Environmental and Experimental Botany 64 (2008) 232–238
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Environmental and Experimental Botany
journal homepage: www.elsevier.com/locate/envexpbot
Leaf gas exchange responses to abrupt changes in light intensity for two invasive
and two non-invasive C4 grass species
Andrea Mojzes a,b,∗ , Tibor Kalapos a
a
b
Department of Plant Taxonomy and Ecology, Institute of Biology, Eötvös Loránd University, Pázmány P. s. 1/C, H-1117 Budapest, Hungary
Institute of Ecology and Botany, Hungarian Academy of Sciences, Alkotmány u. 2-4, H-2163 Vácrátót, Hungary
a r t i c l e
i n f o
Article history:
Received 1 October 2007
Received in revised form 10 June 2008
Accepted 12 June 2008
Keywords:
Bothriochloa ischaemum
Chrysopogon gryllus
Cynodon dactylon
Photosynthetic induction
Sorghum halepense
Water use efficiency
a b s t r a c t
Transient and steady state responses of leaf gas exchange (photosynthesis (A) and stomatal conductance
to water vapor (gs )) to marked changes in photosynthetic photon flux density (PPFD) were studied for two
invasive [Cynodon dactylon (L.) Pers. and Sorghum halepense (L.) Pers.] and two non-invasive, native [Bothriochloa ischaemum (L.) Keng and Chrysopogon gryllus (Torn.) Trin.] perennial C4 grass species from semiarid
temperate grasslands or croplands. Following an abrupt drop in PPFD from 1300 to 270 ␮mol photon
m−2 s−1 , the two invasive species reduced gs to a greater extent than A, resulting in higher intrinsic photosynthetic water use efficiency (PWUE = A/gs ) at low, compared to high-light conditions. For non-invasives,
a comparable drop in gs and A led to invariant PWUE, which was lower than that for the invasive group
under low light. The duration and speed of stomatal closure was similar for the four species. However,
unlike the other grasses, the noxious weed S. halepense exhibited a negligible net loss in PWUE during
the high-to-low light transition. Responses of the native B. ischaemum were mostly intermediate between
those of the two invasive species and the non-invasive C. gryllus, which is in agreement with the species’
ecological intermediacy: non-invasive but often reaches local dominance following a disturbance. With
a sudden reverse change in PPFD photosynthetic light induction was not faster for invasives than for
non-invasives. These results indicate more efficient water use under variable light for invasive compared
to non-invasive perennial C4 grasses which may contribute to their success in semiarid temperate habitats with a heterogeneous light regime. Yet, rapid photosynthetic light induction appears to be of less
importance in such environments.
© 2008 Published by Elsevier B.V.
1. Introduction
Stomata regulate CO2 and water vapor exchange between the
leaf interior and the outside atmosphere in such a way that CO2
Abbreviations: A, net photosynthetic rate; Ci , mesophyll air space CO2 partial
pressure; costPWUE , water cost integrated over the high-to-low PPFD transient; gs ,
stomatal conductance to water vapor; PPFD, photosynthetic photon flux density;
PWUE, intrinsic photosynthetic water use efficiency (A/gs ); PWUE270 and PWUE1300 ,
steady state PWUE at 270 and 1300 ␮mol photon m−2 s−1 PPFD, respectively; sA/Ci ,
magnitude of stomatal limitation during photosynthetic light induction; tAdrop and
tgdrop , duration of continuous decline of A and gs in response to drop in PPFD,
respectively; tArise and tgrise , time needed for photosynthetic induction and stomatal
opening following PPFD increase, respectively; tlag close , time lag of stomatal closure
behind the decline of A; tlag open , time lag of stomatal opening behind the increase
of A; gdrop , speed of stomatal closure; A, relative amplitude of photosynthetic
response; gs , relative amplitude of stomatal response.
∗ Corresponding author at: Department of Plant Taxonomy and Ecology, Institute
of Biology, Eötvös Loránd University, Pázmány P. s. 1/C, H-1117 Budapest, Hungary.
Tel.: +36 1 381 2187; fax: +36 1 381 2188.
E-mail address: [email protected] (A. Mojzes).
0098-8472/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.envexpbot.2008.06.003
is supplied for photosynthesis at a minimum associated water loss
(Nobel, 1983, pp. 448–454; Jones, 1992, pp. 285–292). Under variable conditions, an efficient optimization can be achieved if stomata
respond sensitively to signals both within the plant and from the
environment. Among environmental factors, photosynthetic photon flux density (PPFD) is particularly subject to rapid and marked
fluctuation in the field, e.g. due to variable cloud cover or canopy
movements caused by wind (Pearcy et al., 1996). Plant species differ in their capacity as to how closely their leaf gas exchange is able
to follow fluctuations in light environment. In different environments, natural selection may have favored different strategies even
within herbaceous plants (Knapp and Smith, 1987; Knapp, 1993;
Ögren and Sundin, 1996).
