Copyright ß Physiologia Plantarum 2006, ISSN 0031-9317
Physiologia Plantarum 127: 343–352. 2006
R E VI EW
Keeping a positive carbon balance under adverse conditions:
responses of photosynthesis and respiration to water stress
Jaume Flexas*, Josefina Bota, Jeroni Galmés, Hipólito Medrano and Miquel Ribas-Carbó
Grup de Recerca en Biologia de les Plantes en Condicions Mediterrànies, Departament de Biologia, Universitat de les Illes Balears, Carretera de
Valldemossa Km 7.5, 07122 Palma de Mallorca, Illes Balears, Spain
Correspondence
*Corresponding author,
e-mail: [email protected]
Received 2 August 2005; revised 10 October
2005
doi: 10.1111/j.1399-3054.2005.00621.x
Drought and salinity (i.e. soil water stress) are the main environmental factors
limiting photosynthesis and respiration and, consequently, plant growth. This
review summarizes the current status of knowledge on photosynthesis and
respiration under water stress. It is shown that diffusion limitations to photosynthesis under most water stress conditions are predominant, involving decreased
mesophyll conductance to CO2, an important but often neglected process. A
general failure of photochemistry and biochemistry, by contrast, can occur only
when daily maximum stomatal conductance (gs) drops below 0.05–0.10 mol
H2O m 2 s 1. Because these changes are preceded by increased leaf antioxidant
activities (gs below 0.15–0.20 mol H2O m 2 s 1), it is suggested that metabolic
responses to severe drought occur indirectly as a consequence of oxidative stress,
rather than as a direct response to water shortage. As for respiration, it is remarkable that the electron partitioning towards the alternative respiration pathway
sharply increases at the same gs threshold, although total respiration rates are less
affected. Despite the considerable improvement in the understanding of plant
responses to drought, several gaps of knowledge are highlighted which should
become research priorities for the near future. These include how respiration and
photosynthesis interact at severe stress, what are the boundaries and mechanisms
of photosynthetic acclimation to water stress and what are the factors leading to
different rates of recovery after a stress period.
Photosynthesis and respiration responses to
drought and salinity
Both drought and salinity stresses reduce the capacity of
plants to take up water from the soil (Munns 2002). At
present, low water availability is the main environmental
factor limiting plant growth and yield worldwide, and
global change will likely make water scarcity an even
greater limitation to plant productivity across an increasing
amount of land (Chaves et al. 2003, Hamdy et al. 2003).
The limitation of plant growth imposed by low water
availability is mainly due to reductions in plant carbon
balance, which is dependent on the balance between
photosynthesis and respiration. Both processes are intimately linked. For instance, it has been shown that transgenic plants with modified respiration also alter their
photosynthetic behaviour (Dutilleul et al. 2003, NunesNesi et al. 2005), and an increased respiration rate
seems necessary for photosynthesis recovery after a period of water stress (Kirschbaum 1988). Of the total
Abbreviations – ABA, abscisic acid; Altox, alternative oxidase; AN, light-saturated net photosynthesis; APX, ascorbate peroxidase; ATP, adenosine triphosphate; Cc, chloroplast CO2 concentration; Ci, sub-stomatal CO2 concentration; ETR, electron
transport rate; GR, glutathione reductase; gs, maximum CO2 stomatal conductance; Rubisco, Ribulose-1,5-bisphosphate
carboxylase/oxygenase; RuBP, Ribulose-1,5-bisphosphate; Vc,max, maximum velocity of carboxylation.
Physiol. Plant. 127, 2006
343
344
Net photosynthesis (μmol CO2 m–2 s–1)
A
30
25
20
15
10
5
0
0
200
400
600
800 1000 1200 1400 1600
Ci (μmol mol–1)
B
Net photosynthesis (μmol CO2 m–2 s–1)
carbon assimilated in photosynthesis, usually more
than half is lost in respiratory processes necessary for
growth and maintenance, but this balance may change
under water stress. For instance, although photosynthesis may decrease up to 100% becoming totally
impaired under severe water shortage, the respiration
rate may either increase or decrease under stress, but
may never become totally impaired (Flexas et al.
