Báo cáo khoa học: "Stomatal and non stomatal limitation of photosynthesis by leaf water deficits in three oak species: a comparison of gas exchange and chlorophyll a fluorescence data"

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Original article stomatal limitation Stomatal and non of photosynthesis by leaf water deficits in three oak species: a comparison of gas exchange and chlorophyll a fluorescence data E D Dreyer Epron INRA-Nancy, Laboratoire de Bioclimatologie et d’Écophysiologie Forestières, Sylviculture et Production, Champenoux, F-54280 Seichamps, France Station de (Received 5 April 1990; accepted 6 June 1990) Summary — Net CO assimilation (A), stomatal conductance for CO (g), intercellular mole fraction 2 2 of CO (C kinetics of chlorophyll a fluorescence, and their half decay time (t their ratio of fluo- ), 1/2 2i ), decrease (R and their adaptive index (A have been monitored on potted trees from 3 ), fd ) p rescence species (Quercus petraea, Q pubescens and Q ilex) grown in a climate chamber and submitted oak to drought. Use of A vs C representations for photosynthesis data revealed an apparent impairment i of mesophyll photosynthesis, together with reduced CO supply to mesophyll due to stomatal clo- 2 sure. But in all species chlorophyll a fluorescence kinetics displayed very similar shapes, constant and stable R and A values until predawn leaf water potential dropped below -4.0 MPa. These fd p 1/2 t observations led to the conclusion that photochemical energy conversion and photosynthetic carbon cycle could be very resistant to leaf water deficits, and that observed decreases in meso- reduction phyll photosynthesis had to be attributed to a possible artefact in C calculation. On the other hand, i the susceptibility of leaves to photoinhibition increased as a consequence of water shortage, espe- cially in Q petraea and Q pubescens. Differences in drought adaptation between the studied species could probably be related to susceptibility to photoinhibition rather than to a direct sensitivity of pho- tosynthesis to leaf water deficits, at least in the range of stress intensities of ecophysiological signifi- cance. fluorescence / oak / stomatal conductance / / chlorophyll photosynthesis / water stress a drought / photoinhibition Résumé — Limitation d’origine stomatique et non stomatique de la photosynthèse de trois de chêne soumises à la sécheresse : comparaison de mesures d’échanges gazeux espèces et de fluorescence de la chlorophylle. Les échanges gazeux foliaires et la fluorescence de la * Correspondence and reprints. Abbreviations : A net CO assimilation rate; A A at saturating Ci; A adaptative index; C i 2 max = = = = t F maximal and terminal intercellular CO molar fraction; dA/dCi carboxylation efficiency; F and p 2 = = fluorescence levels; g stomatal conductance for CO LWC leaf water content; P inorganic i ; 2 = = = phosphate; PPFD photosynthetic photon flux density; PSII and PSI photosystem II and I; R fd = = = ratio of fluorescence decrease; t fluorescence half-decay time; a apparent quantum yield of 1/2 = = photosynthesis; ψ predawn leaf water potential; Δw : leaf to air water vapour molar fraction differ- = wp ence
chlorophylle ont été étudiés lors d’une sécheresse édaphique imposée en conditions contrôlées, sur de jeunes plants de Quercus patraea, Q pubescens et Q ilex. L’analyse des relations entre assimila- tion nette de CO (A) et fraction molaire intercellulaire calculée de CO (C semble indiquer que 2 2i ) l’inhibition de A résulté à la fois d’une fermeture des stomates, mais aussi d’une altération des pro- a mésophylliens de la photosynthèse. Par contre, la forme des cinétiques de fluorescence de la cessus chlorophylle réalisées in vivo ainsi que les valeurs de t (temps de demi décroissance), R (rapport fd 1/2 de décroissance de fluorescence) ou de A (index d’adaptation) n’ont pas été affectées tant que le p déficit hydrique foliaire n’avait pas atteint un niveau élevé (potentiel hydrique de base inférieur à -4,0 MPa). Ceci semble indiquer une grande résistance de l’appareil photosynthétique au déficit hydrique foliaire. Par contre, l’étude de la réaction de la photosynthèse aux forts éclairements a révélé une sensibilité accrue à la