It is well documented that plants kept in darkness or under
low light intensity for hours and then transferred to high (saturating) light intensity require a period of time to reach their
steady state photosynthetic rate (Pearcy et al., 1996). Compared
to C3 plants, in C4 species such photosynthetic light induction
includes additional biochemical components: the activation of C4
cycle enzymes (e.g. PEP carboxylase, pyruvate Pi -dikinase) and the
A. Mojzes, T. Kalapos / Environmental and Experimental Botany 64 (2008) 232–238
buildup of the high-energy metabolite pool supporting transport of
substances between the mesophyll and the bundle sheath (Furbank
and Walker, 1985; Horton and Neufeld, 1998; Sage and McKown,
2006). Sage and McKown (2006) argue that C4 plants are less efficient in utilizing short sunflecks in light-limited environments if
compared to C3 plants due to structural and functional constraints
associated with C4 photosynthesis. Rapid stomatal opening or the
maintenance of high stomatal conductance in periods between
sunflecks may play an important role in exploiting subsequent sunflecks (Kirschbaum and Pearcy, 1988; Tinoco-Ojanguren and Pearcy,
1992; Ögren and Sundin, 1996). However, in high-light environments where water is often in short supply, such as in semiarid
grasslands, rapid closure of stomata with sudden decline in irradiance helps to improve water use efficiency, although at the expense
of certain loss in carbon gain. The dumb-bell-shaped stomata of
grasses (Poaceae) possess higher capacity to track environmental
changes at lower cost than the kidney-shaped stomata of other vascular plants (Hetherington and Woodward, 2003). For grass species
dominating such open habitats numerous studies have investigated
the response of photosynthesis and stomatal conductance to water
stress (e.g. Stuart et al., 1985; Williams and Black, 1994; Kalapos
et al., 1996; Pugnaire and Haase, 1996; Awada et al., 2002; Morse
et al., 2002; Colom and Vazzana, 2003; Guenni et al., 2004; Chen
et al., 2005; Xu et al., 2006), but only a few have addressed the
response to changes in the light environment (Knapp, 1993; Fay and
Knapp, 1993, 1995; Dias-Filho, 2002). In the variable light environment of the tallgrass prairie Knapp (1993) reported light response
of stomata to be faster and of smaller amplitude for C4 grasses than
for C3 grasses, and interpreted that as an important determinant
of the success of C4 species in environments with periodic water
limitation.
Invasive species often rapidly colonize disturbed habitats where
canopy is discontinuous (Holm et al., 1977; Zólyomi and Fekete,
1994; Grace et al., 2001; Hamerlynck, 2001). Such species are
expected to face higher degree of environmental heterogeneity
including more variable light climate than non-invasive species.
This may require a rapid and efficient response of physiological processes to the environment, and thus limitation of these processes
by the environment could be minimized. For example, Hamerlynck
(2001) attributed an important role to close stomatal tracking of
variable irradiance in the invasive C3 tree Ailanthus altissima in its
highly successful establishment in urban habitats. C4 species are
often weeds in semiarid temperate regions, and are rarely successful in closed vegetation dominated by C3 plants (Long, 1983;
Sage and Monson, 1999, pp. 313–373). In this study, we investigated
whether stomatal regulation is faster and more efficient in invasive
compared to non-invasive C4 grasses. Two hypotheses were tested.
(1) When PPFD decreases abruptly, invasive C4 grasses are more
efficient in water saving than non-invasive ones during the transient phase and/or in the new steady state under low light. (2) When
PPFD reaches a high level again following a steady state in low light,
invasive C4 grasses obtain higher carbon gain than non-invasive
relatives through more rapid photosynthetic induction.
2. Materials and methods
2.1. Species studied, sampling and growth conditions
We studied two invasive alien and two non-invasive, native
perennial C4 grass species typical of semiarid temperate foreststeppe vegetation or in replacing croplands in Hungary. The
invasive species were Cynodon dactylon (L.) Pers. and Sorghum
halepense (L.) Pers., both ranked among the most serious weeds of
the world (Holm et al., 1977, pp. 25–61). S. halepense is a major weed
of tall dense crops like corn or sugarcane. C. dactylon also inhab-
233
its communities with spatially heterogeneous irradiance, such as
woodlands or mesquite savannas in the USA (Grace et al., 2001)
and open and shaded meadows in India (Kaul and Sapru, 1973),
although in Hungary it mostly occurs in shortgrass steppe or on
disturbed ground (Soó, 1973, pp. 429–430). The two non-invasive
native species in our study were Chrysopogon gryllus (Torn.) Trin.
and Bothriochloa ischaemum (L.) Keng., which are typical of xeric
grasslands in Hungary (Soó, 1973, pp. 445–446). B. ischaemum often
reaches local dominance after a disturbance in short grasslands on
loess or sand substrate (Zólyomi and Fekete, 1994). Each species
belongs to the NADP-ME C4 biochemical subtype except C. dactylon, which is a NAD-ME C4 species (Sage and Monson, 1999, pp.