2005). Therefore, it is imperative to increase our
knowledge on the physiological bases of regulation of
photosynthesis and respiration under water scarcity in
order to be able to improve plant yield in semiarid
regions and to face the climate change-driven increase
in global aridity.
There has been some controversy regarding the main
physiological targets responsible for photosynthetic
impairment under drought and/or salinity (Flexas and
Medrano 2002, Lawlor and Cornic 2002). However,
there is now substantial consensus that reduced CO2
diffusion from the atmosphere to the site of carboxylation is the main cause for decreased photosynthesis
under most water stress conditions (Chaves and
Oliveira 2004, Flexas et al. 2004a). Such reduced leaf
diffusive capacity is due to at least two components that
are regulated almost simultaneously: stomatal closure
and reduced mesophyll conductance. Although the former has been known for a long time to be one of the first
responses of plants to soil water shortage, the latter has
only recently been recognized as an equally important
cause for reduced CO2 diffusion, under drought (Flexas
et al. 2002, Warren et al. 2004) and salinity (Centritto
et al. 2003). There are some observations suggesting that
fine and rapid regulation of mesophyll conductance to
CO2 in response to varying environmental conditions
could be related to the expression and/or regulation of
plasma membrane aquaporins (Flexas et al. unpublished
data, Hanba et al. 2004, Uehlein et al. 2003). The
variability of mesophyll conductance impairs the traditional use of photosynthesis responses to substomatal
CO2 concentration (AN–Ci curves) as a tool to evaluate
the incidence of metabolic impairment under water
stress conditions (Flexas et al. 2004a). An example of
these limitations is shown with severely stressed grapevines in Fig. 1. Using a typical AN–Ci analysis (Long and
Bernacchi 2003), the maximum velocity of carboxylation (Vc, max) appeared to be 167 mmol m 2 s 1 in irrigated plants and only 9.5 mmol m 2 s 1 in severely
stressed plants showing no net photosynthesis at ambient CO2 and saturating light (Fig. 1A). These results
suggest a 94% inhibition of initial Rubisco activity.
However, when decreased mesophyll conductance to
CO2 is taken into account, and the analysis is performed
on a chloroplast CO2 (Cc) concentration basis (Fig. 1B),
30
25
20
15
10
5
0
0
200
400
Cc
600
800
1000
(μmol mol–1)
Fig. 1. The response of net photosynthesis to (A) substomatal CO2
concentration (Ci) and (B) CO2 concentration chloroplast in wellirrigated (filled symbols) and severely water stressed grapevines (empty
symbols). The curves were performed in plants growing outdoors in 50 l
pots, at 1500 mmol photons m 2 s 1 and 35 C, in June 2005 at the
University of the Balearic Islands. The results shown are of two replicate
plants for each treatment.
then Vc, max resulted 193 mmol m 2 s 1 in control
plants and 173 mmol m 2 s 1 in the stressed ones (i.e.
only a 10% inhibition of Rubisco activity). Recently,
alternative methods for AN–Ci analysis taking into
account variations in mesophyll conductance have
been proposed (Ethier and Livingston 2004, Manter
and Kerrigan 2004).
Although restricted CO2 diffusion across leaves is
likely to be the most usual cause for decreased photosynthesis rates under water stress, metabolic impairment
may also occur, particularly under severe stress.
