REVIEW

Keeping a positive carbon balance under adverse conditions:responses of photosynthesis and respiration to water stressJaume Flexas*, Josefina Bota, Jeroni Galmes, Hipolito Medrano and Miquel Ribas-Carbo

Grup de Recerca en Biologia de les Plantes en Condicions Mediterranies, 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: jaume.flexas@uib.es

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 factorslimiting photosynthesis and respiration and, consequently, plant growth. Thisreview summarizes the current status of knowledge on photosynthesis andrespiration under water stress. It is shown that diffusion limitations to photosynth-esis under most water stress conditions are predominant, involving decreasedmesophyll conductance to CO2, an important but often neglected process. Ageneral failure of photochemistry and biochemistry, by contrast, can occur onlywhen daily maximum stomatal conductance (gs) drops below 0.05–0.10 molH2O m�2 s�1. Because these changes are preceded by increased leaf antioxidantactivities (gs below 0.15–0.20 mol H2O m�2 s�1), it is suggested that metabolicresponses 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 remark-able that the electron partitioning towards the alternative respiration pathwaysharply increases at the same gs threshold, although total respiration rates are lessaffected. Despite the considerable improvement in the understanding of plantresponses to drought, several gaps of knowledge are highlighted which shouldbecome research priorities for the near future. These include how respiration andphotosynthesis interact at severe stress, what are the boundaries and mechanismsof photosynthetic acclimation to water stress and what are the factors leading todifferent rates of recovery after a stress period.

Photosynthesis and respiration responses todrought and salinity

Both drought and salinity stresses reduce the capacity ofplants to take up water from the soil (Munns 2002). Atpresent, low water availability is the main environmentalfactor limiting plant growth and yield worldwide, andglobal change will likely make water scarcity an evengreater limitation to plant productivity across an increasingamount of land (Chaves et al. 2003, Hamdy et al. 2003).

The limitation of plant growth imposed by low wateravailability is mainly due to reductions in plant carbonbalance, which is dependent on the balance betweenphotosynthesis and respiration. Both processes are intim-ately linked. For instance, it has been shown that trans-genic plants with modified respiration also alter theirphotosynthetic behaviour (Dutilleul et al. 2003, Nunes-Nesi et al. 2005), and an increased respiration rateseems necessary for photosynthesis recovery after a per-iod of water stress (Kirschbaum 1988). Of the total

Abbreviations – ABA, abscisic acid; Altox, alternative oxidase; AN, light-saturated net photosynthesis; APX, ascorbate perox-

idase; 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

Physiologia Plantarum 127: 343–352. 2006 Copyright � Physiologia Plantarum 2006, ISSN 0031-9317

carbon assimilated in photosynthesis, usually morethan half is lost in respiratory processes necessary forgrowth and maintenance, but this balance may changeunder water stress. For instance, although photosynth-esis may decrease up to 100% becoming totallyimpaired under severe water shortage, the respirationrate may either increase or decrease under stress, butmay never become totally impaired (Flexas et al.2005). Therefore, it is imperative to increase ourknowledge on the physiological bases of regulation ofphotosynthesis and respiration under water scarcity inorder to be able to improve plant yield in semiaridregions and to face the climate change-driven increasein global aridity.

There has been some controversy regarding the mainphysiological targets responsible for photosyntheticimpairment under drought and/or salinity (Flexas andMedrano 2002, Lawlor and Cornic 2002). However,there is now substantial consensus that reduced CO2