568–576).
Plants were collected from semiarid temperate forest steppe
vegetation or from arable land on loess or sand 20 km E (loess)
or 60 km SE (sand) of Budapest between July and October 2002
(C. gryllus), in 2002 and 2004 (C. dactylon and S. halepense), or in
2003 and 2004 (B. ischaemum). Three individuals per species, along
with their soil monoliths, were excavated, placed into 4-l plastic
pots and transferred to an on-campus growth facility in Budapest.
Each pot contained the tillers of one grass species only. Shoots
were cut back to 1 cm above soil surface immediately after transplanting and thereafter regularly every 3 months so that we could
maintain manageable shoot size and comparable shoot age during
the full course of measurements. Plants were grown in a growth
room under natural irradiance supplemented by a 1000-W artificial halogen light source over a daily photoperiod of 12 h from
April to October and 9 h from November to March. Each plant was
allowed to grow in the growth room for at least 1 year—with shoot
cutback once every 3 months—before leaf gas exchange measurements were made. Pots were rotated every 3 weeks to minimize
the effects of possible heterogeneous light distribution on the
bench. Mean PPFD in the height of grass foliage, which included
the irradiance from the 1000 W halogen lamp, varied between 440
and 810 ␮mol photon m−2 s−1 in summer, and between 135 and
180 ␮mol photon m−2 s−1 in winter on clear days. Air temperature
and humidity was measured hourly by using an HOBO Pro RH/Temp
device (Onset Computers Inc., Bourne, MA, USA). Mean air temperature was 24.0 ± 4.4 and 18.5 ± 2.3 ◦ C in summer and winter,
respectively, relative air humidity varied between 20 and 80% during the day. Plants were watered regularly (every second or third
day), and supplied with mineral nutrients (0.5 ml per pot, 13% N,
4.5% P2 O5 , 6.5% K2 O plus micronutrients, Vitaflora, Hungary) at 3week intervals. Plants remained in vegetative stage throughout the
leaf gas exchange measurement periods.
2.2. Leaf gas exchange measurements
Laboratory leaf gas exchange measurements were made
between 7 and 16 April 2004, and in June or July 2004 and 2005
by using an open system infrared gas analyzer (ADC LCA-4 with
PLC4-B leaf chamber, Analytical Development Co., BioScientific
Ltd., Hoddesdon, UK). Plants collected in 2002 and 2003 were
measured in 2004 or 2005, while those collected in 2004 were
measured in 2005. Measurements were performed on three individuals per species (n = 3, except for C. gryllus) with one to four
replicates per individual. Each replicate measurement was conducted on a group of two to four fully developed leaf blades (the
second or third leaf count from the top). Each leaf was measured
only once. In C. gryllus only two individuals were available (n = 2)
due to dieback in the third pot. Gas exchange rates were calculated for unit leaf area, where one sided leaf surface area was
obtained by multiplying the sum of leaf widths with chamber
length (2.5 cm). Each replicate measurement consisted of three
consecutive phases as follows. (Phase 1) Leaves were first incubated
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under 1300 ␮mol photon m−2 s−1 PPFD white actinic light until
steady state gas exchange (A, gs , Ci ) readings were achieved (photosynthetic light induction, this usually took 50–80 min). (Phase
2) PPFD was decreased to 270 ␮mol photon m−2 s−1 in one step
(taking 10–15 s) by placing three neutral density filters in the
light path without opening the leaf chamber, and then illumination was maintained at this level until the new steady state
leaf gas exchange rate was reached (usually in 25–30 min). The
application of neutral density filters enabled us to change light
intensity without affecting spectral composition. (Phase 3) PPFD
was returned to 1300 ␮mol photon m−2 s−1 in one step by removing the neutral density filters from the light path, thus we could
measure photosynthetic light induction (after 25–30 min) on leaves
previously incubated to the same high PPFD (during Phase 1).
