded. The reasons for large
discrepancies can be multiple, from errors in the original published data to inadequateness of the parameters assumed in the
model for some particular species (e.g. the fixed respiration value
chosen, the common Rubisco kinetics, etc.). After filtering there
was still some dispersion, especially for species with large photosynthesis rates, but overall a very good correlation (r = 0.98) lying
close to the 1:1 relationship was obtained between calculated
and measured AN (Fig. 1). Despite the good agreement between
calculated and measured AN in the selected species, to further
validate the assumptions of our simulations we performed a sensitivity analysis on the differences of Rubisco kinetics parameters
for two species, one pteridophyte (Equisetum) and one angiosperm
(Oryza) for which Rubisco kinetics were available from the literature. Reported values for Equisetum of Kc = 503 ␮mol mol−1 [49],
Ko = 367 mmol mol−1 [49] and * = 43.8 ␮mol mol−1 [50], and for
Oryza of Kc = 253 ␮mol mol−1 [51], Ko = 349 mmol mol−1 [51] and
* = 46.4 ␮mol mol−1 [52], were used to simulate AN compared to
those obtained by using standard values for tobacco published by
Bernacchi et al. [20]. Our results show that in the case of Equisetum
AN was overestimated by 5%, meanwhile in Oryza by 7%. In view of
these two independent comparisons, we consider that the model
used (with its simplifications and despite its limitations), overall
predicts reasonably well the photosynthetic rates. Thus, it seemed
useful to compare relative changes in these rates in different scenarios.
Using this system, we can first compare the response to increasing CO2 of two different simulations, one with high and one with
low diffusional conductances, both with identical Rubisco (Fig. 2).
The values for temperature in this theoretical cases was assumed
to be 25 ◦ C. In a leaf with Vc,max and Jmax = 150 ␮mol m−2 s−1 , and
gs and gm = 0.5 mol m−2 s−1 , large increases of AN from 100 to
400 ␮mol CO2 mol−1 air are predicted, and more modest increases
thereafter. At 700 ␮mol CO2 mol−1 air, AN may increase by 19.7% in
this case in the absence of any acclimation (Fig. 2A, dotted line).
With this large leaf conductance, gradual acclimation resulting in a
decrease of gs (Fig. 2A, solid line) or both gs and gm (Fig. 2A, dashed
line) in response to increased CO2 has only a marginal effect on AN ,
decreasing the expected benefit at 700 ␮mol CO2 mol−1 air by 3.6%,
so that the final increase may be only 16.1%. In contrast, in a leaf
with the same Vc,max and Jmax , but much lower conductances (gs
and gm = 0.1 mol m−2 s−1 ), the much larger increases of AN are predicted in the absence of acclimation, consisting of a 61.5% increase
at 700 ␮mol CO2 mol−1 air and photosynthesis rates almost as large
45
as those in the high conductance leaf at 1500 ␮mol CO2 mol−1 air
(Fig. 2B, dotted line). However, the effect of gs or both gs and gm
acclimation in this leaf is much larger, so that an acclimation of
both imposes a penalty of up to 42.7% on the expected increases
at 700 ␮mol CO2 mol−1 air, the final result being an increase of just
18.8%, i.e. only marginally higher than that of the high conductance
plant. However, it is unlikely that two species with large differences
in conductances have similar Vc,max and Jmax , as in nature a common
co-variation of these parameters is observed [14,31,53–55]. Hence,
we repeated the simulation for the low conductance species, but
assuming much lower Vc,max and Jmax = 50 ␮mol m−2 s−1 (Fig. 2C).
Although the expected increases in absolute rates became more
modest (Fig. 2C, dotted line), the relative effect of acclimation
of conductances was also smaller, so that the resulting expected
increase in AN at 700 ␮mol CO2 mol−1 air was only 19.2%. On the
other hand, the photosynthetic enhancement at high CO2 , as well
as its partial cancelation by acclimation, is more evident at high
than at low temperatures (Fig. 3). From this theoretical simulation
we can learn that: (1) in the absence of conductance acclimation,
the relative increase in photosynthesis at high CO2 is much larger
in species with lower, than in species with higher, conductances,
as already reported by [56]; (2) such an increase may be reduced
if the species has lower rates, or downregulates photosynthetic
capacity (Vc,max and Jmax ); (3) the reduction in the potential benefit of high CO2 on AN will be approximately doubled, if in addition
to acclimation of gs (as expected), there is also down regulation
of gm under high CO2 . (4) acclimation of gs and gm has a higher
impact in low conductance species regardless of their biochemistry,
resulting in an overall percentage increase of AN at high CO2 which
is only marginally higher than that shown by high conductance
species; and (5) both the benefits in the absence of acclimation and the partial cancelation of such benefits by acclimation
are more evident as temperatures increase, for which differences
among species are expected to be larger in high-temperature
climates.
