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article Original A comparison of photosynthetic responses to water stress in seedlings from 3 oak species: Quercus petraea (Matt) Liebl, Q rubra L and Q cerris L G Aussenac D E Epron Dreyer bioclimatologie et d’écophysiologie forestières, INRA Laboratoire de Nancy, Champenoux, 54280 Seichamps, France responses of oak seedlings (Quercus petraea, Q rubra and Q cerris) to Summary — Photosynthetic investigated using gas-exchange and chlorophyll fluorescence. Decreases in predawn drought were leaf water potential (ψ led to pronounced reductions in both stomatal conductance (g and net ) w ) wp (A). CO assimilation rate 2 In contrast, the maximal photochemical efficiency of photosystem II (PS II) measured predawn (F remained unaffected until complete cessation of CO assimilation. Re- ) m / v 2 sponses of PS II photochemical efficiency (ΔF/F to increasing photon flux density (PFD) were de- ) m’ termined for leaves of both control and water-stressed seedlings. Drought resulted in a stronger re- duction of ΔF/F at a given PFD in Q rubra and Q petraea, but not in Q cerris, and led to an . m overreduction of the primary electron acceptor pool (decrease in photochemical quenching, q ). p Such behavior could explain the observed increase in sensitivity to photoinhibition when these 2 species were water-stressed. In contrast, drought did not promote such an increase in the suscepti- bility of Q cerris leaves to photoinhibition. chlorophyll fluorescence / oak / photosynthesis / drought / photoinhibition Résumé — Comparaison de la réponse au déficit hydrique de la photosynthèse de semis de 3 espèces de chêne : Quercus petraea (Matt) Liebl, Q rubra L et Q cerris L. La réponse de la photosynthèse à la sécheresse a été étudiée sur des semis de chêne (Quercus petraea, Q rubra et Q cerris) par des mesures d’échange gazeux et de fluorescence de la chlorophylle. La diminution du potentiel hydrique de base (ψ a entraîné une réduction importante de la conductance stomatique ) wp (g et de l’assimilation nette de CO (A). Par contre, l’efficience photochimique maximale du PS II ) w 2 mesurée en fin de nuit (F n’a pas été affectée tant qu’un arrêt complet de l’assimilation de CO ) m / v 2 * Correspondence. Abbreviations: A: net CO assimilation rate; g stomatal conductance to water vapour; ψ pre- 2 : w : wp dawn leaf water potential; π osmotic potential at full turgor; ψ water potential at turgor loss; D: : 0 : wtl leaf water deficit; PS II: photosystem II; QA: primary electron acceptor; F and F initial and maxi- 0 : m mal fluorescence; F maximal photochemical efficiency of PS II in the dark-adapted state; ΔFI : m /F v F photochemical efficiency of PS II in a light-adapted state; F photochemical efficiency of : m’ : m’ /F v open PS II reaction centers in a light-adapted state; qp: photochemical fluorescence quenching; PFD: photon flux density.
Des réponses de l’efficience photochimique du PS II (ΔF/F à une augmenta- ) m’ n’était pas intervenu. tion de la densité de flux quantique (PFD) ont été établies pour des feuilles de semis irrigués et sou- mis à sécheresse. Le déficit hydrique a entraîné une plus forte réduction de ΔF/F à un PFD donné m’ pour Q rubra et Q petraea, s’accompagnant d’une plus forte réduction du pool d’accepteurs primaires Ce comportement pourrait expliquer d’électrons (diminution du quenching photochimique, ). p q l’augmentation de la sensibilité à la photo-inhibition des feuilles des plants soumis à sécheresse de ces 2 espèces. Au contraire, la sécheresse n’a pas entraîné de différence de réduction du pool d’accepteurs primaires d’électrons, ni de la sensibilité à la photo-inhibition des feuilles de Q cerris. fluorescence de la chlorophylle / chêne / photosynthèse / sécheresse / photo-inhibition Drought-induced stomatal closure is now INTRODUCTION well documented. In many recent studies it has been reported to be the primary factor Oak species are distributed over a large promoting the decrease in net assimilation geographic range and display great varia- rates during drought (kaiser, 1987; Comic tions in their abilities to tolerate periods of et al, 1989). Moreover, the photosynthetic restricted water supply. This latter factor apparatus and in particular, the potential probably plays a major role in the control photochemical activity of PS II, has been of the distribution of the various oak spe- shown to be highly insensitive to rapid leaf