Báo cáo khoa học: "Photochemical efficiency of photosystem II in rapidly dehydrating leaves of 11 temperate and tropical tree species differing in their tolerance to drought"

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Original article Photochemical efficiency of photosystem II in rapidly dehydrating leaves of 11 temperate and tropical tree species differing in their tolerance to drought OE E D Dreyer Epron Yog Matig INRA-Nancy, Laboratoire de Bioclimatologie et d’Écophysiologie Forestières, 1 Station de Sylviculture et Production, Champenoux, F-54280 Seichamps, France; 2 IRA-Garoua, Centre de Recherches Agronomiques, Garoua, Cameroon 10 July 1992; accepted 7 September 1992) (Received of PS II photochemical efficiency to rapid and severe leaf dehydration Summary — Responses tested on 11 different tree species differing in their ability to withstand periods of drought. were Seedlings of Quercus robur, Q petraea, Q pubescens, Q rubra, Q cerris and Q ilex, and Dalbergia sissoo, Eucalyptus camaldulensis, Acacia holosericea, Azadirachta indica and Populus candicans were grown in a greenhouse at Nancy. Fifty to 60 leaf disks were punched from a few well-watered and dark-adapted seedlings and left to dehydrate in complete darkness for up to 6 h. Chlorophyll flu- orescence induction kinetics were recorded with a PAM fluorometer (modulated red light below 1 μmol.m s actinic light 220 μmol.m saturating white flashes, 4 000 μmol.m All spe- -2 -1 , , -1 .s -2 ). -1 .s -2 cies displayed a remarkable stability for initial and maximal fluorescence F and F for PS II photo- , m 0 chemical efficiency of dark-adapted disks, and after 10 min at 220 μmol m s up to relative water -2 -1 , losses largely above the turgor loss point. Decreases in the latter were the first observed signs of dysfunction at leaf relative water losses of 0.23-0.40 depending on the species. They were gener- = ally accompanied by significant decreases in the photochemical efficiency of open reaction centers, which revealed increased PS II thermal deexcitation. No correlation between evolution of either of these parameters and known tolerance to drought could be detected among tested species. It is concluded that sensitivity of the photosynthetic apparatus to leaf dehydration in the absence of irra- diance plays a very minor role in the adaptation of species to drought. Photosynthesis decline in re- sponse to water stress under natural conditions is probably the consequence of stomatal closure and possibly of high levels of irradiance and temperature. photosynthesis / chlorophyll fluorescence / PS II photochemical efficiency / water stress / de- hydration / oak species / tropical tree species Abbreviations: D: relative leaf water loss; D 0 at full turgor; D relative leaf water loss at turgor : tl = loss; π osmotic potential at full turgor; ψ ψ leaf water potential, actual value and at turgor w wtl and : : 0 loss; PS II: photosystem II; F maximal fluorescence; F initial fluorescence; F F F v mv ;: 0m -F /F : m : 0 = PS II photochemical efficiency of dark-adapted leaves; F F and F maximal fluorescence, steady , m’ ’: 0 state and basic fluorescence after 10 min induction at 220 μmol m s photon flux density; F F -2 -1 v’ m’ = - F ΔF = F F; ΔF/F PSII photochemical efficiency measured after a 10-min induction period m m’ -: ; 0’ at 220 μmol m s F photochemical efficiency open photosynthetic reaction centers under -2 -1 v’ ; /F : m’ the same conditions; qp: photochemical quenching under the same irradiance; DW: dry weight; FW: fresh weight; FW initial fresh weight; LSW: leaf specific weight. : i