To check for potential effects of conductance(s) acclimation in
different species, a survey was made of photosynthetic parameters
in previously characterized plants (Supplementary Table 1). A few
simplifications were made to obtain a number of simulated scenarios for a future climatic condition of +4 ◦ C (above the temperature
originally used in each study, and summarized in Supplementary
Table 1) and 700 ␮mol CO2 mol−1 air: (1) each species is analyzed
using its own previously estimated values for Vc,max and Jmax ; (2)
the Rubisco kinetic parameters, respiration rate, and temperature
dependencies for all parameters are assumed equal for all species,
as described in Section 2; (3) in cases where acclimation of gs is considered, this is assumed to be reduced by 20% following Ainsworth
and Long [6]; (4) in cases where acclimation of gm is considered, its
reduction is taken as 35%, which is the mean of previously reported
values for several species (Supplementary Table 2); and (5) in cases
where acclimation of Vc,max and Jmax is considered, their reduction
is assumed to be 15% and 5%, respectively, following Ainsworth and
Long [6]. The scenarios are described in Section 2.
When increased temperature is considered (Table 1, Scenario
0) only small increases of photosynthesis are expected, between
0-7%. Instead, when combining higher temperature and CO2 , large
increases of AN are predicted (Table 1, Scenario 1), ranging from
43.7% in the species showing the largest photosynthesis (Cistus
albidus) to 75% in one of the species showing the lowest rates (the
fern Asplenium scolopendrium). On average, the angiosperm and
gymnosperm species surveyed have an increase of about 55%, while
ferns would increase their photosynthesis by 66%. Considering
down regulation of gs by 20% of its current value (Table 1, Scenario 2; Fig. 4A) or of gm by 35% its current value (Table 1, Scenario
3); there is a similar dampening of the high CO2 -induced increase
in photosynthesis. Hence, in the representative seed plants the
46
J. Flexas et al. / Plant Science 226 (2014) 41–48
Fig. 4. (A) Percentage of increase in CO2 assimilation rate (AN ) at ambient CO2 concentration of 700 ␮mol mol−1 air compared to 400 ␮mol mol−1 , for angiosperm and
fern species in the different scenarios evaluated. See text or Table 1 legend for
a description of the scenarios. Values are means and bars indicate the ± SE, n = 7.
(B) Percentage of increase in CO2 assimilation rate (AN ) as a function of total conductance to CO2 (stomatal + mesophyll conductance, gtotal ). Each point represents a
single species.
Fig. 3. Simulation of the effect of increasing temperature on the net CO2 assimilation rate (AN ) at 400 or 700 ␮mol mol−1 CO2 . At 700 ␮mol mol−1 CO2 , the effect
of stomatal acclimation, and stomatal plus mesophyll conductance acclimation to
0.1 mol m−2 s−1 , are shown. Same theoretical species as in Fig. 2.
expected increase would be now reduced to 44% only, while in ferns
it will decline to 53%. Simultaneous acclimation of both conductances (Table 1, Scenario 4) results in roughly a two-fold reduction
of the beneficial CO2 effects on photosynthesis compared to the
acclimation of a single conductance only (Scenarios 2 and 3). If
both conductances are reduced under increased CO2 , the expected
increase of AN would be similar in angiosperms, gymnosperms
and ferns, being only 38%. Finally, considering additionally downregulation with decreases of Vc,max and Jmax (Table 1, Scenario 5), the
expected increase of AN would be reduced on average to 33–35%,
also without differences among plant groups. Therefore, as graphically seen in Fig. 4A, moving from Scenario 0 (only temperature is
increased) to Scenario 1 (both temperature and CO2 are increased)
results in a dramatic predicted increase in photosynthesis. However, progressively incorporating in the model the down-regulation
of gs , gm , Vc,max and Jmax (i.e. moving from Scenario 1 to Scenario 5)
largely reduced the original expectation for increased AN (Fig. 4A),
to about two thirds the estimations without acclimation when all
the parameters are considered acclimated. It can also be seen in
Fig. 4A that, on average, ferns present somewhat larger increases
in photosynthesis than seed plants in most scenarios, although this
difference is the biggest in Scenario 1, i.e. when no acclimation is
considered. This is simply because, as already anticipated by the
theoretical leaf analysis (Fig. 1), the expected relative increases in
photosynthesis are larger in low conductance species, like ferns but
also some angiosperms and many gymnosperms. Indeed, there is a
negative correlation between the total leaf conductance of a given
species and the expected % increase in photosynthesis under simulated climate change conditions (Fig. 4B). However, considering
possible acclimation of both diffusional and biochemical parameters to high CO2 cancels the apparent % benefit of low conductance
species, so that all the species end up with a similar predicted
increase in photosynthesis.