Combining data from our laboratory and from the literature, we have recently shown that metabolic impairment of photosynthesis only occurs when maximum
Physiol. Plant. 127, 2006
daily stomatal conductance drops below 0.05–0.10 mol
H2O m 2 s 1, regardless of the species analysed and of
the water status of leaf tissues (Bota et al. 2004, Flexas
et al. 2004a, 2004b). Such metabolic impairment is not
orchestrated, but rather global. In this sense, it is
remarkable that all the photochemical (chlorophyll
content and fluorescence estimates of maximum photochemical efficiency) and biochemical (contents of ATP,
RuBP and total soluble protein; activities of Rubisco,
sucrose phosphate synthase, fructose bisphosphatase
and nitrate reductase) components of photosynthesis
analysed up to now share the same stomatal conductance threshold for their eventual impairment under
water stress (Flexas et al. 2004a, 2004b). For Rubisco
at least, the effects of severe water stress on its initial
activity can be reproduced by inducing stomatal closure
to levels below the indicated threshold trough addition
of abscisic acid (ABA) to the nutrient solution of
unstressed plants or by cutting the petiole in air to
rapid dehydrate leaves (Flexas et al. unpublished
results). Altogether, the results suggest that metabolic
impairment of photosynthesis under severe stress is
caused by reduced CO2 availability in the mesophyll,
rather than by leaf tissue dehydration.
By using a totally different approach, such as a
photosynthesis limitation analysis, Grassi and Magnani
(2005) have recently confirmed the same patterns of
photosynthesis response to drought in a study during 3
consecutive years in field-grown ash and oak trees. In
their results during summer stress development, whenever gs was higher than 0.1 mol H2O m 2 s 1,
biochemical limitations to photosynthesis were not
detectable, and the sum of limitations imposed by
stomatal closure and decreased mesophyll conductance
accounted for the entire decrease in photosynthesis.
Stomatal conductance accounted for approximately
two-third of the observed decline, whereas mesophyll
conductance accounted for the other one-third of
decrease. At more severe stress conditions, when gs
was below 0.1 mol H2O m 2 s 1, biochemical
limitations were detectable, although they never
accounted for more than 15% of total photosynthetic
limitations.
Studies examining the effects of water stress on
respiration are scarcer than those analysing photosynthetic responses. Generally, the respiration rate
decreases during water stress, due to reduced
photosynthate assimilation and growth needs.
However, this behaviour may be somewhat species
dependent, and respiration rate can also increase, particularly under severe water stress (Flexas et al. 2005,
Ghashghaie et al. 2001). Nevertheless, the total
respiration rates are usually kept within a narrower
Physiol. Plant. 127, 2006
range than those of photosynthesis during water stress
development, resulting in a progressively increased
respiration to photosynthesis ratio, i.e. a decreased
carbon balance (Flexas et al. 2005). Plants possess
two respiratory pathways: the energy-conserving,
cyanide-sensitive, cytochrome pathway and the
energy-wasteful, cyanide-resistant, alternative pathway
(Lambers et al. 2005). We have recently demonstrated
in soybean that severe water stress induces a sharp
decrease in the cytochrome respiration rate concomitant with a similar increase in the alternative
respiration rate, so that total respiration rate remains
quite constant (Ribas-Carbo et al. 2005). Moreover,
these changes are not accompanied by increases in
alternative oxidase protein content, indicating that
these changes may be regarded as a biochemical
regulation. The most surprising aspect is that the
observed changes in mitochondrial electron partitioning
during water stress only occur when stomatal conductance drops below 0.1 mol H2O m 2 s 1, thus in
coincidence with the observed threshold for
photosynthesis metabolic impairment. The gs threshold coincidence for decreased leaf ATP contents
and increased activity of ATP non-synthesizing alternative oxidase (see mirror patterns in Fig. 2A) suggests
that increased alternative respiration pathway may
account for, at least partly, the decrease in ATP
contents. This view may be regarded as opposite to
the drought-induced impairment of chloroplast
photophosphorylation proposed by Tezara et al.
(1999).