photo-inhibition chez Q petraea et Q pubescens lors d’une sécheresse éda- phique, contrairement à ce qui a été observé pour Q ilex. Les différences d’adaptation à la séche- resse existant en conditions naturelles entre ces 3 espèces pourraient être due à une sensibilité ac- crue à la photo-inhibition plutôt qu’à une sensibilité directe de l’appareil photosynthétique au déssèchement foliaire, du moins dans la gamme des déssèchements les plus fréquemment rencon- trés conditions naturelles. en / fluorescence / chêne / conductance / séche- photosynthèse / stress hydrique stomatique resse / photo-inhibition INTRODUCTION Some studies with chloroplastic suspen- sions or enzyme extracts have reported the occurrence of both reductions in photo- European oak species grow in habitats dif- chemical processes (Boyer, 1976) and in fering widely in the frequence of drought ribulose-biphosphate carboxylase-oxygen- Quercus petraea (subgenus occurrence. activity (Vu et al, 1987). Lepidobalanus section robur), as a meso- ase phytic mid European species is rather sen- Leaf gas exchange measurements and sitive to water shortage, while Q pubes- analysis using diffusion models (Jones, cens (subgenus Lepidobalanus section 1973, 1985; Farquhar and Sharkey, 1982) robur) grows in much drier soils. Q ilex have frequently led to the result that leaf (subgenus Lepidobalanus section ilex), a water deficits impair both mesophyll ability Mediterranean sclerophyllous xerophyte, and diffusion to to assimilate , 2 CO 2 CO is sometimes submitted to long periods of chloroplasts (Jones and Fanjul, 1983; Tes- water deficits accompanied by high levels key et al, 1986; Cornic et al, 1987; Grieu et of solar irradiance. al, 1988). In these studies, net assimilation was analysed as a function of calculated Differences in drought tolerance be- intercellular CO mole fraction (C in al- ); i 2 tween species may be partly due to differ- most all stress situations, reductions ential sensitivities of photosynthetic pro- seemed to occur at fairly constant C val- i cesses in leaves to tissue dehydration. But ues, therefore displaying both diffusional it is still unclear whether water shortage and biochemical limitations of photosynthe- and resulting leaf water deficits have direct sis (Jones, 1973, 1985; Comic et al, 1983). effect on the mesophyll processes of pho- However, recent results suggest that this tosynthesis (photochemical energy conver- model may be misleading, due to artefacts sion and/or carbon metabolism), or only in- in C (Terashima et al, 1988). icalculation direct effects via stomatal closure and subsequent limitations of CO diffusion to In order to test potential limitations in- 2 duced by water stress on carbon assimila- chloroplasts.
Avignon, France) and Q pubes- Ventoux, tion of leaves in vivo on our 3 oak species, near Willd(seed origin: Mont Ventoux). cens we compared the results obtained with gas Three-year-old (Q pubescens and Q ilex) or exchange measurements and with chloro- 4-year-old (Q petraea) saplings were grown in phyll a fluorescence kinetics. 7-I plastic pots on a 1:1 (v/v) mixture of brown fluorescence Chlorophyll kinetics, a peat and sandy soil, in a naturally illuminated based the Kautsky effect, allow the as- greenhouse; they were fertilised 4 times a year on during the growing season with a complete nutri- be made of possible impair- sessment to ent solution (N,P,K; 7,6,9; Solugene), and were ments in: watered twice week with deionized water. a energy conversion at PSII level (variable - fluorescence); and in the transfer of electrons from the first Experimental time course - to the photosynthetic carbon re- acceptors duction cycle (fluorescence decrease) One week before each experiment, the plants (Krause and Weis, 1984; Briantais et al, were transferred to a growth cabinet with follow- 1986). In this study, we analysed the ing day/night conditions: 16/8 h; air temperature, shapes of fluorescence decrease which is 22/16 °C; relative humidity, 70/95 %. Photosyn- thetic photon flux density (PPFD) at the top of related to the onset of both photochemical the plants was maintained at 300 μmol m s -2 -1 and non photochemical quenching, and provided by neon lamps. Ambient CO molar2 