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Background and Aims Predicting both plant water status and leaf gas exchange under various environmental conditions is essential for anticipating the effects of climate change on plant growth and productivity. This study developed a functional-structural grapevine model which combines a mechanistic understanding of stomatal function and photosynthesis at the leaf level (i.e. extended Farqhuhar-von Caemmerer-Berry model) and the dynamics of water transport from soil to individual leaves (i.e. Tardieu-Davies model). [br/] Methods The model included novel features that account for the effects of xylem embolism (f(PLC)) on leaf hydraulic conductance and residual stomatal conductance (g(0)), variable root and leaf hydraulic conductance, and the microclimate of individual organs. The model was calibrated with detailed datasets of leaf photosynthesis, leaf water potential, xylem sap abscisic acid (ABA) concentration and hourly whole-plant transpiration observed within a soil drying period, and validated with independent datasets of whole-plant transpiration under both well-watered and water-stressed conditions. [br/] Key Results The model well captured the effects of radiation, temperature, CO2 and vapour pressure deficit on leaf photosynthesis, transpiration, stomatal conductance and leaf water potential, and correctly reproduced the diurnal pattern and decline of water flux within the soil drying period. In silico analyses revealed that decreases in g(0) with increasing f(PLC) were essential to avoid unrealistic drops in leaf water potential under severe water stress. Additionally, by varying the hydraulic conductance along the pathway (e.g. root and leaves) and changing the sensitivity of stomatal conductance to ABA and leaf water potential, the model can produce different water use behaviours (i.e. iso- and anisohydric). [br/] Conclusions The robust performance of this model allows for modelling climate effects from individual plants to fields, and for modelling plants with complex, non-homogenous canopies. In addition, the model provides a basis for future modelling efforts aimed at describing the physiology and growth of individual organs in relation to water status.

Résumé Les techniques de circulation extracorporelle (CEC) peuvent être utilisées dans les défaillances respiratoires graves des syndromes de détresse respiratoire aiguë (SDRA) avec trois objectifs : 1) assurer une oxygénation satisfaisante en court-circuitant le poumon malade grâce à une circulation veinoveineuse à haut débit ; cette technique assure sans difficulté l’épuration de CO2 ; 2) assurer avant tout une élimination partielle de CO2 dans le but de protéger le poumon d’une ventilation mécanique dangereuse. Des débits sanguins quatre à cinq fois plus faibles sont suffisants avec une circulation veinoveineuse ou artérioveineuse sans pompe ; 3) exceptionnellement, la prise en charge d’une défaillance cardiaque associée peut nécessiter une circulation veinoartérielle à haut débit. Des études physiologiques détaillées et des essais cliniques sont indispensables pour mieux connaître les indications de ces techniques.

Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH3) may coexist at the plant level. Moreover, management practices such as mowing, hay production and grazing may change the composition of the sward and hence the source-sink relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to understand the exchange of ammonia between grasslands and the atmosphere better, especially regarding the location and magnitude of sources and sinks. Fluxes of atmospheric NH3 within a grassland canopy were assessed in the field and under controlled conditions using a dynamic chamber technique (cuvette). These cuvette measurements were combined with extraction techniques to estimate the ammonium (NH4+) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (Cp). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH3 within the different compartments of the grassland: the soil, the litter or senescent "litter leaves", and the functioning "green leaves". A set of six field experiments and six laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes. The results show that the cuvette measurements agree with the extraction technique in ranking the strength of compartment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH3 with a compensation point around 0.1–0.4 μg m−3 and an NH3 flux of 6 to 7 ng m−2 s−1. Cutting of the grass did not increase the NH3 fluxes of the green leaves. The litter was found to be the largest source of NH3 in the canopy, with a Cp of up to 1000 μg m−3 NH3 and an NH3 flux up to 90 ng m−2 s−1. The litter was found to be a much smaller NH3 source when dried (Cp=160 μg m−3 and FNH3=35 ng m−2 s−1 NH3). Moreover emissions from the litter were found to vary with the relative humidity of the air. The soil was a strong source of NH3 in the period immediately after cutting (Cp=320 μg m−3 and FNH3=60 ng m−2 s−1), which was nevertheless always smaller than the litter source. The soil NH3 emissions lasted, however, for less than one day, and were not observed with sieved soil. They could not be solely explained by xylem sap flow extruding NH4+. These results indicate that future research on grassland-ammonia relationships should focus on the post-mowing period and the role of litter in interaction with meteorological conditions.