diffusion from the atmosphere to the site of carboxyla-tion is the main cause for decreased photosynthesisunder most water stress conditions (Chaves andOliveira 2004, Flexas et al. 2004a). Such reduced leafdiffusive capacity is due to at least two components thatare regulated almost simultaneously: stomatal closureand reduced mesophyll conductance. Although the for-mer has been known for a long time to be one of the firstresponses of plants to soil water shortage, the latter hasonly recently been recognized as an equally importantcause for reduced CO2 diffusion, under drought (Flexaset al. 2002, Warren et al. 2004) and salinity (Centrittoet al. 2003). There are some observations suggesting thatfine and rapid regulation of mesophyll conductance toCO2 in response to varying environmental conditionscould be related to the expression and/or regulation ofplasma membrane aquaporins (Flexas et al. unpublisheddata, Hanba et al. 2004, Uehlein et al. 2003). Thevariability of mesophyll conductance impairs the tradi-tional use of photosynthesis responses to substomatalCO2 concentration (AN–Ci curves) as a tool to evaluatethe incidence of metabolic impairment under waterstress conditions (Flexas et al. 2004a). An example ofthese limitations is shown with severely stressed grape-vines in Fig. 1. Using a typical AN–Ci analysis (Long andBernacchi 2003), the maximum velocity of carboxyla-tion (Vc, max) appeared to be 167 mmol m�2 s�1 in irri-gated plants and only 9.5 mmol m�2 s�1 in severelystressed plants showing no net photosynthesis at ambi-ent CO2 and saturating light (Fig. 1A). These resultssuggest a 94% inhibition of initial Rubisco activity.However, when decreased mesophyll conductance toCO2 is taken into account, and the analysis is performedon a chloroplast CO2 (Cc) concentration basis (Fig. 1B),

then Vc, max resulted 193 mmol m�2 s�1 in controlplants 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 intoaccount variations in mesophyll conductance havebeen proposed (Ethier and Livingston 2004, Manterand Kerrigan 2004).

Although restricted CO2 diffusion across leaves islikely to be the most usual cause for decreased photo-synthesis rates under water stress, metabolic impairmentmay also occur, particularly under severe stress.Combining data from our laboratory and from the litera-ture, we have recently shown that metabolic impair-ment of photosynthesis only occurs when maximum

A

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Fig. 1. The response of net photosynthesis to (A) substomatal CO2

concentration (Ci) and (B) CO2 concentration chloroplast in well-

irrigated (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.

344 Physiol. Plant. 127, 2006

daily stomatal conductance drops below 0.05–0.10 molH2O m�2 s�1, regardless of the species analysed and ofthe water status of leaf tissues (Bota et al. 2004, Flexaset al. 2004a, 2004b). Such metabolic impairment is notorchestrated, but rather global. In this sense, it isremarkable that all the photochemical (chlorophyllcontent and fluorescence estimates of maximum photo-chemical efficiency) and biochemical (contents of ATP,RuBP and total soluble protein; activities of Rubisco,sucrose phosphate synthase, fructose bisphosphataseand nitrate reductase) components of photosynthesisanalysed up to now share the same stomatal conduc-tance threshold for their eventual impairment underwater stress (Flexas et al. 2004a, 2004b). For Rubiscoat least, the effects of severe water stress on its initialactivity can be reproduced by inducing stomatal closureto levels below the indicated threshold trough additionof abscisic acid (ABA) to the nutrient solution ofunstressed plants or by cutting the petiole in air torapid dehydrate leaves (Flexas et al. unpublishedresults). Altogether, the results suggest that metabolicimpairment of photosynthesis under severe stress iscaused by reduced CO2 availability in the mesophyll,rather than by leaf tissue dehydration.

By using a totally different approach, such as aphotosynthesis limitation analysis, Grassi and Magnani(2005) have recently confirmed the same patterns ofphotosynthesis response to drought in a study during 3consecutive years in field-grown ash and oak trees. Intheir results during summer stress development, when-ever gs was higher than 0.1 mol H2O m�2 s�1,biochemical limitations to photosynthesis were notdetectable, and the sum of limitations imposed bystomatal closure and decreased mesophyll conductanceaccounted for the entire decrease in photosynthesis.Stomatal conductance accounted for approximatelytwo-third of the observed decline, whereas mesophyllconductance accounted for the other one-third ofdecrease. At more severe stress conditions, when gs

was below 0.1 mol H2O m�2 s�1, biochemicallimitations were detectable, although they neveraccounted for more than 15% of total photosyntheticlimitations.