(Light induction in Phase 1 took about twice as long in Phase 3
because plants were under moderate light in the growth room
or laboratory prior to measurement.) Stationary net photosynthetic rate (A) achieved during Phase 3 was considered the steady
state rate at 1300 ␮mol photon m−2 s−1 PPFD. The high and low
light intensity that were applied during the measurement closely
matched the average PPFD the studied species experience in
their forest steppe habitat when they are exposed to full sun
(1400–1740 ␮mol photon m−2 s−1 ) and partially shaded by shrub
canopy (270–430 ␮mol photon m−2 s−1 ), respectively (A. Mojzes,
unpublished data). Leaf gas exchange parameters were calculated
during the measurement using the equations of von Caemmerer
and Farquhar (1981), and automatically saved every 30 s. Standard
measurement conditions in the leaf chamber were provided by the
system component ADC LC4-LMC-002/B Leaf Chamber Microclimate Control Unit (Analytical Development Co., BioScientific Ltd.,
Hoddesdon, UK), and were as follows: 350 vpm CO2 concentration
in the incoming air, 25 ± 0.3 ◦ C air temperature and 40% relative air
humidity.
2.3. Data analysis
From standard gas exchange data measured (net photosynthetic
rate (A), stomatal conductance to water vapor (gs ), and mesophyll
air space CO2 partial pressure (Ci )) the following variables were
obtained for comparisons.
(1) To describe the dynamics of the transition between stages 1
(initial high-light phase) and 2 (low-light steady state) we calculated:
tgdrop time (s) needed for the partial closure of stomata,
measured as the duration of continuous decline of
gs after neutral density filters were inserted into the
light path;
gdrop speed of stomatal closure (mmol H2 O m−2 s−1 min−1 )
obtained as the slope of the linear regression line fitted to data points (gs vs. time) in the continuously
declining part of the response (see Fay and Knapp,
1993, 1995 for references);
tAdrop duration of the continuous decline of net photosynthetic rate after neutral density filters were inserted
into the light path (s, calculated in the same way as
tgdrop );
tlag close time lag of stomatal closure behind the decline of
A in response to decreased PPFD (s). Calculated as
(tgdrop − tAdrop );
costPWUE water cost associated with asynchronous decrease
of A and gs following a drop in PPFD. Difference
of the intrinsic photosynthetic water use efficiency
(PWUE, mmol CO2 mol−1 H2 O, calculated as A/gs ,
Fig. 1. Time courses of photosynthetic rate (A) and stomatal conductance to water
vapor (gs ) for Sorghum halepense leaves incubated under moderate light (one
replicate measurement). Leaves were illuminated with 1300 ␮mol photon m−2 s−1
PPFD during Phase 1, then during Phase 2 PPFD was abruptly decreased to
270 ␮mol photon m−2 s−1 (indicated by the downward arrow), and after steady state
gas exchange was achieved PPFD was increased to 1300 ␮mol photon m−2 s−1 again
(shown by the upward arrow, Phase 3).
Hemsley and Poole, 2004, p. 236.) from its highlight steady state value were integrated over the
course of its decline and subsequent monotonous
increase following the PPFD drop from 1300 to
270 ␮mol photon m−2 s−1 . As time courses showed
certain noise (see Fig. 1), the calculation was as follows. The initial high-light steady state PWUE was
obtained as the mean of the last 20–26 consecutive
data points (10–13 min) prior to the drop in illumination (similar to the procedure used by Horton and
Neufeld, 1998 to obtain Amax ). The transient part of
the curve was smoothed by using moving average
with a frame of three recorded data, and area portions
determined by successive data points and the initial
PWUE value was calculated and summed. During this
response, PWUE may have reached values above initial. In such cases the difference from steady state
was considered negative, thus decreasing the overall water cost. This variable estimated the amount of
water lost through stomata in a photosynthetically
inefficient way during the transition period when,
due to the time lag of stomatal closure behind the
decline of A, water loss was associated with marginal
carbon gain under low light.
(2) To characterize photosynthetic light induction when PPFD was
suddenly increased to 1300 ␮mol photon m−2 s−1 for leaves in
steady state gas exchange under low light (Phase 3):
tArise time (s) taken to reach 90% of steady state A, determined
by fitting a sigmoid (Boltzman) function to the time course
of A following PPFD increase. During light induction, photosynthetic rate usually increases following a sigmoid
course, particularly when initial stomatal conductance is
low (Kirschbaum and Pearcy, 1988; Pearcy et al., 1991);
tgrise time needed to reach 90% of steady state gs (s). Calculation
as described for tArise ;
tlag open delay of stomatal opening behind the increase of A to
90% steady state value (s). Calculated as the difference
tgrise − tArise ;
sA/Ci magnitude of stomatal limitation during photosynthetic
light induction. Using data where both A and Ci increased
monotonously in the course of light induction, A was plotted against Ci and the slope of the fitted linear regression
line was used to assess stomatal limitation. When pho-
A. Mojzes, T. Kalapos / Environmental and Experimental Botany 64 (2008) 232–238
tosynthetic rate during light induction is restricted by
insufficient openness of stomata rather than slow activation of photosynthetic enzymes and the buildup of
metabolic pools, the increase of A with elevating Ci is steep
and the slope may even approach that of the linear part of
the steady state CO2 response of photosynthesis (Pearcy
et al., 1996).
