Summary — The water relations of 30-year-old Quercus petraea were studied for three consecutive growing seasons. Whole tree specific hydraulic conductances (gL) were computed from sap flow densities (dF)/leaf water potential (Ψ leaf ) relationships. gL was clearly reduced with the development of the drought. The

— Bud-burst, leaf area index (LAI), transpiration, soil water content and radial growth of a 35-year-old Quercus petraea stand were measured during 5 successive years (1989)(1990)(1991)(1992)(1993). At the beginning of 1992, half of the stand was thinned. The increase of stand transpiration during spring was linearly correlated to the development of LAI. During the second part of the season, although LAI continued to increase because of rhythmic shoot development, transpiration was strongly reduced as soil water content decreased. The transpiration/potential evapotranspiration (T/PET) ratio decreased sharply as soon as relative extractable water (REW) dropped below 0.4. Likewise, cumulated stand transpiration varied among years because of variability in soil water availability, LAI and canopy structure. A linear relationship, similar to the one observed for weekly variations, was noted between T/PET and LAI; maximum LAI ranged from 3.3 to 6 in this ring-porous species. Seasonal circumference measurements showed that 43% of the annual increment was achieved before leaf development, hence before canopy transpiration and CO 2 assimilation were started. Tree ring area was significantly correlated to the cumulated transpiration; a water-use efficiency variable was defined at both tree and stand level. transpiration / leaf area index / drought / circumference increment / Quercus petraea Résumé — Variations intraet interannuelles de transpiration, d'indice foliaire et de croissance radiale d'un peuplement de chêne sessile (Quercus petraea). Le débourrement, l'indice foliaire (LAI), la transpiration, la teneur en eau du sol et la croissance en circonférence d'un peuplement de Quercus petraea âgé de 35 ans ont été mesurés pendant 5 années successives (1989 à 1993, fig 6). Au début de l'année 1992, la moitié du peuplement a été éclaircie. L'augmentation de la transpiration du peuplement au printemps était linéairement corrélée à LAI (fig 2). Au cours de la seconde partie de la saison, même si LAI continuait à augmenter en raison de la croissance rythmique des pousses, la transpiration était fortement réduite par la sécheresse édaphique. Le rapport transpiration/ETP diminuait rapidement dès que la fraction disponible de l'eau du sol chutait en dessous de 0,4 (fig 3). De même, la transpiration cumulée du peuplement variait entre les années avec la disponibilité en eau du sol, le LAI et la structure du couvert. Une relation linéaire similaire à celle observée à l'échelle hebdomadaire a été mise en évidence entre T/ETP et LAI pour une gamme de LAI s'étendant de 3,3 à 6,0, selon les années et la densité des traitements (fig 8). Les mesures d'accroissement en circonférence au cours de la saison ont montré que 43 % de l'accroissement annuel était réalisé avant le développement des feuilles (fig 4), donc avant la reprise de transpiration et d'assimilation du carbone. La surface de chaque cerne était significativement corrélée à la transpiration cumulée au cours de la saison de végétation (fig 10). Une variable d'effi...

Summary — The vulnerability to water-stress induced cavitation and the petiole leaf specific conductivity (LSC) have been studied on excised branches of Quercus petraea, Q pubescens, Q robur and Q rubra. Seasonal evolution of xylem embolism in the petioles and twigs of mature Q petraea has been followed together with increasing soil water deficit. Field experiments showed that Q petraea suffered from embolism damage in both petioles and twigs after heavy drought. Large differences in terms of vulnerability to cavitation and LSC have been found between species. Q pubescens presented the highest LSC and the lowest vulnerability together with Q petraea. Q robur was found to be more vulnerable than Q petraea although with comparable LSC. Q rubra was the most vulnerable species and exhibited the lowest LSC. It was concluded that these species could be classified according to how their hydraulic mechanism is conceived to resist cavitation events : Q pubescens was the most resistant followed in order by Q petraea, Q robur, and Q rubra. Results are discussed in terms of plant segmentation and drought resistance.

