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Summary1. We present measurements of CO 2 fluxes over 2 years above and within a young Beech stand in the east of France. This site is part of the Euroflux network set up to monitor fluxes over representative European forests. 2. The net ecosystem carbon (C) exchange was derived from continuous eddy flux measurements. Major components of the total flux (i.e. soil and above-ground biomass respiration and assimilation of leafy branches) were measured independently using chambers. The main C stocks (i.e. root, stem and branch biomass) were also quantified. 3. Daily minima of CO 2 flux were typically around -20 µ mol CO 2 m -2 s -1 during the period of full leaf expansion, while night-time ecosystem respiration varied between 5 and 15 µ mol CO 2 m -2 s -1. The seasonal pattern of net ecosystem assimilation was very close to that of net assimilation at the single branch scale. The seasonal variation of net ecosystem exchange was closely related to leaf expansion and soil water content during the dry year of 1996. 4. Measurements of ecosystem respiration (eddy flux) were corrected for CO 2 storage within the stand. This C flux showed a seasonal pattern, the maximum rates (4-7 g C m -2 day -1 ) occurring in spring and summer, and appeared to be correlated with soil temperature. Temporal variation of soil respiration showed the same pattern, and effects of both temperature and soil drying were found. Annual soil respiration was ≈ 70% of ecosystem respiration. Root respiration was 60% of the total below-ground respiration. 5. Annual net C exchange was -218 and -257 g C m -2 in 1996 and 1997, respectively, corresponding to net C uptake by the forest. These values are much lower than the annual biomass increment (stems and large roots) of the stand: 427 and 471 g C m -2 year -1 , respectively. The difference may be explained by a release of CO 2 from the decomposition of woody debris. 6. Ecosystem C loss by respiration was 800-1000 g C m -2 year -1 . Gross C gain was 1000-1300 g C m -2 year -1 . Ecosystem respiration therefore played a major role in the annual C balance of this forest.

We assessed the effects of irradiance received during growth on the vulnerability of Fagus sylvatica L. xylem vessels to water-stress-induced embolism. The measurements were conducted on (1) potted saplings acclimated for 2 years under 100% and 12% incident global radiation and (2) branches collected from sun-exposed and shaded sides of adult trees. Both experiments yielded similar results. Light-acclimated shoots were less vulnerable to embolism. Xylem water potential levels producing 50% loss of hydraulic conductivity were lower in sun-exposed branches and seedlings than in shade-grown ones (-3·0 versus -2·3 MPa on average). The differences in vulnerability were not correlated with differences in xylem hydraulic conductivity nor vessel diameter. Resistance to cavitation was correlated with transpiration rates, midday xylem and leaf water potentials in adult trees. We concluded that vulnerability to cavitation in Fagus sylvatica may acclimate to contrasting ambient light conditions.

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The opening of ligand-gated ion channels in response to agonist binding is a fundamental process in biology. In ATP-gated P2X receptors, little is known about the molecular events that couple ATP binding to channel opening. In this paper, we identify structural changes of the ATP site accompanying the P2X2 receptor activation by engineering extracellular zinc bridges at putative mobile regions as revealed by normal mode analysis. We provide evidence that tightening of the ATP sites shaped like open 'jaws' induces opening of the P2X ion channel. We show that ATP binding favours jaw tightening, whereas binding of a competitive antagonist prevents gating induced by this movement. Our data reveal the inherent dynamic of the binding jaw, and provide new structural insights into the mechanism of P2X receptor activation.

ATP-gated P2X receptors are trimeric ion channels, as recently confirmed by X-ray crystallography. However, the structure was solved without ATP and even though extracellular intersubunit cavities surrounded by conserved amino acid residues previously shown to be important for ATP function were proposed to house ATP, the localization of the ATP sites remains elusive. Here we localize the ATP-binding sites by creating, through a proximity-dependent "tethering" reaction, covalent bonds between a synthesized ATPderived thiol-reactive P2X2 agonist (NCS-ATP) and single cysteine mutants engineered in the putative binding cavities of the P2X2 receptor. By combining whole-cell and single-channel recordings, we report that NCS-ATP covalently and specifically labels two previously unidentified positions N140 and L186 from two adjacent subunits separated by about 18 Å in a P2X2 closed state homology model, suggesting the existence of at least two binding modes. Tethering reaction at both positions primes subsequent agonist binding, yet with distinct functional consequences. Labeling of one position impedes subsequent ATP function, which results in inefficient gating, whereas tethering of the other position, although failing to produce gating by itself, enhances subsequent ATP function. Our results thus define a large and dynamic intersubunit ATPbinding pocket and suggest that receptors trapped in covalently agonist-bound states differ in their ability to gate the ion channel.affinity labeling | chemical modification | purinergic receptor P 2X receptors are oligomeric ATP-gated ion channels selective to cations (1) and are involved in physiological processes as diverse as synaptic transmission, the response to inflammation, and pain perception (2). Upon ATP binding, structural rearrangements of the subunit interface (3-5) lead to the opening of the ion channel (6-8), but the entire molecular sequence of events that couple ATP binding to channel opening remains unknown. The recent X-ray structure of the P2X4 receptor in a closed resting state represents in this regard a decisive step (9). It confirms the trimeric stoichiometry of the ion channel, in agreement with the fact that there are three activatable ATP-binding sites (10), and provides a structural context to interpret functional data (11).Early studies based on mutagenesis data have identified highly conserved extracellular residues important for ATP function and have proposed that ATP binding occurs through the extracellular domain (12-19), presumably at the subunit interface (15,17). When mapped on the crystal structure, most of these residues are observed to line a large and deep intersubunit cavity shaped like an open "jaw" and located approximately 45 Å away from the ion channel domain (9). This observation thus suggests that these residues participate in ATP binding; however, the crystal structure was solved in the absence of ATP, and therefore no direct evidence support this hypothesis to date.We used the proximity-dependent "tethering" approach (20) to localiz...