In sum, the response of photosynthesis to soil water
shortage can be divided into two distinct phases: during
the first stage, characterized by daily maximum stomatal
conductances above 0.05–0.10 mol H2O m 2 s 1,
photosynthesis is mostly limited by restricted CO2 diffusion (decreased stomatal conductance plus decreased
mesophyll conductance); during the second stage, characterized by stomatal conductances below that threshold, a general metabolic impairment eventually occurs
(i.e. depending on the species and conditions). Plant
respiration rate, by contrast, remains within narrower
ranges during stress, but the electron partitioning
between the cytochrome and alternative pathways also
changes in coincidence with the stomatal conductance
threshold for photosynthetic impairment. Within this
frame, some questions remain unanswered, which will
be the focus of this article:
(1) What are the causes for the simultaneous photochemical and biochemical dysfunction of photosynthesis
under severe water stress? Moreover, as mentioned earlier, because respiratory metabolism can interfere with
photosynthetic metabolism, and the change in electron
345
% of Controls (ATP)
or % partitioning (Altox)
120
ATP vs respiration partitioning
A
Catalase activity
B
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100
40
50
20
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
250
GR activity
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
C
APX activity
D
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500
200
% of Controls
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400
150
300
100
200
50
100
% of Controls
0
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1200
700
α-Tocopherol content
E
F
SOD activity
600
1000
500
800
400
600
300
400
200
200
100
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Stomatal conductance (mol H2O m–2 s–1)
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Stomatal conductance (mol H2O m–2 s–1)
Fig. 2. Literature survey on the relationship during drought or salinity between stomatal conductance and (A) electron partitioning during respiration
(empty symbols) and ATP content (filled symbols) (B–E) the activity of several antioxidant enzymes in leaves (B, catalase; C, glutathione reductase; D,
ascorbate peroxidase; E, superoxide dismutase), and (F) a-tocopherol content. Except for stomatal conductance (mol H2O m 2 s 1), all parameters are
expressed as percentage of maximum values to facilitate comparison, due to the large variability in the units given in the original references. Data for
chloroplast and cytosol SOD are not distinguished in most of the original references. Data have been compiled from the following references. (A)
electron partitioning from Ribas-Carbo et al. (2005), ATP content from Lawlor and Khanna-Chopra (1984) and Tezara et al. (1999, 2002). (B) Mittler
and Zilinskas (1994), Lima et al. (2002), Pinheiro et al. (2004), Jeyaramraja et al. (2005) and Sofo et al. (2005). (C) Loggini et al. (1999), Hernández
et al. (1999), Van Heerden and Krüger 2002), Pinheiro et al. (2004) and Jeyaramraja et al. (2005). (D) Hernández et al. (1999), Lima et al. (2002), Van
Heerden and Krüger (2002), Pinheiro et al. (2004), Jeyaramraja et al. (2005) and Sofo et al. (2005). (E) Mittler and Zilinskas (1994), Hernández et al.
(1999), Lima et al. (2002), Pinheiro et al. (2004), Jeyaramraja et al. (2005) and Sofo et al. (2005). (F) Moran et al. (1994), Bartoli et al. (1999), MunnéBosch et al. (1999, 2003), Munné-Bosch and Alegre (2000), Herbinger et al. (2002) and Ratnayaka et al. (2003). The species included in these studies
are Triticum aestivum, Pisum sativum, Glycine max, Coffea canephora, Melissa officinalis, Cistus clusii, Cistus albidus, Rosmarinus officinalis, Melissa
officinalis, Gossypium hirsutum, Olea europaea, Camelia sinensis and Helianthus annuus.
partitioning during respiration occurs at the same conductance threshold where photosynthetic metabolism is
impaired, can the cyanide-resistant alternative respiratory pathway play any role in the observed photosynthetic responses under severe stress?
346
(2) Provided the strong dependency of photosynthetic
metabolism on stomatal conductance, is there any
opportunity for photosynthesis acclimation to drought
and/or salinity? And if so, which are the main physiological features leading to such acclimation?