cies. Some species have evolved very dehydration in the dark for Q petraea (Ep- specialized adaptive features which are ron and Dreyer, 1992) and for a large spec- thought to enable better survival under trum of species (Dreyer et al, 1992). Rapid drought, such as sclerophylly, restricted leaf dehydration does not affect photochem- area of individual leaves and thick cuti- istry above degrees of dehydration only cles. However, even among the more rarely attained under natural conditions. mesophytic and deciduous species, some Still, the question remains as to whether important differences in tolerance to gradually increasing drought can affect the drought appear. For instance, thorough photosynthetic processes when it is im- ecological studies showed that Q robur posed under medium or high irradiance. In and Q petraea had different water supply particular, the relationship between water- requirements, the former being more sen- stress intensity and light-induced disorders sitive to drought and, as a consequence, in PS II activity still has to be clearly as- more prone to drought-induced decline sessed. Chlorophyll a fluorescence may be (Becker and Lévy, 1982). Nevertheless, used to estimate quantum efficiencies of the physiological mechanisms involved in PS II under diverse environmental con- this differentiated water stress tolerance straints (Baker, 1991) and is therefore a are still poorly understood. Efficiencies o f useful tool to study physiological conse- soil water extraction and of water trans- quences of drought on photosynthetic elec- port pathways in the trees probably play a tron transport. major role and differ significantly among To test responses of different oak spe- species (Abrams, 1990; Cochard et al, cies to a combination of water stress and 1992; Bréda et al, 1993). In addition, the high irradiance, we subjected potted seed- ability to maintain significant rates of CO 2 lings to a gradually increasing drought and assimilation and to keep a functional pho- monitored predawn leaf water potential, to-synthetic apparatus during drought gas exchange and photochemical efficien- may have an important role in this re- spect. cy of PS II. Selected species were Q cer-
A (Hansatech, UK). electrode leaf-disk 2 O European species known to be SE ris, a stream of water-vapor saturated air, maintained relatively drought tolerant, Q petraea, an at 23°C, and with ambient CO was sufficient to , 2 important mesophytic timber species of W or heating of leaf tissues. prevent dehydration Europe and Q rubra, a NE American spe- every 10 min from 135 to PFD changed was cies probably slightly more sensitive to 230, 460, 890, 1300 and 1750 μmol m s -2 -1. cerris has the thickest leaves drought. Q Then, the leaf disk was exposed to a PFD of 1750 μmol m s for 135 min and finally put in -2 -1 and bears a high amount of trichomes; Q the dark for 45 min to determine long-term petraea has been shown to be less prone changes in maximal photochemical efficiency. to drought-induced embolism than Q rubra (Cochard et al, 1992). Leaf water status MATERIAL AND METHODS Predawn leaf water potential (ψ was meas- ) wp ured with a pressure chamber on a single leaf of Seedlings of Quercus petraea (Matt) Liebl (Fo- each seedling, while relative water content was rêt de la Reine, Toul, NE France), Q cerris L estimated from 2 disks punched through this (commercial seedlots) and Q rubra L (Féné- leaf prior to introduction into the pressure cham- trange, NE France) were grown in a naturally il- ber. luminated greenhouse from March to Septem- The 2 leaf disks (2 cm were immediately ) 2 ber 1990, in 5-I pots filled with a 1:1 (v/v) weighed (W used for fluorescence measure- ), f mixture of sand and blond peat, fertilized with ments, rehydrated by floating on distilled water 2.0 g of Nutricote 100 (N/P/K: 13/13/13) and for 4 h at 4 °C in the dark to determine saturated complemented with a mixture of oligoelements, weight (W and oven-dried for 24 h at 80 °C to ) s and 4 g of magnesium chalk. The plants were determine dry weight (W Relative water con- ). d irrigated daily. One week before the onset of tent was calculated as RWC - s )/(W d - f (W the experiments, the seedlings were transport- = W and leaf water deficit expressed as D ); d ed into a growth cabinet with the following day/ = 1-RWC. night conditions: 16/8 h; relative humidity, 70/ 95%; air temperature, 22/16 °C. Photosynthetic potential at full turgor (π