Résumé — Comparaison des effets d’une déshydratation rapide sur l’efficience photochimi- que du photosystème II de 11 espèces ligneuses présentant des degrés variables de résis- tance à la sécheresse. Les diminutions d’efficience photochimique du photosynthèse II en réponse à une déshydratation rapide et sévère de feuilles, ont été comparées sur 11 espèces d’arbres connues pour présenter des degrés variables de tolérance à des conditions de sécheresse. Des semis de dif- férents chênes (Quercus robur, Q petraea, Q pubescens, Q rubra, Q cerris et Q ilex), d’espèces tropi- cales (Dalbergia sissoo, Eucalyptus camaldulensis, Acacia holosericea, Azadirachta indica) et de peuplier (Populus candicans) ont été élevés en serre à Nancy. Cinquante à soixante disques foliaires ont été prélevés sur des plants bien alimentés en eau et préalablement maintenus à l’obscurité. Ils ont transpiré librement à l’obscurité pendant des temps variables pouvant aller jusqu’à 6 h. Leur degré de déshydratation a été estimé par leur teneur en eau relative au moment des mesures. Les ci- nétiques d’induction de fluorescence ont été enregistrées sur chacun de ces disques en utilisant un fluoromètre modulé PAM (densités de flux de photons : lumière modulée rouge : < 1 μmol.m lu- ; -1 .s -2 mière actinique : 220 μmol m lumière saturante : 0,7 s à 4 000 μmol m Toutes les es- ; -1 .s -2 ). -1 .s -2 pèces ont présenté une remarquable stabilité de la fluorescence de base et de la fluorescence maxi- male, ainsi que de l’efficience photochimique du photosystème II tant maximale qu’après une induction à 220 μmol m et ce jusqu’à des teneurs en eau largement en deçà de celles corres- , -1 .s -2 pondant à la perte de turgescence. Les premiers signes de dysfonctionnement observés ont consisté en une baisse de l’efficience photochimique à 220 μmol m qui a débuté à des déficits de teneur , -1 .s -2 en eau relative de l’ordre de 0,23 à 0,40 suivant l’espèce. Cette baisse était généralement accompa- gnée d’une diminution de l’efficience photochimique des centres ouverts révélant ainsi une augmenta- tion significative de la déexcitation thermique du PS II. Mais aucune corrélation n’a pu être étab lle entre la réponse de ces paramètres à la déshydratation et la tolérance globale des espèces à la sé- cheresse. La sensibilité de l’appareil photosynthétique foliaire à la déshydratation elle-même ne joue sans doute qu’un rôle mineur dans l’adaptation des espèces aux déficits d’alimentation hydrique. Les diminutions de photosynthèse observées en réponse à l’épuisement progressif des réserves hydri- ques du sol en conditions naturelles sont vraisemblablement dues à une fermeture des stomates, ac- compagnée parfois par une action des fortes irradiances et des températures élevées. photosynthèse / fluorescence chlorophyllienne / efficience photochimique du PSII / stress hydrique / déshydratation / chêne / espèce tropicale INTRODUCTION synthetic apparatus appears to be rather dehydration (Kaiser, 1987) and tolerant to many authors claim that the main effect of Water availability plays a major role in the water strees is to induce stomatal closure distribution of tree species all over the and to limit photosynthesis via reduced world. But the physiological basis of the supply of CO to chloroplasts (Comic et al, 2 observed differences in tolerance to water 1989; Chaves, 1991).The use of chloro- shortage still has to be clarified. In particu- phyll a fluorescence is one of the different lar, survival and growth of trees under con- techniques suitable for studying photosyn- ditions of low water availability imply opti- thesis tolerance to environmental con- mization of water use through stomatal straints. From such measurements it has regulation, high photosynthetic efficiency been shown that PS II displays good stabil- in leaves during the short periods of water ity up to very low levels of water content in availability and long-term survival of these leaves. Rapidly dehydrated leaves show a leaves during periods of stress. constant basic fluorescence F and a high 0 Does tolerance of the photosynthetic maximal photochemical efficiency F m /F v (Ögren and Öquist, 1985; Comic et al, apparatus to leaf dehydration play any role in these stress adaptations? The photo- 1987; Epron and Dreyer, 1992). Epron and