Physiol. Plant. 127, 2006
(3) Finally, because whole-life plant carbon balance
depends on the balance between photosynthesis and
respiration not only during the periods of stress imposition, but also during the periods after re-watering, how
fast is photosynthetic recovery on re-watering? Does it
depend on the severity of the previous stress and/or on
acclimation factors?
Causes for simultaneous photochemical and
biochemical dysfunction of photosynthesis
under severe water stress: downregulation
or damage?
This question is important, not only for the improvement
of our basic understanding of why plants decrease
photosynthesis under water stress, but also from a
more practical perspective, because downregulation
may be expected to reverse more rapidly than damage
on re-watering. But, do we have at present any evidence
to distinguish between metabolic downregulation and
damage to the photosynthetic apparatus?
The fact that all photochemical and biochemical parameters analysed up to now share a common gs threshold
for their inhibition under stress does not favour the idea
of downregulation. For instance, if CO2 was the most
limiting factor for photosynthesis under severe stress, a
downregulation of leaf photochemistry may be expected
to balance the light and dark reactions of photosynthesis,
but there may be no apparent reason to downregulate
Rubisco activity or chlorophyll content as well. On the
other hand, below the indicated gs threshold, the metabolic impairment may appear general but stochastic.
Depending on the studies and species analysed, the
impairment of a given metabolic process can vary
between 0 and 100% (Flexas et al. 2004a, 2004b). This
suggests that metabolic impairment may depend on the
environmental conditions and/or on acclimation factors,
but the number of data available is too scarce to define
any clear pattern. A possible explanation, based on the
fact that water stress is often accompanied by high light
intensity, would be that general metabolic disruption
occurs as a consequence of secondary oxidative stress
developed under severe water stress and high light intensity. In this case, impairment may certainly be dependent
on both environmental conditions (i.e. excess light) and
acclimation factors (e.g. antioxidant capacity of leaves).
The following paragraphs are devoted to analyse current
evidences favouring that actually oxidative stress is
occurring at severe water stress and is linked to the
observed metabolic impairment.
First, the fact that mitochondrial electron partitioning
towards the alternative pathway increases in
Physiol. Plant. 127, 2006
coincidence with the gs threshold for photosynthetic
impairment strongly suggests oxidative conditions. The
involvement of increased alternative oxidase activity in
the antioxidant defence of plants has been recognized in
two ways (Lambers et al. 2005, Mittler 2002, Purvis
1997, Wagner and Krab 1995). First, it maintains
mitochondrial electron transport, hence preventing the
formation of reactive oxygen species. Second, it contributes to ascorbate synthesis. Ascorbate is an important
molecule protecting plants from oxidative stress, and its
synthesis is localized in the mitochondria. One of the
enzymes involved in ascorbate synthesis, galactone-glactone dehydrogenase, is an intrinsic component of
mitochondrial complex I (Millar et al. 2003).
Therefore, maintaining the activity of mitochondrial
electron transport chain is essential for plant defence
under oxidative stress, and alternative oxidase may
well accomplish this function under severe water stress,
where impaired growth makes cytochrome pathwayassociated ATP synthesis not viable. Alternatively, it
could be that stress-induced senescence and programmed cell death occur at the indicated gs threshold.
It is also known that plants lacking the alternative
oxidase are more susceptible to programmed cell
death, since alternative oxidase induction prevents it
(Vanlerberghe et al. 2002). Very recently, Bartoli et al.
(2005) could demonstrate that increased alternative
oxidase activity in drought-stressed plants resulted in
enhanced photosynthetic electron transport under
these conditions. The effect was more significant at
high light intensities, which suggested it was actually
due to reduced harmful effects of excess light due to the
enhancement of the alternative pathway.
At the moment of writing previous reviews (Flexas
et al. 2004a, 2004b), the studies available in which the
response of typical antioxidant systems to water stress
was assessed together with gas exchange analysis were
insufficient to provide a clear picture regarding the gs
threshold for their activation in response to stress.