calculated the half decay time t the ratio , 1/2 fraction averaged 475 ± 25 μmol mol . -1 of fluorescence decrease (R Lichthen- ; fd Measurements performed during May were thaler et al, 1986) and an adaptative index 1989 for Q pubescens, June 1989 for Q petraea reflecting the degree of integrity of photo- and July 1989 for Q ilex. For each species, 2 synthetic membranes Strasser et al, ; p (A control saplings were watered daily and 4 or 5 1987). In addition, water stress often pro- plants were exposed to water shortage by with- holding irrigation for about 20 d. Small amounts motes susceptibility to photoinhibition of water were added to the pots when needed, (Krause, 1984). Susceptibility to photoin- to avoid death of plants. Predawn leaf water po- hibitory damages has therefore been com- tential, net COassimilation rate and chlorophyll 2 pared in our species and related to the lev- fluorescence kinetics were studied 2 d a week el of drought tolerance. for the water-stressed plants and only 1 d a week for the control. At the end of the stress pe- The aims of these experiments were to riod, a twig of 2 control and of 2 or 3 drought- give an insight into the mechanisms of stressed plants was exposed for 4 h to a PPFD stress reactions, and to compare them in of 2 000 μmol m s provided by a sodium -2 -1 the 3 tree species known for their differ- lamp (SON-T-400W, Philips) in order to assay ences in drought tolerance. susceptibility to photoinhibition. An electric fan was used to prevent thermal injury to the leaves. Apparent quantum yield of photosynthesis (a) and chlorophyll fluorescence were used to quan- MATERIAL AND METHODS tify possible photoinhibitory effects. To investi- gate the effect of rapid dehydration on chloro- phyll fluorescence kinetics, 20 leaf discs were Plant material and growth conditions punched from a twig of a well-watered plant of Q petraea. Five leaf discs were kept on a wet filter paper and 15 were submitted to dehydration in air for several h. This stress treatment was im- The oak species studied were Quercus petraea Liebl (seed origin: Forêt Domaniale d’Amance, posed in darkness at room temperature (≈ near Nancy, France), Q ilex L (seed origin: Mont 20 °C).
PPFD) response curves, C was maintained at a Water relations μmol mol in a 1% O air and PPFD was -1 2 950 from 0 to 100, 200, 300 changed every 30 min μmol m s (A, PPFD) response curves were -2 -1 . Predawn leaf water potential (ψ was meas- ) wp run before and 30 min after the high-illumination ured using a pressure chamber. Leaf water con- tent (LWC) was estimated after over-drying a treatment. leaf disk during 48 h at 60 °C. Each value of As defined by Jones (1973, 1985), (A, C re- ) i LWC is the mean of 3 replicates. sponse curves outline the mesophyll photosyn- thetic capacity (demand functions). The supply functions, defined as the lines with an x-axis in- tercept equal to C E /(g + E/2)] and a neg- [1 - a Gas exchange measurements ative slope equal to -(g + E/2) (Guehl and Aussenac, 1987), give an estimate of diffusive Whole leaf gas exchange was measured in an limitations to COassimilation. Stomatal and 2 open system designed in the laboratory. Net mesophyll components of A limitation can be evaluated by considering the displacement of COassimilation (A) and transpiration (E) rates 2 were monitored with a differential infra-red gas those 2 functions on the same (A, C graph. ) i analyser for both CO and water vapour (Binos, The initial slope of the (A, C response curve ) i 2 (dA /d Cwas calculated as an estimate of car- Leybold Heraeus). Two or 3 leaves (Q pubes- ) i boxylation efficiency. Apparent quantum yield of 10 leaves (Q ilex) cens and Q petraea) or = were enclosed in a 2-I assimilation chamber, in photosynthesis (a) was computed as the initial slope of the (A, PPFD) response, obtained in a which air temperature (T leaf-to-air water va- ), a pour molar fraction difference (Δw) and ambient 1% Oair mixture to limit photorespiration. 