Overall, the lessons that can be extracted from this simulation study are: (1) acclimation of stomatal conductance to climate
change conditions results in a significant reduction of the expected
benefits of high CO2 in photosynthesis under light saturation; (2) a
potential acclimation of mesophyll conductance to CO2 besides gs
cannot be neglected when attempting to model photosynthesis in a
climate change scenario, as it roughly doubles the reduction which
occurs with acclimation of stomatal conductance alone; (3) species
with lower diffusional conductances have greater enhancement of
J. Flexas et al. / Plant Science 226 (2014) 41–48
photosynthesis than species with high conductances under those
conditions, although they will be also more negatively affected by
potential acclimation of gs and/or gm ; (4) it is important to consider
the variability among species and plant groups in their acclimation
of both conductances to climate change conditions. In this sense,
species with low gs and gm , and which lack acclimation, would
show much larger increases in photosynthesis than species with
high conductances with acclimation.
Interestingly, according to recent work by Brodribb’s lab
[13,57–59], ferns and possibly also conifers would not change their
gs in response to high CO2 – but gm may decline, see Gago et al.
[14] and Supplemental Table 2 – for which their most likely scenario would be Scenario 3. In contrast, studies suggest that most
angiosperms would reduce both gs and gm , in addition to downregulation of Vc,max and Jmax [61–63], for which their most likely
scenario would be Scenario 5. In this purely hypothetical situation,
light-saturated photosynthesis could increase by ca. 50% in ferns
and conifers but only by about 30% in angiosperms (Table 1). It is
unclear whether this fact alone would be sufficient to favor ferns
and conifers in comparison to other seed plants to the extent of
increasing the range of distribution of these groups, which were
dominant prior to the Angiosperm burst, during the Mesozoic era
when atmospheric CO2 concentration was much large than nowadays [57,60,64]. However, other studies suggest that ferns and
conifers may also show decreased gs under high CO2 associated
with changes in leaf anatomy and stomatal density – i.e. the ‘morphological acclimation’ – [11], while others would suggest that they
could possess even the ‘physiological down-regulation’, just like
seed plants [15,16]. On the other hand, to the best of our knowledge, only one recent study has attempted to grow a fern species
under high CO2 and analyze its gs and photosynthesis response
[11], while no published work has addressed the gm response to
growing under high CO2 conditions in ferns, and very few studies
in seed plants [17,34,37,38]. Preliminary results growing the fern
N. exaltata under high CO2 showed a decrease of gs by 22% and of
gm by just 11% (Flexas and Morales, unpublished). In view of the
lack of sufficient knowledge on gs , and especially gm , in response to
climate change in different species, and its potential importance to
determine the boundaries of future species fitness and distribution,
studies are urged in which different species belonging to distant
phylogenetic groups are grown under high CO2 (and temperature)
to check for potential differences in the acclimation of diffusional
conductances to CO2 . Moreover, models that account for dynamic
acclimation of gm , gs and other parameters to climate change variables (temperature, CO2 , etc.) should be developed for a better
understanding of vegetation dynamics under climate change.
Acknowledgements
This work was partly supported by the Plan Nacional, Spain, contracts AGL2009-11310 (A. Díaz-Espejo), BFU2011-23294 (J. Flexas
and J. Gago), contracts AGL2009-07999 (J. Galmés), BFU2011-26989
(F. Morales), FPI grant from AGL2008-04525-C02-01, AGL201130408-C04-01, (S. Martorell), the FONDECYT N 1120965 (R.E
Coopman) UE Innovine Project (Combining innovation in vineyard
management and genetic diversity for a sustainable European viticulture (Call FP7-KBBE-2012-6, Proposal N◦ 311775-INNOVINE))
(F. Morales) and Gobierno de Aragón (A03 research group) (F.
Morales).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.plantsci.
2014.06.011.
47
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