(3) For the comparison of performance under the two steady states
(i.e. at 1300 and 270 ␮mol photon m−2 s−1 PPFD):
gs relative amplitude of stomatal
response: the difference between
the steady state gs at 270 and
1300 ␮mol photon m−2 s−1
PPFD
(means of the last 20–26 successive readings of gs in Phases 2
and 1, respectively) expressed as
a percentage of gs at 1300 ␮mol
photon m−2 s−1 , %);
A relative amplitude of photosynthetic
response: the difference between
steady state photosynthetic rate at
270 and 1300 ␮mol photon m−2 s−1
PPFD, expressed as a percentage of
A at 1300 ␮mol photon m−2 s−1 , %).
At 270 ␮mol photon m−2 s−1 PPFD, A
was calculated as described for gs ,
while at 1300 ␮mol photon m−2 s−1
PPFD, it was obtained from the sigmoid (Boltzman) function fitted for
the time course of A during Phase 3;
PWUE270 and PWUE1300 steady state PWUE at 270 and
PPFD,
1300 ␮mol photon m−2 s−1
respectively (A/gs , mmol CO2 mol−1
H2 O, calculated as means of the last
20–26 successive readings of PWUE
in the Phases 2 and 3, respectively).
When two to four replicate measurements were performed
on the same individual the above variables were averaged for
the individual, and the means (as independent samples) were
used for statistical analyses. For interspecific comparisons oneway analysis of variance (ANOVA) with species as fixed effect
was applied with subsequent Tukey HSD post hoc tests to analyze significant differences among means. For each species, paired
t-tests were used to compare the duration or magnitude of simultaneous or related processes (i.e. tgdrop and tAdrop , gs and A,
tArise and tgrise , tgdrop and tgrise , tlag close and tlag open , PWUE270 and
PWUE1300 , A at 270 and 1300 ␮mol photon m−2 s−1 PPFD, gs at
270 and 1300 ␮mol photon m−2 s−1 PPFD). These analyses were
also repeated for species groups by merging individual means
for C. dactylon and S. halepense (invasives, n = 6), and those for
B. ischaemum and C. gryllus (non-invasives, n = 5). Unpaired ttests were applied to compare means of the two groups. When
data did not meet the normality assumption, non-parametric
Mann–Whitney U-test was conducted. Paired t-tests were executed for within-group comparisons. For each statistical test, the
significance level was p = 0.05, and differences were considered
marginally significant when 0.05 < p < 0.1. Linear regressions and
exploratory statistics were completed by using the Statistica 7.0
package (StatSoft Inc., Tulsa, USA).
3. Results
For each grass species, the stomatal conductance and photosynthetic rate closely tracked changes in PPFD and the time course
235
of responses was similar for each species (see Fig. 1 for an example). The partial closure of stomata after the drop in light level
took 248–379 s, while reopening with increased PPFD lasted for
189–354 s on average. The speed of stomatal closure (gdrop ) was
three times higher for C. dactylon (35.7 mmol H2 O m−2 s−1 min−1 ),
than for C. gryllus (11.2 mmol H2 O m−2 s−1 min−1 ), while S.
halepense and B. ischaemum showed intermediate values (22.2 and
23.2 mmol H2 O m−2 s−1 min−1 , respectively). However, tgdrop did
not differ significantly between species (Fig. 2a). In response to
marked decline in PPFD, net photosynthetic rate (A) decreased
suddenly, within 123–160 s. It was followed by the partial closure
of stomata (decline in gs ) with 88–256 s delay (Fig. 2b). Consequently, during this transient phase, intrinsic photosynthetic water
use efficiency (PWUE = A/gs ) dropped quickly to a minimum, then
slowly increased again to a value matching or even exceeding the
prior steady state value with subsequent partial closure of stomata.
No statistically significant difference appeared in costPWUE among
species. However, costPWUE was somewhat below zero, resulting in
a gain rather than a loss of PWUE during the transient phase for
S. halepense, while a small positive value was found for the other
three grasses (Fig. 2c).