2 CO molar fraction (C were controlled. A gas ) a 2 stream of 2 I min was provided continuously -1 and monitored by a mass flow controller. A fan Chlorophyll a fluoresence homogenized the air inside the chamber. CO 2 measurements molar fraction of the air in the chamber (C was ) a controlled by injecting pure CO into the main 2 flux of CO free air. Air with a low oxygen con- 2 The slow induction transients of in vivo chloro- centration (1% O was obtained when needed, ) 2 phyll fluorescence were measured at room tem- II- CO free air + 95% N . 2 mixture of 5% 2 from a perature with the apparatus described by Lich- lumination provided from the growth cabinet tenthaler and Rinderle (1988). Fluorescence of was increased to 400 μmol m s with a sodi- -2 -1 30-min dark -adapted leaf disks was excited by um lamp (SON-T 400W, Philips), and monitored an He-Ne laser (215, Spectra Physics; 5 mW, λ with a quantum sensor (Li 190SB, LiCor). Regu- 632.8 nm) using 1 arm of a 3-arm glass-fibre = lations and data acquisition were monitored by optic, and guided by the other arms to detecting an application stored in a computer (AT3, IBM) photodiodes (SD 444-41-11-261, Silicon Detec- via a data logger (SAM 80 AOIP). The means of tor Corp). The exciting red light at leaf surface 5 successive measurements were computed amounted to≈ 400 μmol ms (80 W m A ). -2 -2-1 and stored every 10 s. Stomatal conductance red cut-off filter (Schott RG 665) was used to ex- for CO (g) and intercellular CO molar fraction 2 2 clude excitation light and interference filters (C) were calculated according to von Caemmer- (Schott DAL, λmax 691 nm or 732.9 nm) were er and Farquhar (1981). applied to sense the fluorescence induction ki- The following conditions prevailed in the as- netics simultaneously in the 690 or 735 nm similation chamber: T 22 °C and Δw, 8 mmol , a spectral regions. Both fluorescence kinetics mol During the establishment of (A, C re- ) a . -1 recorded with a-2-chann recorder el were sponse curves, PPFD was maintained at 400 (BS316 W + W, Electronic Inc). μmol m s and C was changed every 15 min - 2 -1 a Fluorescence decrease was analysed using from 950 to 800, 650, 500, 350, 200 and 50 following indices: half decay time (t eg the , 1/2 μmol mol (A, C response curves were run . -1 ) a time needed to reach the level (F F ratio pt - )/2, 45 min after illumination, and values of A and g p (F - ) )/F t F of fluorescence decrease (R fd = were recorded at the end of the period at 350 =1 - [(1 and stress adaptative index p (A + μmol mol During the establishment of (A, . -1
R + R 690)]). All of these were com- ψ attained ≈ -3.0 MPa in Q petraea, and fd fd 735)/(1 wp puted from manual measurements on chart re- ≈ -4.0 MPa in Q pubescens and Q ilex. cordings. During drought stress each measure- During drought, A and g decreased in ment was replicated 3 times, and made before parallel, which led to a linear relationship onset of illumination. For the photoinhibition and was an indication of a close coupling study, 2 chlorophyll fluorescence kinetics were recorded for each twig before high illumination between both parameters (fig 3). But in treatment, 30 min after and 1 night later. well watered Q ilex and Q pubescens plants, this relationship did not remain line- ar at high conductances; in this case A RESULTS was probably limited by other factors. The intial slopes (S) of these relationships, which give an estimate of instant water use Plant water status Predawn leaf water potential (ψ of all ) wp decreased rapidly after approxi- plants 1 wk of water deprivation. Small mately amounts of water were added to maintain ψ between -2.0 and -4.0 MPa. ψ wp wp time-course was similar for Q petraea or Q pubescens, but displayed a steeper de- crease for Q ilex (fig 1). Leaf water content (LWC) was lower (45% approximately) in Q ilex leaves than in Q petraea or Q pubescens (60 and 55% respectively). Because of a high interindi- vidual variability, no significant reduction in LWC could be observed during drought, excepted when ψ decreased below -4.0 wp MPa. LWC then decreased to 45% Q pe- traea leaves, 40% in Q pubescens and 35% in Q ilex. Effects of drought on net CO 2 assimilation (A), stomatal conductance (g) and (A, Ci) relationships Both A and g decreased in response to de- creasing ψ (fig 2). The high interindividu- wp al variability observed at high ψ was not wp due to variations in water status. Stomatal closure and inhibition of A started between -1.0 and -2.0 MPa in all tested species. A and g reached values near to zero when