Although patterns of seedling selection by herbivores are strongly influenced by plant age and the expression of anti‐herbivore defence, it is unclear how these characteristics interact to influence seedling susceptibility to herbivory. We tracked ontogenetic changes in a range of secondary metabolites (total phenolics, alkaloids and cyanogenic glycosides) commonly associated with seedling defence for nine sympatric British grassland species. Although there was marked variation in concentrations of secondary metabolites between different species, we found a consistent increase in the deployment of phenolics, alkaloids and cyanogenics with seedling age for six of the seven dicotyledonous species examined. The two grass species by contrast exhibited low levels of secondary metabolites across all developmental stages, possibly due to an investment in structural (silica phytoliths) defence. Our results corroborate species‐specific patterns of seedling herbivory observed in field studies, and offer some explanation for the relatively high sensitivity to herbivore attack frequently observed for relatively young seedlings compared with their older conspecifics. Our results also support predictions made by the growth–differentiation balance hypothesis regarding ontogenetic changes in resource allocation to anti‐herbivore defence for a range of potential chemical defences and across a range of sympatric plant species presumably subject to broadly similar selective pressures at the regeneration stage.

-The stomatal control of embolism in Fagus sylvatica L. was analysed in response to crown position and experimental changes of trunk hydraulic resistance. On one mature beech tree deep cuts were made in the trunk to increase the resistance to water transfert. We followed the changes in leaf and xylem water potential and stomatal conductance after the cuts at three levels within the canopy. We characterised vulnerability to cavitation for branches taken from two levels of irradiance (sun-exposed branches and shaded ones). Some differences appeared between shade and sun-exposed branches. When the leaf water potential dropped, stomatal conductances decreased earlier and faster in the shade branches. These results are well correlated with vulnerability to cavitation, shade branches being more vulnerable than sun-acclimated branches. Xylem water potential levels producing fifty percent loss of hydraulic conductivity were lower in sun-exposed branches than in shade grown ones (-3.1 MPa vs. -2.5 MPa on average). Xylem water potentials that induced stomatal closure were above the threshold-value inducing cavitation both for shade and sun-exposed branches. We confirmed that vulnerability to cavitation in Fagus sylvatica can acclimate to contrasting ambient light conditions, and we conclued that stomatal response to water stress occured early and sufficiently fast to protect xylem from dysfunction. beech (Fagus sylvatica L.) / xylem embolism / stomatal regulation / irradiance / acclimation Résumé -Variations de l'architecture hydraulique du hêtre (Fagus sylvatica L.) : contrôle de l'embolie du xylème par les stomates. Nous avons analysé le contrôle stomatique du développement de l'embolie chez Fagus sylvatica L. en fonction de l'éclaire-ment des branches et suite à un changement de la résistance hydraulique du tronc. Nous avons fait des entailles dans le tronc d'un hêtre de façon à augmenter la résistance au transfert de l'eau. Nous avons suivi les variations de potentiels hydriques foliaire et de xylème et la conductance stomatique à trois niveaux dans le houppier. Nous avons caractérisé la vulnérabilité à la cavitation de branches de pleine lumière et d'ombre. Lorsque le potentiel hydrique a diminué, la conductance stomatique des branches d'ombre a diminuée le plus tôt et le plus fortement. Ce résultat est bien corrélé avec la vulnérabilité à la cavitation des branches. Les branches d'ombre sont plus vulnéra-bles que les branches de lumière ; ainsi le potentiel hydrique de xylème induisant 50 % d'embolie est plus négatif en plein éclairement qu'à l'ombre (-3,1 MPa contre -2,5 MPa). Le potentiel de xylème induisant la fermeture des stomates est supérieur au potentiel induisant la cavitation à la lumière comme à l'ombre. Nous avons confirmé que la vulnérabilité du hêtre s'acclimate aux conditions d'éclaire-ment et que les stomates protègent le xylème d'un dysfonctionnement.hêtre (Fagus sylvatica L.) / embolie / régulation stomatique / éclairement / acclimatation