Steady state intrinsic PWUE was higher at 270 than at
1300 ␮mol photon m−2 s−1 PPFD for the invasive C. dactylon and
S. halepense, but was not significantly different for the noninvasive B. ischaemum and C. gryllus (Fig. 3a). This was due to
the greater reduction in gs than in A in response to decreased
light intensity for the two invasive grasses (gs > A), although
this difference was only marginally significant for C. dactylon
(p = 0.072). In contrast, for the two non-invasive species gs and
A were statistically similar (Fig. 3b). Species did not differ in
steady state intrinsic PWUE under either light condition (Fig. 3a).
Under low light, there was no significant difference in either gs or
A among species (Fig. 3c and d). Under high-light, steady state A
of C. dactylon exceeded that of C. gryllus and tended to be higher
(marginal significance) than that of S. halepense and B. ischaemum
(Fig. 3c). The 8.9–51.3% higher gs for C. dactylon than for the
other three species at 1300 ␮mol photon m−2 s−1 PPFD probably
does not explain this difference as interspecific differences in gs
were statistically non-significant (Fig. 3d). With PPFD reduction
to 270 ␮mol photon m−2 s−1 , A diminished significantly for each
species except for C. gryllus (for which the drop was also obvious, but
statistically not significant probably due to the small sample size
(n = 2), Fig. 3c). The magnitude of change in A (A) was greatest for
C. dactylon (Fig. 3b). Likewise, C. dactylon exhibited the most marked
and significant decline in gs among species, while the reduction
of this variable in response to decreased light intensity was only
marginally significant for S. halepense and B. ischaemum, and nonsignificant for C. gryllus (Fig. 3d). Neither gs nor A differed
significantly between these three species (Fig. 3b). Consequently,
at 270 ␮mol photon m−2 s−1 PPFD C. dactylon maintained only 36%
of its A at 1300 ␮mol photon m−2 s−1 PPFD, while for the rest of the
species this proportion was 53.5–55.4%.
When the light intensity was elevated again to 1300 ␮mol
photon m−2 s−1 after steady state at 270 ␮mol photon m−2 s−1 , the
duration of both stomatal opening and photosynthetic induction
(reaching 90% of the high-light steady state values in gs and A,
respectively) were statistically similar for all species studied. Nevertheless, tgrise was 46.7% lower (marginally significant, p = 0.072)
for S. halepense than for C. gryllus (Fig. 2d). The delay of stomatal
opening behind the enhancement of A was 123 and 83 s for C.
dactylon and C. gryllus, respectively, while it was negligible for S.
halepense and B. ischaemum. However, differences in tlag open values were not significant among species (Fig. 2b). Intrinsic PWUE
did not display a characteristic pattern during the low-to-high-light
switch, as it reached gradually a new steady state. This is probably
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A. Mojzes, T. Kalapos / Environmental and Experimental Botany 64 (2008) 232–238
Fig. 2. Gas exchange variables during transient states associated with abrupt changes in PPFD. (a) tgdrop on PPFD decline from 1300 to 270 ␮mol photon m−2 s−1 and tgrise in
response to subsequent reverse change in PPFD; (b) tlag close and tlag open ; (c) costPWUE ; (d) tgrise and tArise when PPFD was increased from 270 to 1300 ␮mol photon m−2 s−1 .
Error bars show ±1 S.E. of two individuals for C. gryllus (n = 2) and three individuals for the rest of the species (n = 3). Species with the same letter above bars are not significantly
different for the given variable (p < 0.05). When two variables are plotted on a graph, lower and upper case letters are used to indicate significance for the different variables.
Within-species significant difference (p < 0.05) is marked by an asterisk. Parentheses and square brackets indicate marginal significance (0.05 < p < 0.1) between two species
or between two associated variables within a species.
explained partly by the smaller delay of stomatal response behind
the abrupt change of A in response to increase than to decrease in
PPFD, although in all four species the difference between tlag open
and tlag close was not significant, Fig. 2b). When PPFD was increased
to 1300 ␮mol photon m−2 s−1 again, Ci temporarily declined as A
increased more rapidly than gs and CO2 supply lagged behind
demand. Then, Ci gradually increased again as stomata opened,
and reached a new steady state value. During this low-to-high-light
transition, A and Ci correlated significantly for one individual of C.
gryllus and for two individuals of C. dactylon and B. ischaemum only.
Thus, interspecific comparison of the slope of the A/Ci relationship
(sA/Ci ) was not possible. Mean values of sA/Ci ranged from 0.047 to
0.067. For each species tgdrop and tgrise was not significantly different
(Fig. 2a).