Q petraea is shown in figure 4. Slopes of efficiency under water shortage (Schulze and Hall, 1982), were 0.24 μmol·mmol in -1 the supply functions were reduced due to stomatal closure with declining ψ but Q ilex, and 0.13 and 0.15 in Q petraea and , wp the demand functions were also modified, Q pubescens. which could indicate that both stomatal of An example ) i (A,C curves response and non stomatal factors contributed to the during drought development obtained on
Effects of drought on chlorophyll a fluorescence All tested species displayed similar shapes for chlorophyll a fluorescence kinetics while well watered, with a fairly large inter- individual variability; Q ilex alone showed slightly lower values for R (4-5), (= fd p A 0.25) and higher t (30 s instead of ≈ 15 s 1/2 for both Q petraea and Q robur; see figs 6 and 7). These differences are probably re- lated to the optical properties of the leaves; in fact, Q ilex leaves exhibit thicker cuti- cules and mesophyll tissues. For all 3 spe- cies, no effect of water stress could be observed on t R ψ values 1/2 fd p or Afor , wp > -3.0 MPa for Q petraea, and -4.0 MPa for Q pubescens. With Q ilex a slight de- crease was observed till -3.5 MPa for fd R and but t did not increase significant- , p A 1/2 ly with the exception of one case (figs 6 and 7). When stress became extremely se- vere, ie in 1 case at ψ < -5.0 MPa for wp both Q petraea and Q pubescens, and in 3 cases < -4.0 MPa for Q ilex, t increased 1/2 strongly while R decreased markedly, fd and seemed less affected. Chlorophyll p A fluorescence kinetics as exemplified in fig- ure 8a then displayed both a decrease in and an increase in peak fluorescence ) p (F steady state fluorescence (F ). t Leaf discs were submitted to rapid de- hydration in vitro in free air and obscurity (LWC was reduced from 70 to 30% in 5 h) to ensure that R fd and t could really , p A 1/2 be affected by strong stresses, and that the previously observed stability was not drought induced decline in A. The maximal an artefact. In this case, both R and fd p A assimilation rate (A decreased 2 CO ) max decreased markedly while t increased 1/2 first, as soon as A and g were inhibited. In strongly (fig 9). But an important difference contrast, the initial slope of the (A, C) re- appeared as compared to in situ dehydra- sponse curves (dA/dC remained con- ) i tion: was not affected (fig 8b). p Flevel stant until ψ values fell to below ≈ -2.0 wp Once again, to be less affected to -3.0 MPa. Nevertheless, we observed a p Aseemed than R and a severe water loss was close relationship between A at 350 μmol , fd -1 mol and dA/dC during drought (fig 5). necessary to induce R decrease. fd i
kinetics exhibited a strong decrease Susceptibility to photoinhibition cence t and the form of the fluo- in level, but 1/2 p F rescence decrease were not affected (fig Results of these experiments are presen- 8c). Recovery after 12 h of darkness fol- ted in table I. High illumination treatments lowing the high illumination treatment was induced a decrease of the apparent quan- less in water-stressed than in well-watered tum yield of photosynthesis (a). Well- plants, especially in Q pubescens. Recov- watered plants of Q petraea displayed a ery was more pronounced in both control larger decrease than Q pubescens and and stressed Q ilex saplings than in the Q ilex. But, when drought was imposed, other species. 70%) in Q a was strongly reduced (> and Q pubescens. In contrast, Q petraea ilex water-stressed plants exhibited ap- DISCUSSION proximately the same reduction in a as well-watered ones. Quercus ilex and Q pubescens exhibited in all species, strongly reduced fd R was similar decreases of net CO assimilation 2 for well-watered Q ilex. Fluores- excepted