and water Osmotic ) 0 photon flux density (PFD) provided by neon turgor loss (ψ were assessed on ) wtl at potential lamps was around 200 μmol m s at the top - 2 -1 well-watered controls by means of a pressure- of the plants. volume analysis using the transpiration method described by Hinckley et al (1980) and Dreyer et al (1990). Three shoots were severed from 3 well-watered seedlings of each species and re- Stress application hydrated overnight through the cut end. Water and experimental design potentials of freely transpiring shoots (ψ were ) w measured at regular time intervals from 0 to -6.0 MPa in a pressure chamber. Shoot weight Drought was imposed on 6 seedlings from each was recorded to calculate shoot water deficit as: species by withholding irrigation for 9 days. Pre- D = 1-[(W W W where W W and fdd - )/(W )], , fi dawn leaf water potential (ψ relative water i - ), wp W represent respectively, shoot weight meas- content, gas exchange and chlorophyll a fluores- d ured immediately after ψ determination, initial cence characteristics were monitored every day w weight of the rehydrated shoot and dry weight of on half of the plants on the last fully developed the shoot. growth flush. Three plants were kept as controls. Responses of photochemical efficiency to increasing PFD and susceptibility to high light stress were studied on 3-4 leaf disks (10 cm ) 2 Gas-exchange measurements punched from either well-watered or water- stressed plants (predawn leaf water potential Stomatal conductance for water vapour (g and ) w -3.0 MPa in the latter case). Each leaf wp ψ = disk net CO assimilation rate (A) were recorded us- 2 inserted into the compartment of a was
(135, 230, 460, 590, 1300 and 1750 μmol PFD ing a portable gas-exchange measurement sys- -2 m ). -1 s Steady-state fluorescence (F) and tem (LiCor 6200, Lincoln, NE, USA). Average (± standard deviation) leaf temperature (t leaf- maximal fluorescence following a saturating ), a to-air difference in vapor mol fraction, CO mole flash (F were recorded and used to compute 2 ) m’ the photochemical efficiency of PS II as: ΔF/F fraction in the air (c and PFD at the leaf sur- m’ ), a (F (Genty et al, 1989). After each 10- face were, respectively, 23.9 (± 0.9)°C, 11.6 (± -F)/F m’ = 3.3) mmol mol 440 (±24) μmol mol and 194 -1 , -1 min period, the actinic light was switched off for (± 22) μmol m s Both A and g were com- -2 -1 . 1 min to allow recording of basic fluorescence w F and to compute photochemical efficiency of puted according to von Caemmerer and Farqu- 0’ open PS II reaction centers as: F = (F har (1981) and expressed on a projected leaf- /F- v’m’ m’ F (Genty et al, 1989). The two parameters area basis (ΔT area meter, ΔT Devices, UK). m’ )/F 0’ Measurements were made 3—4 h after the onset are related by the equation: ΔF/F m’ qF pv’ ·/F ; m’ = where q is the photochemical quenching, ie the of the light period. p fraction of open PS II reaction centers. Decreas- q are generally ascribed to increased re- es in P duction of the primary acceptor Q while de- , A fluorescence Chlorophyll a crease of F are thought to reveal enhanced . m’ /F v’ measurement thermal deexcitation of PS II (Baker, 1991). To test the effects of high light stress we compared F before exposure to light and af- m /F v Chlorophyll a fluorescence of PS II was meas- ter a complete PFD response curve followed by ured using a pulse amplitude modulated fluo- an additional 135 min at 1750 μmol m s and -2 -1 rometer (PAM 101, Walz, Germany) as previ- 45 min darkness. ously described (Epron and Dreyer, 1992). Leaf disks (2 cm were punched from overnight ) 2 dark-adapted seedlings. Initial fluorescence when all PS II reaction centers were open, ), o (F RESULTS obtained using a weak light (less than 1 was μmol m s from a light-emitting diode (λ -2 -1 ) , max 650 nm; pulse duration, 1 μsec; frequency, 1.6 Drought progression kHz). Maximum fluorescence (F when all PS ) m and plant water status II reaction centers were closed, was recorded during a flash of saturating white light (4000 μmol m s Maximal photochemical efficien- -2 -1 ). During the first 4 days, soil water content cy of PS II, ie, in the dark-adapted state, was decreased from 0.5 to 0.2 g of dry weight -1 calculated according to Genty et al (1987) as: without any significant decrease in pre- mo /F-F vm F= /F . ) (F dawn leaf water potential ψ Thereafter, . wp Photochemical efficiency of PS II was deter- ψ declined steadily and reached values mined during the establishment of light re- wp below -6.0 MPa 5 days later. sponse curves, after 10 min at each successive