We repeated our analyses for species groups as well when the
species were merged into two groups: invasives (C. dactylon and S.
halepense) and non-invasives (B. ischaemum and C. gryllus). Only
two variables were significantly different between groups: both
gs and PWUE270 were higher for invasives than for non-invasives
(Fig. 4a and b). Within groups, gs exceeded A, and PWUE270 surpassed PWUE1300 for invasives, while no such differences appeared
in the non-invasive group. In both groups, stomatal closure took
Fig. 3. Steady state gas exchange variables under high (1300) and low (270 ␮mol photon m−2 s−1 ) PPFD: (a) intrinsic photosynthetic water use efficiency (PWUE = A/gs ); (b)
percentage decline in A (A, %) and gs (gs , %) in response to decline in PPFD from 1300 to 270 ␮mol photon m−2 s−1 ; (c) net photosynthetic rate (A); (d) stomatal conductance
(gs ). Labels as in Fig. 2.
A. Mojzes, T. Kalapos / Environmental and Experimental Botany 64 (2008) 232–238
Fig. 4. Selected gas exchange variables for the invasive group (C. dactylon and S.
halepense) and the non-invasive group (B. ischaemum and C. gryllus) of species studied: (a) percentage decline in A (A, %) and gs (gs , %) in response to drop in PPFD
from high (1300) to low (270 ␮mol photon m−2 s−1 ); (b) intrinsic photosynthetic
water use efficiency (PWUE = A/gs ) under high and low PPFD. For a given variable, significant difference (p < 0.05) between groups is indicated by different letters, while
within-group significant difference is marked by an asterisk. Lower and upper case
letters are used for different variables on the same graph. Error bars show ±1 S.E. of
six and five individuals (n = 6 and 5) in the invasive and in the non-invasive group,
respectively.
significantly longer than the decline of A (tgdrop > tAdrop , by 80.1%
for invasives and 127% for non-invasives), and steady state values of both gs and A were significantly higher under high than
under low light. Stomatal opening lasted for 50% longer than photosynthetic induction in the invasive group, while tArise and tgrise
were statistically similar for non-invasives. In line with interspecific comparisons, we did not find significant differences between
tlag open and tlag close , and between tgdrop and tgrise respectively in
either the invasive or the non-invasive group.
4. Discussion
In accordance with the prior hypothesis, the group of the invasive S. halepense and C. dactylon achieved higher steady state
intrinsic water use efficiency under low light than the group of the
non-invasive C. gryllus and B. ischaemum. This was a consequence
of a greater reduction in gs than in A with a drop in PPFD from 1300
to 270 ␮mol photon m−2 s−1 for the two invasive species, which
resulted in an enhanced PWUE at low, compared to high, light conditions. Considering the similar carbon gain under low light for the
four species, water spared in such a way, may allow these two invasive grasses to better exploit periods of moderate irradiance, and
thus confer advantage for them particularly in semiarid environments.
Reduction in A with light level drop was similar for the three
NADP-ME grasses in our study, but it was greatest for the NAD-ME
C. dactylon. This suggests that the maintenance of a high carbon assimilation rate under moderate light might be associated
with the higher efficiency of CO2 -concentrating mechanism and
hence greater quantum yield of NADP-ME than NAD-ME C4 species
237
(Pearcy and Ehleringer, 1984), rather than to the species’ invasive
potential. However, an improved steady state PWUE270 for C. dactylon suggests that a marked reduction in photosynthetic rate with
drop in PPFD does not necessarily mean a lower degree of shade
tolerance for NAD-ME than for NADP-ME C4 species. C. dactylon
achieved high PWUE270 mainly through marked stomatal closure
with concomitant reduction in A, even exceeding the other invasive
grass S. halepense in gs .
In response to an abrupt decline in PPFD the great amplitude
of reduction in gs for C. dactylon was associated with a high speed
of stomatal closure, even exceeding literature values reported for
other C3 or C4 grasses (12.4–26.1 mmol H2 O m−2 s−1 min−1 , Knapp,
1993; Fay and Knapp, 1993, 1995). In contrast, gdrop of the other
three species studied here remained within or close to this range.