not very different from that of the previous species. During the entire experiment a was observed between de- close coupling in A and g. Parallel decreases in A creases and g in response to decreasing ψ have wp often been reported (Wong et al, 1985; Teskey et al, 1986; Di Marco et al, 1988). A/g increased during drought progression, and reached constant values with a higher water use efficiency (dA/dg) for Q ilex than for Q petraea or Q pubescens under limit- ed water supply. Alteration of (A, C) relationships showed that apparently both stomatal and non stomatal factors contributed to the limi- tation of A. The maximal rate of net CO 2 (A) and stomatal conductance for CO2 rate assimilation at high C (A was first af- i max ) (g) with increasing drought. Due to a large fected. According to von Caemmerer and interindividual variability, no unequivocal Farquhar (1981) and Farquhar and Shar- difference in sensitivity could be detected, key (1982), this could mean a decrease in even if Q petraea showed earlier re- the rate of regeneration of ribulose 1,5 bi- sponses to decreasing ψ In Q ilex, de- . wp sphosphate (RUP which could be limited ) 2 creases in A and g were steep, with higher by reduced photophosphorylation associa- initial values, but the overall evolution was
et al, 1986; Ögren and Öquist, Teskey ted with electron transport, or by a starva- 1985; Kirschbaum, 1987; Cornic et al, tion in stromal P (Sharkey, 1985). The de- i 1987; Grieu et al, 1988). Farquhar and crease in dA/dC could result from a de- i Sharkey (1982) have also reported that the crease in carboxylation efficiency (von first effects of water stress were a reduc- Caemmerer and Farquhar, 1981).Earlier tion of A while dA/dC initially un- i was results showed similar alterations in (A, C , max ) i affected. relationships (Jones and Fanjul, 1983;
In order to obtain additional information this apparent mesophyll limitation of net on 2 CO assimilation, we studied the decrease of in vivo fluorescence during the onset of drought. Surprisingly, half decay time (t ), 1/2 ratio of fluorescence decrease (R and ) fd stress adaptation index were not af- ) p (A fected by drought until reached values wp ψ < -4.0 MPa, that is well below turgor loss in these species (-2.0 in Q petraea, -2.8 in Q pubescens and -2.4 in Q ilex under sim- ilar conditions; Dreyer et al, 1990). The ab- sence of an effect of water stress on the in- itial rise in fluorescence has been frequently reported (Ögren and Öquist, 1985; Genty et al, 1987; Toivonen and Vi- dauer, 1988; di Marco et al, 1988), indic- ating that PSII photochemistry is quite resistant to leaf water deficits. The ab- sence of a decrease in F levels in relation p to water stress which we observed is in agreement with this view. The evolution of R and A under leaf water stress has sel- fd p dom been documented; however, Schwab et al (1989) showed the stability of R in fd Spinacia oleracea and in resurrection plants until relative water content declined to 40%. In addition to the same fd R stability we also observed a remarkably constant half decay time (t In fact, the decrease ). 1/2 in fluorescence following the peak re- p F sults both from photochemica quenching and non photochemical quenching ) p (Q ). np (Q The former is due to reoxidation of the primary electron acceptor of PSII dur- ing the onset of carbon reduction, and the latter results largely from thermal de- excitation of PSII associated with the build- ing up of transthylakoidal proton gradients and to a lesser extent from the transfer of excitation energy from PSII to PSI (Krause and Weis, 1984; Briantais et al, 1986; Krause et al, 1988). The remarkable stabil- ity of both R and t observed in our ex- fd 1/2 periments could be an argument in favour of a stability of both and This hy- p Q . np Q
the intensity of thermal de-excitation of agreement with the observa- is in pothesis PSII. Electron flow could be maintained at tions of Stuhlfauth et al, 1988 (with Digital- low values of C through photorespiratory i lanata). Constancy of these parameters is CO recycling (Osmondinet hel, initial An- a 1980; , 2 implies a stability of both the electron flow slope dré, 1986). A reduction t from PSII to the primary acceptors, and of F to F decline (ie an increase in t pobserved during drought with higher/2 ) 1 t il- was Di Marco et al, 1988 (with luminations by Triticum durum); Genty et al, 1987 (with Gossypium hirsutum); Ögren and Öquist, 1985 (with Salix sp); Epron and Dreyer (unpublished observations with Populus sp). The stability we obtained with our oak species may therefore not be a general feature under different conditions and in other species. The results obtained from gas exchange and chlorophyll fluorescence studies there- fore appear contradictory: the evolution of (A, C relationship indi- ) i - cated the appearance of mesophyll limita- tions of photosynthesis during drought; and conversely, fluorescence data showed - the absence of any major impairment in