Axial water flow in the trunks of mature oak trees (Quercus petraea (Matt.) Liebl. and Q. robur L.) was studied by four independent techniques: water absorption from a cut trunk, sap flowmeters, heat pulse velocity (HPV) and thermoimaging. Estimation of the total water flow with sap flowmeters, HPV and water absorption yielded comparable results. We concluded from dye colorations, thermograms and axial profiles of sap flow and heat pulse velocity that, in intact trunks, most of the flow occurred in the current-year ring, where early-wood vessels in the outermost ring were still functional. Nevertheless, there was significant flow in the older rings of the xylem. Total water flow through the trunk was only slightly reduced when air embolisms were artificially induced in early-wood vessels, probably because there was little change in hydraulic conductance in the root-leaf sap pathway. Embolization of the current-year vessels reactivated transport in the older rings.

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The validity of actinometry to monitor oxygen atom concentration in O2-N2 microwave discharges created by surface wave is investigated. The plasma is created with a gas flow in a quartz tube of inner diameter 16 mm at pressures in the Torr range. First, it is shown that the reliability of actinometry can be deduced from the longitudinal profile of the actinometry signal. Second, absolute concentrations of oxygen atoms are estimated from the experimental actinometry signal and agree satisfactorily with concentrations simultaneously measured by vacuum ultraviolet (VUV) absorption downstream from the plasma. Moreover, upon varying the nitrogen percentage (0%–100%), it is evidenced that the actinometry signal is proportional to the concentration measured by VUV absorption. Furthermore, it is evidenced that the oxygen dissociation rate is only 2% in pure oxygen plasmas, while it reaches 15% (433 MHz) or 30% (2450 MHz) for mixtures containing more than 20% of nitrogen. This drastic increase in [O] upon the addition of N2 is extensively discussed and, finally, attributed to a decrease in the recombination frequency of oxygen atoms on the quartz wall, in the presence of nitrogen.

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Summary — Water relations were analysed in adult oaks (Quercus petraea and Q robur) during a period of water shortage in a simplified lysimeter. Sap flux densities and stomatal conductance were reduced by = 70% at maximal drought intensity. Predawn leaf water potential then ranged from -1.7 to -2.0 MPa. The slightly lower transpiration observed in pedunculate oaks could be ascribed to their smaller crown development. Nevertheless, no significant difference in stomatal conductance could be observed between the two species. They also had the same percent loss of conductivity (= 80%) in petioles at maximal drought intensity when midday leaf water potential had dropped to = -3.0 MPa. Finally, good agreement was found between observed losses of hydraulic conductivity during in situ dehydration and the vulnerability curves obtained under laboratory conditions. The shifts in maximal conductivity observed in some droughted trees probably accentuated discrepancies between field and laboratory data. However, a correction procedure helped overcome these artifacts. Abbreviations: F d : sap flux density (dm 3 ·dm -2 ·h -1 ); g s : midday stomatal conductance to water vapor (cm·s -1 ); k i : initial hydraulic conductivity of petioles (kg·m·s -1 ·MPa -1 ); K max : maximal hydraulic conductivity of petioles after 2 flushes at high pressure (kg·m·s -1 ·MPa -1 ); ψ wm : midday leaf water potential (MPa); ψ wp : predawn leaf water potential (MPa). étaient alors compris entre -1,7 et -2,0 MPa. Une transpiration légèrement plus faible observée pour le chêne pédonculé a été interprétée comme résultant de différences dans le statut social des 2 espèces. Toutefois, aucune différence significative de conductance stomatique n'a pu être mise en évi-dence entre les 2 espèces, qui apparaissent toutes 2 comme assez tolérantes à la sécheresse. Au plus fort de la sécheresse, les 2 espèces ont montré des pourcentages d'embolie de l'ordre de 70 à 80% dans leurs pétioles, alors que le potentiel hydrique foliaire minimum atteignait -3,0 MPa.