Studies examining the effects of water stress onrespiration are scarcer than those analysing photosyn-thetic responses. Generally, the respiration ratedecreases during water stress, due to reducedphotosynthate assimilation and growth needs.However, this behaviour may be somewhat speciesdependent, and respiration rate can also increase, parti-cularly under severe water stress (Flexas et al. 2005,Ghashghaie et al. 2001). Nevertheless, the totalrespiration rates are usually kept within a narrower

range than those of photosynthesis during water stressdevelopment, resulting in a progressively increasedrespiration to photosynthesis ratio, i.e. a decreasedcarbon balance (Flexas et al. 2005). Plants possesstwo respiratory pathways: the energy-conserving,cyanide-sensitive, cytochrome pathway and theenergy-wasteful, cyanide-resistant, alternative pathway(Lambers et al. 2005). We have recently demonstratedin soybean that severe water stress induces a sharpdecrease in the cytochrome respiration rate con-comitant with a similar increase in the alternativerespiration rate, so that total respiration rate remainsquite constant (Ribas-Carbo et al. 2005). Moreover,these changes are not accompanied by increases inalternative oxidase protein content, indicating thatthese changes may be regarded as a biochemicalregulation. The most surprising aspect is that theobserved changes in mitochondrial electron partitioningduring water stress only occur when stomatal con-ductance drops below 0.1 mol H2O m�2 s�1, thus incoincidence with the observed threshold forphotosynthesis metabolic impairment. The gs thres-hold coincidence for decreased leaf ATP contentsand increased activity of ATP non-synthesizing alterna-tive oxidase (see mirror patterns in Fig. 2A) suggeststhat increased alternative respiration pathway mayaccount for, at least partly, the decrease in ATPcontents. This view may be regarded as opposite tothe drought-induced impairment of chloroplastphotophosphorylation proposed by Tezara et al.(1999).

In sum, the response of photosynthesis to soil watershortage can be divided into two distinct phases: duringthe first stage, characterized by daily maximum stomatalconductances above 0.05–0.10 mol H2O m�2 s�1,photosynthesis is mostly limited by restricted CO2 diffu-sion (decreased stomatal conductance plus decreasedmesophyll conductance); during the second stage, char-acterized by stomatal conductances below that thresh-old, a general metabolic impairment eventually occurs(i.e. depending on the species and conditions). Plantrespiration rate, by contrast, remains within narrowerranges during stress, but the electron partitioningbetween the cytochrome and alternative pathways alsochanges in coincidence with the stomatal conductancethreshold for photosynthetic impairment. Within thisframe, some questions remain unanswered, which willbe the focus of this article:(1) What are the causes for the simultaneous photoche-mical and biochemical dysfunction of photosynthesisunder severe water stress? Moreover, as mentioned ear-lier, because respiratory metabolism can interfere withphotosynthetic metabolism, and the change in electron

Physiol. Plant. 127, 2006 345

partitioning during respiration occurs at the same con-ductance threshold where photosynthetic metabolism isimpaired, can the cyanide-resistant alternative respira-tory pathway play any role in the observed photosyn-thetic responses under severe stress?

(2) Provided the strong dependency of photosyntheticmetabolism on stomatal conductance, is there anyopportunity for photosynthesis acclimation to droughtand/or salinity? And if so, which are the main physiolo-gical features leading to such acclimation?

SOD activity

Stomatal conductance (mol H2O m–2 s–1) Stomatal conductance (mol H2O m–2 s–1)

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C D

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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

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

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

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ltox)

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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), Hernandez

et al. (1999), Van Heerden and Kruger 2002), Pinheiro et al. (2004) and Jeyaramraja et al. (2005). (D) Hernandez et al. (1999), Lima et al. (2002), Van

Heerden and Kruger (2002), Pinheiro et al. (2004), Jeyaramraja et al. (2005) and Sofo et al. (2005). (E) Mittler and Zilinskas (1994), Hernandez 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), Munne-

Bosch et al. (1999, 2003), Munne-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.

346 Physiol. Plant. 127, 2006

(3) Finally, because whole-life plant carbon balancedepends on the balance between photosynthesis andrespiration not only during the periods of stress imposi-tion, but also during the periods after re-watering, howfast is photosynthetic recovery on re-watering? Does itdepend on the severity of the previous stress and/or onacclimation factors?