Decreases in ψ led to increases in measured on well-watered seedlings are wp leaf water deficit, D after an initial period of presented in table I. Q cerris displayed sig- marked variability. But the relationship be- nificantly lower &0 and ψ while the other pi; , wtl tween D and ψ displayed some interspe- 2 species behaved similarly. A discrepancy wp cific differences: for a given value of ψ Q between these data and the D - ψ rela- w, wp rubra displayed higher deficits than the tionship, as presented in figure 1, ap- other 2 species (fig 1). For example, a ψ peared for all species: D for a given value wp of about -3 MPa was accompanied by a D of ψ was always higher (lower water con- w for ≈ 0.26 in Q cerris and Q petraea, but of tent) during the progression of dehydration ≈ 0.30 in Q rubra. than during the establishment of pressure- volume relationships with well-watered at full turgor (π and Osmotic ) 0 potential seedlings. This may be due either to shifts at turgor loss (ψ leaf water ) wtl potential
in osmotic potential induced by the drought treatment or to oversaturation of the leaf disks. Drought effects on stomatal conductance and net CO assimilation rate 2 Stomatal conductance to water vapor (ψ ) w and net CO assimilation rates (A) dis- 2 played large species-related differences on well-watered seedlings (fig 2): Q petraea reached the highest rates of A and g fol- , w lowed by Q cerris and Q rubra. All 3 spe- cies exhibited similar rates of change in A and g ψ decreased. Decreases in A w wp as Drought effects began above -1.0 MPa but were gradual. on photochemical efficiency of PS II Values close to zero were obtained in all ψ reached -3.5 MPa. The cases when wp decline of g was much steeper, reaching Responses of maximal photochemical effi- w values below 0.025 mol m s at -1.5 -1 -2 ciency of PS II (F to declining ψ are ) m /F v’ wp MPa in all species. Differences in the de- shown in figure 3. F remained high m /F v cline rates of g and A may be due to CO and constant (= 0.81) in all species until 2 w limitation of A and supra-optimal stomatal ψ dropped below -4.0 MPa. Such low wp conductance on well-watered seedlings. potentials correspond to values of D >
0.35. The observed decreases resulted from both a decrease in F and an in- m crease in F (data not shown). Low F m /F v 0 values of 0.5 were reached at the lowest ≈ water potentials. It is worth noting that these decreases began at stress intensi- ties for which net assimilation rates were almost nil. Some marked species-related differences were clear: the drought- induced decline appeared at lower water potentials in Q cerris than in the other 2 species. Drought effects on light-response curves of PS II photochemical efficiency Responses of PS II photochemical effi- ciency (ΔF/F to increasing PFD are ,) m shown in figure 4. As expected, ΔF/F , m was high at low irradiance, and decreased steadily with increasing PFD in both con- trols and water-stressed seedlings. Final values of controls (at 1750 μmol m s -2 -1) were ≈ 0.20 for Q petraea and Q rubra, but 0.10 for Q cerris. Water stress had < strong consequences in the 2 former spe- cies, inducing much lower ΔF/F at a giv- m’ en irradiance as compared to controls. In Q cerris no significant difference was ob- served between both situations. The same relationship between ΔF/F and F or m’ m’ /F v’ q was observed in all species indepen- p the control and the water-stressed (-3.0 dently of the drought treatment (fig 5), MPa) seedlings already tested for PFD re- which may be interpreted as the mainte- sponses (see above). Large decreases of nance of the same equilibrium, at a given F as measured after 45 min of dark- m /F v efficiency, between thermal deexcitation of ness were detected in all cases. But these PS II and the reduction status of the pri- decreases pronounced on were more mary acceptor pool. stressed seedlings than on well-watered ones for both Q petraea and Q robur, but not for Q cerris, for which the decreases Drought effects on response reached the same extent in both cases. to high irradiance Decreases in F were always the con- m /F v sequence of both a slight increase in F0 Table II shows the effects of 135 min of (about 10%) and a strong decrease in F m exposure to high light on F for both (30% minimum). m /F v