Since then, a few studies have appeared, which allow
now plotting the response of such antioxidant systems
to decreasing gs during water stress imposition (Fig. 2).
As occurred with the components of photosynthetic
metabolism components, a single pattern of response
is revealed regardless of the species analysed and study
conditions. Such a pattern consists in a lack of
response of these processes when gs decreases from a
maximum to about 0.15 mol H2O m 2 s 1, followed
by abrupt increases of the activity of all antioxidant
systems below that threshold. The pattern is much
more clear for those systems increasing five- to 10fold in response to stress (ascorbate peroxidase activity,
347
Acclimated plant
Non-acclimated plant
Plant carbon gain
Irrigated plant
Acclimated plant
A
% Carbon loss
Non-acclimated plant
Time after water stress imposition
B
ETR (% of the controls)
superoxide dismutase activity and a-tocopherol content) than for those responding to a lesser extent (glutathion reductase and catalase activities). The fact that
the gs threshold for depressed photosynthetic metabolism is somewhat smaller than for increased antioxidant response strongly supports the idea that the former
may be a result of oxidative stress occurring in severely
stressed plants in which antioxidant capacity is not
enough to cope with the generation of reactive oxygen
species.
Altogether, there are some evidences supporting the
idea that drought- and/or salt-induced depressions of the
photosynthetic capacity are associated with secondary
oxidative stress developing as a result of combined
water stress and excess light. Further work will be
needed to confirm this hypothesis, including the comparison of plants stressed at different light intensities and
the analysis of the response of transgenic plants differing
in the expression of mitochondrial alternative oxidase or
antioxidant enzymes.
100
90
80
70
60
50
Acclimation of photosynthesis to long-term
drought or salinity: can plants maintain a
positive carbon gain without water?
The photosynthetic responses described above come, in
general, from studies in which water stress was applied
to plants over relatively short periods. Therefore, most of
the observed patterns may correspond to immediate
responses to water stress, which correspond with functional reductions. However, under natural conditions,
water stress develops much more gradually, over periods comprising weeks or months. Acclimation to water
stress comprises all those responses, involving gene
expression and modification of plant physiology and
morphology, and taking place in days to weeks, which
leads to a homeostatic compensation for the initial
negative effects of water stress. Acclimated plants
would improve their water relations and photosynthesis
in respect to non-acclimated plants, which may lead to
lower decreases in carbon gain and growth losses
(Fig. 3A). Despite the obvious importance of the acclimation processes, very few studies have addressed the
question, and hence, the physiological mechanisms
involved in acclimation are mostly unknown.
Responses to prolonged soil water stress include
increases in stomatal conductance and photosynthetic
capacity (Stewart et al. 1994), increased concentration
of several photosynthetic enzymes along with a
decrease in their specific activity (Büssis et al. 1998,
Pankovic et al. 1999) and the maintenance of a high
thylakoid electron transport rate (Pankovic et al. 1999).
However, none of those studies undertook the
348
50
75
100 125 150 175 200 225 250
Cc (μmol mol–1)
Fig. 3. (A) Theoretical diagram showing how photosynthetic acclimation may reduce the impact of water stress on decreased plant carbon
gain. (B) The response of photosynthetic electron transport rate, as
determined by modulated chlorophyll fluorescence analysis, and CO2
concentration in the chloroplasts (estimated as in Flexas et al. 2002) in
tobacco leaves non-acclimated (filled symbols) and acclimated (empty
symbols) to three different levels of water stress. The plants were grown
in a greenhouse in 4 l pots, in spring 2004 at the University of the
Balearic Islands. Plants were irrigated at field capacity until they had
totally developed at least for leaves (non-acclimated leaves). Then they
were subjected to three different water regimes: 100, 40 and 15% field
capacity and maintained at these levels during 6 weeks. New leaves
(acclimated leaves) developed during that period, at the end of which
their photosynthesis was measured and compared with non-acclimated
leaves (these had been measured a few weeks before to ensure similar
leaf age). The results average standard error of six replicates per
treatment.
comparison, under similar stress conditions, of plants
and/or leaves acclimated and not acclimated to water
stress.