Causes for simultaneous photochemical andbiochemical dysfunction of photosynthesisunder severe water stress: downregulationor 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 para-

meters analysed up to now share a common gs thresholdfor their inhibition under stress does not favour the ideaof downregulation. For instance, if CO2 was the mostlimiting factor for photosynthesis under severe stress, adownregulation of leaf photochemistry may be expectedto balance the light and dark reactions of photosynthesis,but there may be no apparent reason to downregulateRubisco activity or chlorophyll content as well. On theother hand, below the indicated gs threshold, the meta-bolic impairment may appear general but stochastic.Depending on the studies and species analysed, theimpairment of a given metabolic process can varybetween 0 and 100% (Flexas et al. 2004a, 2004b). Thissuggests that metabolic impairment may depend on theenvironmental conditions and/or on acclimation factors,but the number of data available is too scarce to defineany clear pattern. A possible explanation, based on thefact that water stress is often accompanied by high lightintensity, would be that general metabolic disruptionoccurs as a consequence of secondary oxidative stressdeveloped under severe water stress and high light inten-sity. In this case, impairment may certainly be dependenton both environmental conditions (i.e. excess light) andacclimation factors (e.g. antioxidant capacity of leaves).The following paragraphs are devoted to analyse currentevidences favouring that actually oxidative stress isoccurring at severe water stress and is linked to theobserved metabolic impairment.

First, the fact that mitochondrial electron partitioning

towards the alternative pathway increases in

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 contri-

butes to ascorbate synthesis. Ascorbate is an importantmolecule protecting plants from oxidative stress, and itssynthesis is localized in the mitochondria. One of theenzymes involved in ascorbate synthesis, galactone-g-lactone dehydrogenase, is an intrinsic component ofmitochondrial complex I (Millar et al. 2003).Therefore, maintaining the activity of mitochondrialelectron transport chain is essential for plant defenceunder oxidative stress, and alternative oxidase maywell accomplish this function under severe water stress,where impaired growth makes cytochrome pathway-associated ATP synthesis not viable. Alternatively, itcould be that stress-induced senescence and pro-grammed cell death occur at the indicated gs threshold.It is also known that plants lacking the alternativeoxidase are more susceptible to programmed celldeath, since alternative oxidase induction prevents it(Vanlerberghe et al. 2002). Very recently, Bartoli et al.(2005) could demonstrate that increased alternativeoxidase activity in drought-stressed plants resulted inenhanced photosynthetic electron transport underthese conditions. The effect was more significant athigh light intensities, which suggested it was actuallydue to reduced harmful effects of excess light due to theenhancement of the alternative pathway.

At the moment of writing previous reviews (Flexaset al. 2004a, 2004b), the studies available in which theresponse of typical antioxidant systems to water stresswas assessed together with gas exchange analysis wereinsufficient to provide a clear picture regarding the gs

threshold for their activation in response to stress.Since then, a few studies have appeared, which allownow plotting the response of such antioxidant systemsto decreasing gs during water stress imposition (Fig. 2).As occurred with the components of photosyntheticmetabolism components, a single pattern of responseis revealed regardless of the species analysed and studyconditions. Such a pattern consists in a lack ofresponse of these processes when gs decreases from amaximum to about 0.15 mol H2O m�2 s�1, followedby abrupt increases of the activity of all antioxidantsystems below that threshold. The pattern is muchmore clear for those systems increasing five- to 10-fold in response to stress (ascorbate peroxidase activity,

Physiol. Plant. 127, 2006 347

superoxide dismutase activity and a-tocopherol con-tent) than for those responding to a lesser extent (glu-tathion reductase and catalase activities). The fact thatthe gs threshold for depressed photosynthetic metabo-lism is somewhat smaller than for increased antioxi-dant response strongly supports the idea that the formermay be a result of oxidative stress occurring in severelystressed plants in which antioxidant capacity is notenough to cope with the generation of reactive oxygenspecies.

Altogether, there are some evidences supporting theidea that drought- and/or salt-induced depressions of thephotosynthetic capacity are associated with secondaryoxidative stress developing as a result of combinedwater stress and excess light. Further work will beneeded to confirm this hypothesis, including the com-parison of plants stressed at different light intensities andthe analysis of the response of transgenic plants differingin the expression of mitochondrial alternative oxidase orantioxidant enzymes.