However, the overall duration of partial stomatal closure with PPFD
drop (determined by both speed and amplitude) was not significantly different between invasive and non-invasive grasses. No
net loss in intrinsic photosynthetic water use efficiency during
the high-to-low light transition observed for S. halepense suggests an efficient light utilization at a negligible water cost for
this species. Furthermore, S. halepense slightly exceeded the noninvasive C. gryllus in the speed of stomatal opening when PPFD was
elevated from 270 to 1300 ␮mol photon m−2 s−1 . These results indicate a highly efficient light-tracking behavior of the stomata for
S. halepense that might contribute to the species’ success in habitats with heterogeneous light climate. Indeed, S. halepense is one of
the most successful weeds in tall monocultures like corn or millet
(Holm et al., 1977, pp. 54–61), where light penetrating the canopy
may be highly variable in time. Stuart et al. (1985) also found sensitive stomatal regulation in response to water stress or increasing
leaf temperature in this species in its natural habitat. Under field
conditions similar to our study, both C. dactylon and S. halepense
were found to be more efficient in accumulating micronutrients,
particularly K+ , than either B. ischaemum or C. gryllus (Kovács et
al., 2002). Potassium plays a key role in stomatal regulation and
photosynthesis (Marschner, 1995, pp. 299–308).
Stomatal response of the non-invasive B. ischaemum to abrupt
decline in PPFD (i.e. gdrop , tlag close ), and its steady state PWUE270
were intermediate between those of the invasive species in our
study (C. dactylon and/or S. halepense) and the non-invasive C. gryllus. Similar to the two invasives, B. ischaemum exhibited at least
marginally significant reduction in both steady state A and gs in
response to decrease in PPFD. However, this species did not achieve
an enhancement in PWUE at low light, like the non-invasive C. gryllus. Also, a sizeable loss in water use efficiency associated with the
reduction in A appeared for B. ischaemum during the high-to-low
light transition. Such intermediacy in physiology between invasives
and non-invasives is consistent with the species’ ecological behavior in terms of its non-invasive character coupled with an ability to
reach local dominance on disturbance in semiarid temperate grasslands (Zólyomi and Fekete, 1994). Compared to a traditional crop
and an introduced exotic C4 grass species in the semiarid temperate
Loess Hills of China, Xu et al. (2006) found intermediate stomatal sensitivity and WUE of biomass production for the native B.
ischaemum when exposed to water stress.
When PPFD was elevated from 270 to 1300 ␮mol photon
m−2 s−1 , A reached the new steady state under similar time interval for all four species. This is in contrast with the hypothesis that
invasive grasses would be faster in increasing assimilation rate in
response to higher light availability that could contribute to a better exploitation of periods of high-light relative to non-invasive
grasses. This, however, should be interpreted by considering that
the species studied here originally inhabit semiarid high-light environments (grasslands or herbaceous crops). In such habitats, water
sparing through rapid and substantial stomatal closure on irradi-
238
A. Mojzes, T. Kalapos / Environmental and Experimental Botany 64 (2008) 232–238
ance drop may be of higher adaptive value than maximizing carbon
gain by faster and more marked photosynthetic induction on PPFD
increase. Earlier studies suggest that stomatal regulation maximizes water use efficiency in C4 species from high light, periodically
water limited habitats (Knapp, 1993; Huxman and Monson, 2003).
Rapid photosynthetic light induction is probably more advantageous for plants living in light-limited environments (such as forest
understorey), as it has already been shown in experimental studies
(e.g. Chazdon and Pearcy, 1986; Ögren and Sundin, 1996; Horton
and Neufeld, 1998). The invasive group studied (and particularly
C. dactylon) exhibited significant delay in stomatal opening relative to the rise of A to the new steady state in response to increased
PPFD. This suggests that after incubation under low light the activation of photosynthetic enzymes and the buildup of metabolite pools
for transfer between mesophyll and bundle sheath are faster than
simultaneous opening of stomata. The sparse significantly positive
relationship between A and Ci during the transition from low to
high-light steady state for the four species and the lack of difference
in sA/Ci between the invasive and the non-invasive groups indicate
similarly minimal stomatal limitation during photosynthetic light
induction for these species irrespective of their invasive capacity.
5. Conclusions
Based on the study of two pairs of species, our results support the
hypothesis that among C4 perennial grasses the closure of stomata
with drop in PPFD is more efficient for invasive than for noninvasive species. The greater magnitude of stomatal closure and/or
enhanced water use efficiency in the low-light steady state enables
invasives to use water more sparingly than non-invasives. These in
turn may translate to higher growth rate and competitive success
for invasive grasses in habitats of heterogeneous light regime. Our
results, however, do not support the hypothesis of faster photosynthetic light induction upon increased PPFD for invasive than for
non-invasive C4 grass species. Nevertheless, these results suggest
one candidate mechanism contributing to the success of invasive
grasses and await further validation on other species and in other
ecosystems.
Acknowledgements
Support from the Hungarian Scientific Research Fund (OTKA
T03828, M41454) is greatly appreciated. We thank three anonymous reviewers for their helpful comments on an earlier version of
the manuscript and Timothy Brian Hoelzle for language corrections.
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