Q robur L (Lepidobalanus, robur), from Ma- Dreyer (1992) suggested that the first - noncourt, near Toul, eastern France (5.50 E, signs of dehydration-induced impairment N; elevation 250 m); 48.40 were increases in a fast relaxing non- Q pubescens Willd (Lepidobalanus, robur), - photochemical quenching of fluorescence, from Mont Ventoux, Avignon, Vaucluse (5.12 E, which appeared at a relative leaf water 44.15 N; elevation 800 m); loss > 0.35 in Q petraea and which was in- Q rubra L (Erythrobalanus, rubraea), from - increase in PS II thermal terpreted as an Schopperten Forest, the Bas Rhin, eastern deexcitation related to reduced electron France (6.25 E, 48.50 N; elevation 250 m); consumption and decreased activity of the Q ilex L (Lepidobalanus, ilex), from Uzès, - carbon reduction and photorespiratory cy- Gard, southern France (4.25 E, 44.05 N; eleva- cles. The question nevertheless remains tion 350 m); open whether these features could be gen- Q cerris (Lepidobalanus, cerris), provided by - eralized to a broader range of species. In Vilmorin, France. particular, it is not clear if differences in Acorns from all these species were col- leaf structural characteristics (such as lected during autumn 1989, stored over winter at chlorophyll content per unit leaf area or - 1 °C, and germinated during March 1990. Q petraea and Q robur are mid-European spe- leaf specific weight), leaf water relations cies which grow under rather well-watered con- (osmotic potential at full turgor or water ditions, while Q pubescens and Q cerris are lo- content at turgor loss), and more generally cated in drier areas. Q ilex is a typical in drought tolerance could be related to sempervirent macchia species with sclerophyl- some modifications in the above-described lous leaves. Q rubra was introduced from north- reactions to dehydration. We therefore eastern America; compared the changes induced by rapid Populus candicans Ait (Salicaceae, section - balsamifera), provided by the Laboratory of For- dehydration in the dark, on PSII photo- est Pathology, INRA-Nancy, originating from chemical efficiency of dark-adapted leaves Northern America and drought-intolerant; and after a 10-min induction period at Azadirachta indica A Juss (Meliaceae) from 220 μmol m s on leaf disks from seed- -2 -1 - Maroua, Northern Cameroon (14.15 E, 10.40 N, lings of a broad range of species, including elevation 400 m, 780 mm rainfall). This species mesophytic oaks, xerophytic oaks and originates from Southern India and is now wide- intertropical species used for reforestation ly used in a Sahelian environment; under semi-arid conditions in northern Dalbergia sissoo (Papilionaceae), from Oua- - Cameroon. dagoudou, Burkina Faso (1.31 W, 12.21 N, ele- vation 304 m, 860 mm rainfall). This species originates from Southern India, and is now being tested in Sahelian environment; a MATERIAL AND METHODS Acacia holosericea (Papilionaceae), from - Mount Molloy, Australia (145.15 E, 16.46 S, ele- vation 380 m, 1150 mm rainfall) is a phyllode- Plant material bearing Acacia shrub originating from Australia, tested in a Sahelian environment; and seed The following species origins Eucalyptus camaldulensis Dehn (Myrtaceae), were - used: from Djarengol, Cameroon (14.15 E, 10.40 N, el- evation 400 m, 780 mm rainfall) is widely used in Quercus petraea (Matt) Liebl (Fagaceae; sub- - a Sudano-Sahelian environment, but seems genus Lepidobalanus, section robur) either 40- poorly adapted to drier climates (Sall et al, 1991). year-old trees growing in a natural stand at Nan- cy-Champenoux; or seed collected in the Forêt The last 4 species were sown during the de la Reine, near Toul, eastern France (5.50 E, 1989. All seedlings were grown in 5-I pots spring 48.40 N; elevation 250 m); a mixture of blond peat and sand (50/50 v/v) on