Acclimation of photosynthesis to long-termdrought or salinity: can plants maintain apositive carbon gain without water?

The photosynthetic responses described above come, ingeneral, from studies in which water stress was appliedto plants over relatively short periods. Therefore, most ofthe observed patterns may correspond to immediateresponses to water stress, which correspond with func-tional reductions. However, under natural conditions,water stress develops much more gradually, over peri-ods comprising weeks or months. Acclimation to waterstress comprises all those responses, involving geneexpression and modification of plant physiology andmorphology, and taking place in days to weeks, whichleads to a homeostatic compensation for the initialnegative effects of water stress. Acclimated plantswould improve their water relations and photosynthesisin respect to non-acclimated plants, which may lead tolower decreases in carbon gain and growth losses(Fig. 3A). Despite the obvious importance of the accli-mation processes, very few studies have addressed thequestion, and hence, the physiological mechanismsinvolved in acclimation are mostly unknown.

Responses to prolonged soil water stress includeincreases in stomatal conductance and photosyntheticcapacity (Stewart et al. 1994), increased concentrationof several photosynthetic enzymes along with adecrease in their specific activity (Bussis et al. 1998,Pankovic et al. 1999) and the maintenance of a highthylakoid electron transport rate (Pankovic et al. 1999).However, none of those studies undertook the

comparison, under similar stress conditions, of plantsand/or leaves acclimated and not acclimated to waterstress.

To the best of our knowledge, only in two studies thisanalysis was provided, and in both, it was suggested thathigher photosynthesis rates in acclimated leaves wasassociated with the maintenance of higher electrontransport rates as compared with non-acclimated leaves(Kitao et al. 2003, Maury et al. 1996). We haveobserved, in tobacco plants maintained for 3 weeks atthree different levels of constant soil water deficit, thatthe thylakoid electron transport rate was also higher in

Cc (μmol mol–1)

50 75 100 125 150 175 200 225 250

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R (

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arbon lossAcc

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Irrigated plant

Non-acclimated plant

Fig. 3. (A) Theoretical diagram showing how photosynthetic acclima-

tion 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.

348 Physiol. Plant. 127, 2006

acclimated leaves (i.e. leaves having unfolded andgrowth during the water stress period) than in non-acclimated leaves (i.e. leaves having unfolded andgrowth prior to the water stress period) at similar valuesof chloroplast CO2 concentration (Fig. 3B). Thisresponse may be unexpected, because photosynthesisunder water stress is generally limited by carboxylation,not by electron transport or RuBP regeneration. On theother hand, it has been shown that the transcription ofseveral genes associated with the structure and functionof photosystems is progressively depressed during waterstress, and it increases again on re-watering (Chaveset al. 2003, Collett et al. 2003).

Such a discrepancy between physiological studies andgene expression analysis is also apparent regarding accli-mation of Rubisco to water stress. Although physiologicaldeterminations in soybean suggest that Rubisco content inleaves increases with acclimation to stress (Pankovic et al.1999), other authors have observed that under stressimposition there is a decrease in the transcription of thelarge subunit of Rubisco in pines (Pelloux et al. 2001) andof the small subunit in tobacco (Kawaguchi et al. 2003).Clearly, further studies are needed in which detailed phy-siological analysis is combined with multiple gene expres-sion using, for instance, DNA microarrays. Up to date,only one study has analysed the relationship betweenphotosynthetic acclimation and multiple gene expressionin Pinus taeda (Watkinson et al. 2003). The results of thisstudy showed that there are different patterns of geneexpression depending on the intensity of water stress,as well as on the number of consecutive cycles of stressand recovery. Therefore, the acclimation patterns mayrespond to a complexity of different conditionings,whose better understanding may be key to increase thecurrent knowledge about the limits and mechanisms forphotosynthesis acclimation to water stress.

The other side of carbon balance undersemiarid conditions: how fast doesphotosynthesis recover after a water stressperiod?