photosynthetic apparatus during leaf water tion of leaf discs, while they showed strong deficit. According to Terashima et al reductions during a severe drought stress (1988), values of C could be overestima- i in situ. As high light treatments induced a ted if patchy stomatal closure occurred in decline in F p (fig 8c), we suggest levels photoinhibitory water-stressed leaves. Non uniform stoma- that damage could have tal closure has been reported in response arisen when severe water stress was im- to ABA application in Helianthus annuus, posed on our saplings in situ and after car- Vitis vinifera and Vicia faba (Downton et al, bon reduction was impaired. During leaf 1988a; Terashima et al, 1988) and in re- disc dehydration, carbon reduction was sponse to water stress in Vitis vinifera, Ne- also impaired but water stress was very rium oleander, Eucalyptus pauciflora and rapidly imposed in darkness. Kaiser (1987) Phaseolus vulgaris (Downton et al, 1988b; has suggested that the inhibition of stromal Sharkey and Seeman, 1989). If C values i enzymes by increasing electrolyte concen- were overestimated, dA/dC and trations or by extremely high protein con- i max A would be underestimated and the apparent centrations induced impairment of carbon non-stomatal inhibition of photosynthesis reduction during severe drought stress, but would be an artefact. Using another that high irradiance density could be re- method, Kaiser (1987) and Comic et al sponsible for photoinhibitory damages un- (1989) showed that apparent quantum der natural drought conditions. yield and maximal rate of photosynthetic Because we could not observe any al- O evolution measured with a CO concen- 2 2 teration in the fluorescence kinetics over tration of up to 5% which overcame diffu- the entire ecophysiologically significant sive resistance did not decline with water range of ψ between 0 and -4.0 (ie, wp stress until there was a severe water loss it appears that our plants did not MPa), (20-40%), indicating a high resistance of suffer from photoinhibition during imposi- the photosynthetic apparatus. Patchy tion of water stress under our light condi- stomatal closure has not yet been studied tions. Powles et al (1984) have shown that in water-stressed oak leaves. Anyway, our maintenance of a minimal level of carbon results seem to indicate that the mesophyll reduction (by photorespiratory CO recy- 2 photosynthetic capacity is rather insensi- cling) prevents photoinhibition in leaves. tive to drought stress in the 3 oak species In leaves exposed to drought, photoinhi- and that observed inhibition of net CO as- 2 bition of photosynthesis by high light treat- similation seemed to be related mostly to ments was more pronounced, especially in stomatal closure and limitations of CO dif- 2 Q petraea and Q pubescens, as has been fusion into the leaves, at least during the previously reported for Salix sp leaves first stages of dehydration. (Ögren and Öquist, 1985). The decrease in When stress became more se- drought the apparent quantum yield of net CO as- 2 -4.0 MPa), both R wp (ψ fd vere p and A < similation and of of chlorophyll p Flevels decreased and t increased, indicating 1/2 fluorescence kinetics show that electron possible damage to the photosynthetic ap- transport, and particularly PSII activity paratus. The same results were obtained were inhibited (Powles, 1984). Recovery with leaf discs of Q petraea submitted to after photoinhibition was lower after 12 h in rapid dehydratation in air. After large water Q petraea and Q pubescens water- losses, F level and to F half decay time t t p F stressed leaves. As recovery from photoin- (t increased. However, ) 1/2 levels were p F hibition is known to be partly due to protein not affected by a rapid in vitro dehydrata- synthesis in the chloroplasts (Greer et al,
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