To the best of our knowledge, only in two studies this
analysis was provided, and in both, it was suggested that
higher photosynthesis rates in acclimated leaves was
associated with the maintenance of higher electron
transport rates as compared with non-acclimated leaves
(Kitao et al. 2003, Maury et al. 1996). We have
observed, in tobacco plants maintained for 3 weeks at
three different levels of constant soil water deficit, that
the thylakoid electron transport rate was also higher in
Physiol. Plant. 127, 2006
The other side of carbon balance under
semiarid conditions: how fast does
photosynthesis recover after a water stress
period?
The carbon balance of a plant during a period of water
stress and recovery may depend as much on the velocity
and degree of photosynthetic recovery, as it depends on
the degree and velocity of photosynthesis decline during
water depletion (Fig. 4). Surprisingly, since early studies
by Kirschbaum (1987), there is a scarcity of studies analysing the capacity of recovery from different water stress
intensities, as well as evaluating the physiological features limiting recovery. There are some indications suggesting that previous water stress intensity is a crucial
factor affecting both the velocity and the extent of
Physiol. Plant. 127, 2006
Fast
recovery
Slow
recovery
Plant carbon gain
acclimated leaves (i.e. leaves having unfolded and
growth during the water stress period) than in nonacclimated leaves (i.e. leaves having unfolded and
growth prior to the water stress period) at similar values
of chloroplast CO2 concentration (Fig. 3B). This
response may be unexpected, because photosynthesis
under water stress is generally limited by carboxylation,
not by electron transport or RuBP regeneration. On the
other hand, it has been shown that the transcription of
several genes associated with the structure and function
of photosystems is progressively depressed during water
stress, and it increases again on re-watering (Chaves
et al. 2003, Collett et al. 2003).
Such a discrepancy between physiological studies and
gene expression analysis is also apparent regarding acclimation of Rubisco to water stress. Although physiological
determinations in soybean suggest that Rubisco content in
leaves increases with acclimation to stress (Pankovic et al.
1999), other authors have observed that under stress
imposition there is a decrease in the transcription of the
large subunit of Rubisco in pines (Pelloux et al. 2001) and
of the small subunit in tobacco (Kawaguchi et al. 2003).
Clearly, further studies are needed in which detailed physiological analysis is combined with multiple gene expression using, for instance, DNA microarrays. Up to date,
only one study has analysed the relationship between
photosynthetic acclimation and multiple gene expression
in Pinus taeda (Watkinson et al. 2003). The results of this
study showed that there are different patterns of gene
expression depending on the intensity of water stress,
as well as on the number of consecutive cycles of stress
and recovery. Therefore, the acclimation patterns may
respond to a complexity of different conditionings,
whose better understanding may be key to increase the
current knowledge about the limits and mechanisms for
photosynthesis acclimation to water stress.
Incomplete
recovery
Re-watering
Time after water stress imposition
Fig. 4. Theoretical diagram showing how photosynthetic recovery after
water stress may strongly impact plant carbon gain (arrows). Three
different possibilities, of increasing impact on plant carbon gain, are
considered: rapid and total recovery (long dashed arrow), slow but total
recovery (shorted dashed arrow) and slow and incomplete recovery
(dotted arrow). For each treatment, total carbon lost is represented by
the areas formed between the initial solid arrow and the corresponding
recovery arrows. The initial carbon loss during water stress imposition is
equal for all treatments (dotted area); the carbon loss during recovery
depends on it velocity and extent (dark grey – fast recovery; pale grey –
slow recovery). The carbon loss area for plants with incomplete recovery
is not represented, because it depends on the length of the crop cycle
and the percentage of recovery.
recovery after re-watering. For instance, grapevines
subjected to mild water stress (i.e. maximum stomatal
conductance above 0.1 mol H2O m 2 s 1) recover completely during the day after re-watering, whereas more
severely stressed plants recovered slowly during the next
week and did not reach the maximum photosynthesis
rates presented before water stress (Flexas et al. 2004b).