DISCUSSION already been reported (Vivin et al, 1993). It is worth noting that, under such conditions, the intrinsic water-use efficiency ratio (A/ Interspecific variability g of Q rubra was higher than that for Q ) w at optimal water supply petraea and Q cerris (35.6, 31.2 and 26.8 μmol CO mol H respectively). Differ- 2 -1 2 O, in leaf structure and their conse- Our species displayed marked differences ences on leaf optical properties and on in behavior at optimal water supply. Net quences photosynthetic efficiency among oak spe- assimilation rates per unit leaf area 2 CO cies clearly need to be better documented; highest on Q petraea and Q cer- (A) were moreover, the impacts of the light regime and lowest on Q rubra. Differences in ris, and microclimate during leaf expansion re- chlorophyll contents, leaf-specific weight quire further elucidation. The data present- and leaf optical properties could partly ex- ed apply to greenhouse-grown seedlings, plain these differences in A. Lin and Ehle- and a direct extrapolation to natural condi- ringer (1982) reported that changes in tions would be questionable. Nevertheless, spectral properties of leaves of papaya as- despite differences in A, maximal photo- sociated with differences in chlorophyll chemical efficiency of PS II was identical in contents were strongly correlated to the all species. rate of net CO assimilation. Leaves of Q 2 rubra seedlings grown in a glasshouse are Water relationships were also very differ- known to have lower chlorophyll contents ent between the tested species. Q cerris and specific leaf weights than the 2 other had the lowest osmotic potential at full tur- species (Dreyer et al, 1992). This latter consequence, the lowest (π and, ) 0 as a gor species also exhibited lower stomatal con- water potential at turgor loss. The observed ductance (g Similar differences in A and values of π were relatively high when com- ). w 0 g between Q petraea and Q rubra have pared to trees grown under natural condi- w
tions, but in agreement with already pub- ofhighest irradiance. We- during periods ber and Gates lished data for (1990) showed the lack of mesophytic oaks grown in permanent photo-inhibitory damage on Q the greenhouse (Dreyer et al, 1990). rubra subjected to drought, despite the strong reduction in A. But, during these General reactions to drought field experiments, water stress never in- duced complete arrest of photosynthetic carbon assimilation. It can be inferred from All species exhibited an abrupt decline in these observations that marked decreases stomatal conductance as soon as ψ de- wp of potential PS II activity may occur only creased from values near 0 to -1.0 MPa. during periods of complete cessation of as- Decreases in A were much more gradual. similation and under intense irradiance. As An important consequence was that intrin- the observed decreases resulted from both sic water-use efficiency increased during increased F and decreased F it can be , m 0 the initial stages of progressive dehydra- concluded that they were the expression of tion, as has been frequently reported some kind of damage to the PS II (Dem- (Schulze and Hall, 1982; Epron and mig and Björkman, 1987). However, these Dreyer, 1990, for other oak species). Our injuries were not directly associated with results suggest a rather good ability of oak leaf dehydration, but rather with excess species to maintain significant rates of A light energy reaching PS II reaction cen- during drought progression, as already ters when CO assimilation was complete- 2 shown by Epron and Dreyer (1990) for pot- ly inhibited in severely stressed seedlings. ted saplings or under natural conditions by Hinckley et al (1978), Bahari et al (1985) The lack of damage to PS II in moderate- and Epron et al (1992). ly stressed leaves (ψ from -1.0 to -4.0 wp MPa), despite a pronounced decrease in A, Recent results suggest that the photo- has already been documented in other spe- synthetic apparatus is rather tolerant to de- cies (Ben et al, 1987; Genty et al, 1987; Di hydration (Kaiser, 1987; Comic et al, Marco et al, 1988). Two complementary 1989; Epron and Dreyer, 1990, 1992), and mechanisms could help protect PS II from that drought effects seem to be mainly me- injury: 1) the quantum yield of PS II photo- diated by stomatal closure, at least at the chemistry may be transiently reduced by in- levels commonly experienced under field creased thermal energy dissipation