The carbon balance of a plant during a period of waterstress and recovery may depend as much on the velocityand degree of photosynthetic recovery, as it depends onthe degree and velocity of photosynthesis decline duringwater depletion (Fig. 4). Surprisingly, since early studiesby Kirschbaum (1987), there is a scarcity of studies ana-lysing the capacity of recovery from different water stressintensities, as well as evaluating the physiological fea-tures limiting recovery. There are some indications sug-gesting that previous water stress intensity is a crucialfactor affecting both the velocity and the extent of

recovery after re-watering. For instance, grapevinessubjected to mild water stress (i.e. maximum stomatalconductance above 0.1 mol H2O m�2 s�1) recover com-pletely during the day after re-watering, whereas moreseverely stressed plants recovered slowly during the nextweek and did not reach the maximum photosynthesisrates presented before water stress (Flexas et al. 2004b).In general, plants subjected to severe water stress recoveronly 40–60% of the maximum photosynthesis rate duringthe day after re-watering, and recovery continues duringthe next days, but maximum photosynthesis rates are notalways recovered (De Souza et al. 2004, Flexas et al.2004b, Kirschbaum 1987). As suggested for the differentresponses observed at severe water stress, secondary oxi-dative stress could be also affecting the velocity andextent of recovery, because it has been shown that anti-oxidant mechanisms raise their concentration and activ-ities on re-watering, and plants presenting higheractivities and/or contents of antioxidants recover maxi-mum photosynthetic rates more rapidly (Mittler andZilinskas 1994, Yan et al. 2003).

Recently, the strong influence of previous water stressseverity on the velocity and extent of photosynthesisrecovery has been excellently illustrated in kidneybean by Miyashita et al. (2005), although the physiolo-gical mechanisms limiting recovery were not assessed.Therefore, current knowledge about physiological lim-itations to photosynthetic recovery after different water

Time after water stress imposition

Pla

nt c

arbo

n ga

in

Re-watering

Fast recovery

Slow recovery

Incompleterecovery

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.

Physiol. Plant. 127, 2006 349

stress intensities and under different environmental con-ditions is scarce. Acquisition of this knowledge wouldbe necessary to improve the understanding of plantresponses to drought and salinity, and on the appliedaspect, it could be crucial for the development of water-saving irrigation schedules in agriculture.

Concluding remarks and future prospects

The current state of the art on the regulation of photo-synthesis and respiration in response to water stress hasbeen summarized. It is suggested that in all but verysevere stress situations, diffusional limitations to CO2

(stomatal plus mesophyll) account for most of theobserved decreases in photosynthesis during waterstress development. At severe stress, metabolic impair-ment of photosynthesis also occurs in some cases, prob-ably due to secondary oxidative stress. The response ofrespiration to severe water stress consists in increasedelectron partitioning to alternative oxidase anddecreased cytochrome pathway, with little effect ontotal respiration rates.

Besides, a number of important gaps of knowledgehave been highlighted. Here, we propose severalresearch priorities that, in our opinion, would helpadvance in the understanding of photosyntheticresponse to drought and salinity:(1) Understanding why metabolic impairment at severewater stress is global rather than orchestrated and why itis eventual (i.e. occurring in some studies but not inothers). Although a role of secondary oxidative stresshas been hypothesized, it would be necessary to proveit, ideally by comparing the response of transgenic plantsdiffering in their antioxidant capacities. It would also benecessary to analyse the role of respiration (particularly ofalternative respiration) in the maintenance/regulation ofphotosynthesis under water stress. Again, transgenicplants differing in the expression of alternative oxidasewould be a key instrument to achieve this goal.(2) Combining detailed physiological analysis withmultiple gene expression during water stress impositionat different intensities, acclimation and recovery peri-ods. Increased availability of DNA microarray techni-ques may provide tools for this kind of analysis. Amultiplicity of such studies under different conditionsand using different species may help unravelling thelimits and the physiological mechanisms involved inphotosynthesis acclimation of plants to water stresssituations differing in intensity, duration and so on.(3) Analysing the recovery of different photosyntheticcomponents on re-watering from different water stressintensities in different plants and conditions. This

knowledge would be of importance for the developmentof deficit irrigation programs, as well as for improvingthe accuracy of ecosystem productivity predictions fromclimate 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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Edited by V. Hurry

352 Physiol. Plant. 127, 2006