In general, plants subjected to severe water stress recover
only 40–60% of the maximum photosynthesis rate during
the day after re-watering, and recovery continues during
the next days, but maximum photosynthesis rates are not
always recovered (De Souza et al. 2004, Flexas et al.
2004b, Kirschbaum 1987). As suggested for the different
responses observed at severe water stress, secondary oxidative stress could be also affecting the velocity and
extent of recovery, because it has been shown that antioxidant mechanisms raise their concentration and activities on re-watering, and plants presenting higher
activities and/or contents of antioxidants recover maximum photosynthetic rates more rapidly (Mittler and
Zilinskas 1994, Yan et al. 2003).
Recently, the strong influence of previous water stress
severity on the velocity and extent of photosynthesis
recovery has been excellently illustrated in kidney
bean by Miyashita et al. (2005), although the physiological mechanisms limiting recovery were not assessed.
Therefore, current knowledge about physiological limitations to photosynthetic recovery after different water
349
stress intensities and under different environmental conditions is scarce. Acquisition of this knowledge would
be necessary to improve the understanding of plant
responses to drought and salinity, and on the applied
aspect, it could be crucial for the development of watersaving irrigation schedules in agriculture.
Concluding remarks and future prospects
The current state of the art on the regulation of photosynthesis and respiration in response to water stress has
been summarized. It is suggested that in all but very
severe stress situations, diffusional limitations to CO2
(stomatal plus mesophyll) account for most of the
observed decreases in photosynthesis during water
stress development. At severe stress, metabolic impairment of photosynthesis also occurs in some cases, probably due to secondary oxidative stress. The response of
respiration to severe water stress consists in increased
electron partitioning to alternative oxidase and
decreased cytochrome pathway, with little effect on
total respiration rates.
Besides, a number of important gaps of knowledge
have been highlighted. Here, we propose several
research priorities that, in our opinion, would help
advance in the understanding of photosynthetic
response to drought and salinity:
(1) Understanding why metabolic impairment at severe
water stress is global rather than orchestrated and why it
is eventual (i.e. occurring in some studies but not in
others). Although a role of secondary oxidative stress
has been hypothesized, it would be necessary to prove
it, ideally by comparing the response of transgenic plants
differing in their antioxidant capacities. It would also be
necessary to analyse the role of respiration (particularly of
alternative respiration) in the maintenance/regulation of
photosynthesis under water stress. Again, transgenic
plants differing in the expression of alternative oxidase
would be a key instrument to achieve this goal.
(2) Combining detailed physiological analysis with
multiple gene expression during water stress imposition
at different intensities, acclimation and recovery periods. Increased availability of DNA microarray techniques may provide tools for this kind of analysis. A
multiplicity of such studies under different conditions
and using different species may help unravelling the
limits and the physiological mechanisms involved in
photosynthesis acclimation of plants to water stress
situations differing in intensity, duration and so on.
(3) Analysing the recovery of different photosynthetic
components on re-watering from different water stress
intensities in different plants and conditions. This
350
knowledge would be of importance for the development
of deficit irrigation programs, as well as for improving
the accuracy of ecosystem productivity predictions from
climate data.
Acknowledgements – This work has been partially financed
by CICYT Projects BFI2002-00772 and BFU2005-03102/BFI
(Plan Nacional, Spain). We are indebted to Prof. Hans
Lambers and all the participants of the Workshop ‘Plant
Water Relations in Seasonally Dry Environments’ for fruitful
discussion.
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