and fertilized with a slow release fertilizer (Nutri- After complete relaxation from this flash, a cote, N/P/K 13/13/13) and a solution of oligo- fluorescence kinetic was induced by an actinic elements. All seedlings were grown in a green- white light (Schott KL1500, Germany; 220 μmol -2 -1 m s After 10 min steady-state fluorescence ). house where irradiance was reduced by 30%. = Temperatures ranged between 10-30 °C for (F) was recorded and a new flash yielded F , m’ temperate and between 15-30 °C for tropical allowing the calculation of an actual PS II photo- chemical efficiency (ΔF/F 220 μmol m s -2 -1 species. Seedlings were watered manually ) at m’ twice a week. from (Genty et al, 1989): The following rating for drought tolerance is suggested, based on species distribution: P candicans < Q rubra, Q robur, Q petraea < immediately switched The actinic light was Q cerris, E camaldulensis < Q pubescens, off, and F recorded, allowing calculation of the 0’ D sissoo < A indica, Q ilex < A holesericea. photochemical efficiency of open PS II reaction (F 1 F Both parameters ). m’ /F 0’ centers m’ /F v’ = - are related by: Dehydration experiments Forty to 60 leaf disks (2.0 in diameter) were where qp is the photochemical quenching coeffi- punched from 2-3 well-grown seedlings, which cient, ie the fraction of open PS II reaction cen- had previously been fully hydrated and dark- ters (Genty et al, 1989; Baker, 1991). Decreas- adapted over a 14-h period. Disks were immedi- es in F are an index for increased PS II m’ /F v’ ately weighed (FW and dehydrated for 0-8 h ), i thermal deexcitation. in the dark at room temperature as described by Epron and Dreyer (1992). Fluorescence induc- tion kinetics were recorded successively on each disk and corresponding values of fresh Leaf characteristics weight (FW) were determined immediately after completion of the kinetics. Dry weight (DW) was Leaf specific weight (LSW, g dm was comput- ) -2 measured after 24 h oven-drying at 80 °C. Rela- ed from disk dry weight and estimated disk area tive leaf water loss (D) was always estimated (0.031 41 dm and averaged for all used disks. ), 2 as: Chlorophyll was extracted from 5 leaf disks per species (15 mm in diameter) in 5 ml dimethylsul- foxide and chlorophyll concentrations were de- termined spectrometrically (Hiscox and Israel- stam, 1979). Fluorescence measurements Fluorescence measurements were carried out Shoot-water relations at ambient CO and temperature on dark- 2 adapted leaf disks with a PAM 101 fluorometer (Walz, Germany). Initial fluorescence (F was Three shoots were selected for each species ) 0 determined by applying a pulsed measuring red and pressure-volume curves established using light (< 1 μmol m s at a frequency of 1.6 -2 -1 ) the free transpiration method as described by kHz, and maximal fluorescence (F by an addi- Hinckley et al (1980) and Dreyer et al (1990). ) m tional saturating flash of white light (0.7 s; 4 000 Each shoot was rehydrated overnight through μmol m s provided by a cold light source -2 -1 ) the cut end, and left to transpire freely on a lab- (Schott KL1500, Germany). The ratio of variable oratory bench. Fresh weight and leaf water po- to maximal fluorescence F that is the maxi- tential were recorded together at regular inter- , m /F v mal PS II photochemical efficiency was calculat- vals till the latter reached -6 MPa. Water poten- ed as (Genty et al, 1987): tial was measured with a pressure chamber, and the main parameters of water relations (os- motic pressure at full turgor, π water potential , 0
turgor loss, ψ relative leaf water loss at tur- at petraea had much higher LSW when , wtl D calculated as in Dreyer et al gor loss, ) tl grown under field conditions. Two species (1990). differed significantly from the others: A hol- sericea has very thick hairy phyllodes, and Q ilex has sclerophyllous waxy leaves. P Analysis of results candicans displayed by far the lowest LSW. Total chlorophyll content expressed For each species, values of F F F ΔF/F , , /F 0 m v m’, m on a leaf area basis varied strongly be- plotted against relative leaf water loss D. were tween 2.36 and 7.35 mg dm Oaks dis- . -2 Optimal values of these parameters were re- played the highest chlorophyll content, with corded. Successive linear regressions were Q rubra slightly lower than the others. In used to determine the range of stability of F , 0 general, tropical species exhibited the low- F F ΔF/F with increasing D, and the , /F m v m’ , m est values (< 3 mg.dm No clear correla- ). -2 threshold values for which statistically