when conditions. In particular, maximal photo- the rate of electron transport exceeds the chemical efficiency (F measured on ) m /, v need of reducing power for CO fixation; 2 dark-adapted oak leaves during rapid de- this was observed at midday under natural hydration, remained constant until very conditions on sun-exposed leaves of oak high leaf-water deficits (D ≈ 0.75) (Epron (Epron et al, 1992); 2) an increasing trees and Dreyer, 1992). In our case, on potted part of the electron flow originating from PS seedlings drying out in a climate chamber, II may be diverted from carboxylation to the decline in F appeared at lower def- m /F v photorespiration, as experimentally demon- icits (ψ≈ -4 MPa, that is D≈ 0.35). wp strated by Comic and Briantais (1991). F was measured at predawn, which m /F v should have allowed overnight relaxation of daily changes in potential PS II activity. Response to PFD and photo-inhibition Under field conditions, Epron et al (1992) observed on stressed trees that predawn Increasing PFD clearly reduced the quan- F was always near optimal values, de- m /F v efficiency of PS II (ΔF/F of both con- ) m’ tum spite dramatic but reversible reductions
responsible for this damage (Kyle, could be trol and water-stressed leaves. However, 1987). However, loss of PS II activity should at all PFD, water stress resulted in lower values of ΔF/F in Q petraea and Q rubra. be observed only if the rate of damage ex- m’ Lower ΔF/F in stressed individuals was m’ ceeded the rate of repair (Baker, 1991). probably induced by low CO availability at 2 Q cerris displayed the least drought- the chloroplast level resulting from stoma- induced sensitivity to high-light stress. Dif- tal closure. The decrease reveals that di- ferences in absorbance existed between version of electron flow to photorespiration our species (Epron, unpublished data), but may not have been sufficient to maintain their magnitudes were too limited to ex- similar rates of PS II photochemistry during plain the observed differences. Demmig et drought in these species. These PFD- al (1988) suggested that the resistance of related reductions were always accompa- Nerium oleander to photodamage when nied both by increased thermal de- exposed to a combination of high light and excitation (reduced F and decreases ) m’ /F v’ water stress was associated with an in- in q ie the pool of primary electron accep- , p creasing ability for radiationless energy tors was gradually reduced. It has to be dissipation. But we did not detect any inter- emphasized that, at any given value of PS specific difference in the ability to dissipate II photochemical efficiency, the balance excess energy when electron transport between the increase in thermal de- was reduced. It has frequently been sug- excitation and the reduction status of the gested that photodamage should be en- pool of primary electron acceptors (Q ) A hanced when the Q pool is highly re- A was similar in the 3 species tested, and on duced (Krause and Weis, 1991). both control and dehydrated leaves. Surprisingly, the Q pool at a given PFD A We demonstrated that drought also in- was reduced more in well-watered Q cerris duced an enhancement in susceptibility to than in the other species, despite similar high-light stresses. Such effects of drought sensitivities to high-light exposure. Some has previously been observed on many other mechanisms, for instance higher species (Nerium oleander, Björkman and rates of recovery (Greer et al, 1986), may Powles, 1984; Q petraea, Q pubescens limit the extent of damage to PS II photo- but not Q ilex, Epron and Dreyer, 1990). In chemical efficiency in this species. In con- our case, high light favored an alteration in trast, differences in sensitivity to high-light PS II reaction centers, as the reduction in exposure between control and dehydrated F resulted from both a decrease in F m /F v m leaves of Q rubra and Q petraea were well and an increase in F (Demmig and 0 correlated to lower PS II photochemical ef- Björkman, 1987). This finding clearly distin- ficiency at a given PFD, ie a higher reduc- guishes the reactions observed here from tion state of Q In Q cerris, the reduction . A the diurnal and reversible decreases in F , 0 state of Q was similar in well-watered and A F and F noted under natural condi- m /F v m water-stressed leaves, which was in agree- tions (Epron et al, 1992). The reasons for ment with the observed lack of increase in this increased sensitivity to high irradiance sensitivity to high light. due to drought are still open to debate. One explanation may be that CO starva- 2 tion induced by stomatal closure allowed CONCLUSION damaging effects of excess excitation de- livered to PS II reaction centers (Powles, Early drought effects seem to be mainly in- 1984). Excess excitation energy may gen- erate highly reactive oxygen species that duced by stomatal limitation to photosyn-