significant declines could be observed were computed. tion was found between LSW and chloro- phyll content. Parameters of shoot-water relations (os- RESULTS motic potential at full turgor, π relative ; 0 leaf water loss at turgor loss D leaf water ; tl potential at turgor loss ψ and leaf water ; wtl Leaf characteristics and water relations potential at D = 0.3) are presented in table II. All species displayed rather high values Leaf characteristics are listed in table I. of π that is low solute contents. The low- , 0 LSW was very variable among the species est values were obtained with the Mediter- studied, and relatively low, due probably to ranean oaks Q ilex and Q cerris. Tropical growth under greenhouse conditions. Q species showed even higher values than
the oaks. Turgor loss occurred at very low and Q ilex). The main features of these re- relative leaf water loss (D between 0.1 ) tl lations were as follows. In P candicans, F 0 and 0.15, and at relatively high leaf water was almost constant over the entire range potentials (ψ ≥ -2.5 MPa). It is interest- wtl of D from 0 to 0.8, while F remained con- m ing to note that the lowest π and ψ and 0 stant till D 0.4, and decreased very grad- wtl = highest D occurred in Q petraea in the tl ually later. A very sharp decline occurred stand. Finally, ψ at a deficit of 0.3 varied only after D 0.75. As a consequence, w = between -2.0 and -3.8 MPa which was F remained rather constant at optimal m /F v largely below the turgor loss point for all values of 0.82. A sharp decline occurred ≈ species. Greenhouse microclimate prob- also only above D ≈ 0.75. ΔF/F was al- m’ ably had a major effect on leaf water rela- most constant at the high values of 0.62 till tions, and π and D would probably have 0 tl D 0.4 and declined sharply thereafter. = been higher under field conditions (Dreyer E camaldulensis presented almost the et al, 1990). Despite a strong interspecific same behaviour with a slight difference: F m variability, no clear trend could be detect- decreased progressively during the whole ed in these results in relation to the eco- range of D, together with F and F , 0 m /F v logical adaptation of species to drought. showed a slow decrease from D 0.4 on. = Nevertheless, final values at D 0.8 were ≈ still around 0.75. The same description Fluorescence measurements also applied to Q ilex, with the strong dif- ference that ΔF/F decreased much earli- m’ er, ie at D = 0.2. Three representative examples of evolu- tion of , , /F 0mv FFF and , m with in- Such m’ ΔF/F feature fits very well with that a creasing dehydration have been indicated already described by Epron and Dreyer in figure 1 (P candicans, E camaldulensis (1992): maintenance of high values of
PS II maximal photochemical efficiency To enable a comparative analysis to be (F up to very strong levels of dehydra- ) m /F v made of the response curves to dehydra- tion, and decline in photosynthetic activity, tion in all species we computed the follow- as estimated by PS II photochemical effi- ing parameters (table III): F ΔF/F , / v ,F m m’ ciency under low irradiance (ΔF/F only (D < 0.2), the at optimal water content ), m’ beyond the turgor loss point. threshold in D below which ΔF/F declined m’ strongly, the minimal value of F and m’ /F v’ The decrease in ΔF/F was also ac- m’ Δ, the magnitude of changes in F dur- m’ /F v’ companied by a decrease in qp, although ing dehydration. species-related differences ap- some peared in the relationship between both Optimal values of F ranged from 73.4 m parameters (fig 2). In fact, in both P candi- to 112.9 respectively depending on spe- cies. These species-related differences cans and E camaldulensis, an almost could be partly attributed to variations in straight relationship appeared, while in Q ilex the first stages of decrease were ac- leaf total chlorophyll content. This was the companied by a maintenance of high qp, ie only fluorescence parameter which could a high oxidation state of the primary elec- be correlated to a leaf structural feature. Maximal values of F averaged 0.800, tron acceptor QA. In the meantime, the m /F v photochemical efficiency of open centers with some significant differences between F decreased till a minimal value was species (range: 0.774 for A indica, and m’ /F v’ reached, and reincreased. The magnitude 0.826 for P candicans. During dehydration, of the changes in F were very differ- F remained almost constant, with only m’ /F v’ m /F v ent between species, the largest being re- slight decreases in a few species. In any corded in Q ilex. case, even at D 0.7, F was still m /F v’ =