Pallardy SG, Parker WC (1985) Pho- Bahari ZA, thesis. Disorders in the photosynthetic ap- tosynthesis, water relations and drought adap- paratus appeared, nevertheless, at higher tation in six woody species of oak-hickory fo- stress intensities in all 3 species but were rests in central Missouri. For Sci 31, 557-569 not mediated by leaf-tissue dehydration. Baker NR (1991) A possible role for photosys- The relationships between water stress, tem II in environmental perturbations of pho- high light and species responses need fur- tosynthesis. Physiol Plant 81, 563-570 ther analysis to elucidate such differential Becker M, Lévy G (1982) Le dépérissement du responses. It appears that the drought- chêne en Forêt de Tronçais. Les causes induced increase in sensitivity to high light écologiques. Ann Sci For 39, 439-444 in Q petraea and Q rubra leaves could be Ben GY, Osmond CB, Sharkey TD (1987) Com- the result of an overreduction of the Q A parisons of photosynthetic responses of Xan- pool under high irradiance. Q cerris, which thium strumarium and Helianthus annuus to did not exhibit such an over-reduction, did chronic and acute water stress in sun and shade. Plant Physiol 84, 476-482 not suffer from increased sensitivity to high irradiance. Differences in the abilities of Björkman O, Powles SB (1984) Inhibition of pho- tosynthetic reactions under water stress: in- photorespiration to compensate reduction teraction with high light level. Planta 161, in CO assimilation between our species 2 490-503 may be able to explain these differences in Bréda N, Cochard H, Dreyer E, Granier A (1993) photoinhibi- drought-induced sensitivity to Seasonal evolution of water transfer in a ma- tion. Still the higher tolerance to photoinhi- ture oak stand (Quercus petraea Matt Liebl) bition in Q cerris at a similar level of Q re- A submitted to drought. Can J For Res (in press) duction has to be clarified. Moreover, Cochard H, Bréda N, Granier A, Aussenac G these responses to drought and light may (1992) Vulnerability to air embolism of three differ on seedlings and trees grown out- European oak species (Quercus petraea (Matt) Liebl, Q pubescens Willd, Q robur L). doors, which are known to present dramat- Ann Sci For 49, 225-233 ically different leaf-specific weight and pig- Comic G, Briantais JM (1991) Partitioning of ment compositions. It remains to be photosynthetic electron flow between CO 2 elucidated to what extent irradiance inten- and O reduction in a C3 leaf (Phaseolus vul- 2 sity during leaf growth may modulate the garis L) at different CO concentrations and 2 stress responses revealed in this work. during drought stress. Planta 183, 178-184 Comic G, Le Gouallec JL, Briantais JM, Hodges M (1989) Effect of dehydration and high light ACKNOWLEDGMENTS on photosynthesis of two C3 plants (Phaseo- lus vulgaris L, Elatostema repens (Lour) Hall f). Planta 177, 84-90 The authors thank JM Gioria and JM Desjeunes for having grown the seedlings. This work has O (1987) Comparison of Demmig B, Björkman been realized in the framework of a research pro- the effect of excessive light on chlorophyll flu- gram on: Water stress, xylem dysfunction and orescence (77 K) and photon yield of O evo- 2 dieback mechanisms in European oaks, funded lution in leaves of higher plants. Planta 171, by ECC DG XII (STEP CT 90 050 C). The com- 171-184 ments and suggestions made by 2 anonymous Demmig B, Winter K, Krüger A, Czygan FC reviewers are gratefully acknow-ledged. (1988) Zeaxanthin and heat dissipation of ex- cess light energy in Nerium oleander ex- posed to a combination of high light and wa- REFERENCES ter stress. Plant Physiol 87, 17-24 Di Marco GA, Massacci A, Gabrielli R (1988) Abrams MD Drought effects on photosynthesis and fluo- (1990) Adaptations and responses drought in Quercus species of North in hard wheat cultivars grown in the to rescence 385-390 America. Tree Physiol7, 227-238 field. Physiol Plant 74,
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