0.40 in Q robur. This threshold value was in all cases above the turgor loss point, and was apparently not related to the known ability of species to withstand drought stress. Finally, the dehydration in- duced changes in F displayed a m’ /F v’ strong interspecific variability; both minimal values (0.25 for Q ilex to 0.57 for E camal- dulensis) and the magnitude of decline (0.44 for Q ilex to 0.17 for Q robur) were very variable. DISCUSSION The results presented here confirm the ob- servations made by a number of authors, showing that the photosynthetic functions are very unresponsive to leaf dehydration (Kaiser, 1987; Comic et al, 1989; Comic and Briantais, 1991; Epron and Dreyer, 1992). In fact, for all species the PS II pho- tochemical efficiency of dark-adapted leaves (F declined strongly only at relative leaf ) m /F v 0.7. The PS II photo- water losses > chemical efficiency at 220 μmol m s -2 -1 decreased sharply below 0.25, ) m’ (ΔF/F that is after turgor loss, and probably at rel- ative leaf water losses where net CO as- 2 similation rates should be almost nil for all the species used here (Epron and Dreyer, 1990). Under such conditions, it is indicat- ed from the results of Comic and Briantais (1991) that photorespiration may consume the electron flow produced by PS II activi- ty; as a matter of fact, all species tested display a C metabolism. 3 The increase in relative leaf water loss beyond this level induced a strong decline in photochemical efficiency at low irradi- (ΔF/F The absence of decrease in around 0.75. Sharp declines were ob- ). m’ ance PS II maximal photochemical efficiency served only when D > 0.7. ΔF/F dis- m’ (F and of increases in F clearly dem- played high values between 0.60 and 0.66 ) m /F v 0 onstrated that the declines could not be at- depending on the species, and remained almost constant until a threshold in D was tributed to decreased potential activity of reached ranging from 0.23 in Q cerris to PS II reaction centers. But in all cases they
detected in PS II photochemical effi- accompanied by a decrease in the were were ciency, both after dark adaptation (which photochemical efficiency of open PS II re- remained ≈ 0.82) and after 10 min at 220 action centers (F which reflected in- ), m’ /F v’ μmol m s (= 0.62). The only significant -2 -1 creased PS II thermal deexcitation (Genty change was detected in F which in fact , m et al, 1989). This in some cases allowed was related to the amount of chlorophyll maintenance of high states of oxidation of per leaf area. This is not surprising since, the primary acceptor Q as revealed by , A as has been demonstrated by Björkman high values of the photochemical quench- and Demmig (1987), maximal quantum ing qp, or at least slowed down the reduc- yield of photosynthesis is identical in all C 3 tion of this acceptor pool. Epron and species and corresponds to an efficiency Dreyer (1992) showed that at this stage, 0.83 electrons issued from PS II per of an efficient recovery of F occurred in m’ /F v’ = a few minutes as soon as the actinic light intercepted photon. had been switched off, which indicates that The reactions to dehydration were simi- the decreases were due to a fast relaxing lar in all species. No significant interspecif- non-photochemical quenching. Highest ic differences could be detected in the sen- levels of D finally resulted in a reincrease sitivity of PS II maximal photochemical F and in a strong decline of qp. of m’ /F v’ efficiency (F Some important differ- ) m /. v ences appeared in the precocity of the de- Despite large differences in leaf struc- cline of photochemical efficiency at 220 species, only minor variations tures among
μmol m s revealing changes in photo- -2 -1 , ACKNOWLEDGMENTS synthetic activity. But surprisingly, the spe- cies which was supposed to display the The authors are most grateful to JM Gioria and best adaptation to drought also showed the JM Desjeunes for growing the seedlings used in earliest decrease (over D 0.34 for this experiment. They also wish to thank the = Centre Technique Forestier Tropical for provid- P candicans, and D 0.25 for Q ilex)! = ing the seeds from tropical species, and the The largest interspecific difference ap- French Ministère de la Coopération et du Déve- peared in the magnitude of changes in F / v’ loppement for travel funds accorded to OEM. F in relation to decreased ΔF/F They . m’ , m’ The comments of 2 anonymous reviewers on an earlier version of this paper are gratefully ac- reflect differences in the magnitude of PS knowledged. II thermal deexcitation while the photo- chemical efficiency decreases. The largest levels were displayed by Q ilex, and REFERENCES helped to maintain high values of qp, that is a high oxidation state of the primary ac- ceptor QA. This feature could be consid- Baker NR (1991) A possible role for photosys- tem II in environmental perturbations of pho- ered as an index for a better tolerance to tosynthesis. Physiol Plant 81, 563-570 relative leaf water losses, but it should be B (1987) Photon yield of kept in mind that the photochemical effi- Björkman O, Demmig evolution and chlorophyll fluorescence 2 O ciency also decreased rather early in this characteristics at 77 K among vascular plants species and that P candicans, one of the of diverse origins. Planta 170, 489-504 most drought-sensitive species tested Chaves MM (1991) Effects of water deficits on here, also displayed rather high values. carbon assimilation. J Exp Bot 42, 1-16 In conclusion, these results emphasize Comic G, Papagiorgiou I, Louason G (1987) Ef- the very poor correlation existing between fect of a rapid and a slow drought cycle fol- drought resistance of different species and lowed by rehydration on stomatal and non the sensitivity of their photosynthetic func- stomatal components of leaf photosynthesis in Phaseolus vulgaris L. J Plant Physiol 126, tions to leaf dehydration. This result is in 309-318 accordance with much other experimental Comic G, Le Gouallec JL, Briantais JM, Hodges evidence. In fact, the decrease in photo- M (1989) Effect of dehydration and high light synthetic activity in response to drought on photosynthesis of two C plants (Phaseolus 3 under natural conditions is probably not re- vulgaris L, Elatostema repens (Lour) Hall f). lated to dysfunction induced by leaf dehy- Planta 177, 84-90 dration, but to stomatal closure, as has Comic G, Briantais JM (1991) Partitioning of been confirmed by direct measurement of photosynthetic electron flow between CO 2 O evolution under saturating CO (Comic 2 2 and O reduction in a C3 leaf (Phaseolus vul- 2 et al, 1989). Stomatal closure leads to low garis L) at different CO concentrations and 2 during drought stress. Planta 183, 178-184 CO concentrations in the chloroplasts, 2 and high irradiance and temperature in- Dreyer E, Bousquet F, Ducrey M (1990) Use of in water relation pressure volume creases associated with drought could in- curves woody shoots: influence of rehy- analysis on duce deleterious effects (Chaves, 1991). dration and comparison of four European oak The ability to withstand such periods of species. Ann Sci For 47, 285-297 high irradiance and high temperature dur- Epron D, Dreyer E (1990) Stomatal and non- ing drought may be the most significant stomatal limitation of photosynthesis by leaf physiological aspect of drought tolerance, water deficits in three oak species: a compar- together with the precocity of drought- ison of gas exchange and chlorophyll a fluo- induced stomatal closure. data. Ann Sci For 47, 435-450 rescence
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