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (A n) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee A n “spot” measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to A n by low light availability (photon flux density, PFD) and stomatal conductance (g s). These gas exchange measurements were carried out during two consecutive coffee growing seasons in a commercial plantation in the Orosi valley of Costa Rica. Levels of A n were related to PFD and g s in order to calculate envelope functions which were used to establish PFD or g s limitations to A n. Under the growing conditions of the present trial, mean leaf A n remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by ~20% at 19% GI. Limitation to A n due to g s was strong in full sun and decreasing with increasing shade levels. On the other hand, limitation due to PFD remained at a similar level for all shade treatments. These different evolutions of limitations of A n by PFD and g s in response to shade explain the absence of a decrease in coffee leaf A n with a shade level up to 55%. Consequently, these results confirm that Arabica coffee is a shade-adapted plant with leaves that can maintain a high photosynthetic performance under low light availability.

Potted seedlings from three oak species (Quercus robur, Q petraea and Q rubra), known to present different sensitivities to temporary soil hypoxia, were submitted to two contrasted irradiance regimes in a greenhouse (100% and approximately 35% of full greenhouse irradiance) and shortly after to waterlogging for 2-4 weeks. The experiment was repeated for 2 years. Leaf gas exchange, chlorophyll content and fluorescence, leaf water potential and photosynthetic capacity were recorded several times during the stress. Biomass increment was estimated at the end of the stress, or after 3 weeks of drainage. Despite its late application, after completion of elongation and expansion of leaves, shading significantly reduced growth and modified photosynthetic activity. It generally reduced net assimilation rates and increased chlorophyll contents. Q rubra seedlings behaved differently: growth decreased and chlorophyll was enhanced as in the two other species but net assimilation rates and photosynthetic capacity were higher in the shade than under full irradiance. This surprising effect probably resulted from high irradiance stress, and was not alleviated by improved fertilization during the second year. Waterlogging induced severe disorders in photosynthesis: net CO2 assimilation, stomatal conductance, photosynthetic capacity: chlorophyll content and growth declined. In addition, predawn values of photochemical efficiency of photosystem Il, which are usually close to the optimum value of 0.83, were significantly decreased. The largest dysfunctions occurred in Q rubra seedlings, and the smallest in Q robur, which confirms that the number of disorders in photosynthesis is an accurate estimate of the differential sensitivity to waterlogging among genotypes. Shading of the seedlings interacted with waterlogging and limited the extent of induced damage. In particular, stomatal closure and net CO2 assimilation rates were proportionally less affected under shade. Similarly, predawn leaf water potential presented smaller decreases. The decline of predawn values of photochemical efficiency occured to a much lesser extent under shade. Finally, the behaviour of seedlings during post-waterlogging stress differentiated significantly the species. None of them recovered completely after 3 weeks. Nevertheless, Q robur behaved best, recovering high levels of predawn water potential, slowly reopening stomata and reincreasing net assimilation rates. Q petraea recovered with more difficulties: predawn leaf water potential decreased in several plants after the end of waterlogging, and photosynthesis recovered very slowly. Q rubra did not recover at all and several individuals died after the end of waterlogging stress., Des semis en pots de trois espèces de chêne (Quercus robur, Q petraea et Q rubra), connues pour présenter d'importantes différences de sensibilité à la présence de nappes d'eau temporaires dans le sol, ont été soumis à deux niveaux d'éclairement (100 et 35 % du rayonnement ambiant) dans une serre à Nancy, puis à une période d'ennoyage de plusieurs semaines. Cette expérience a été répétée 2 années de suite. Les échanges gazeux foliaires, les teneurs en chlorophylle, la fluorescence de la chlorophylle, le potentiel hydrique et la capacité de photosynthèse ont été mesurés plusieurs fois au cours de la contrainte. La biomasse accumulée a été estimée à la fin de l'ennoyage, ou après 3 semaines de ressuyage du sol. Malgré son application tardive après l'expansion des feuilles et l'arrêt de croissance en hauteur, l'ombrage a induit des baisses de croissance et d'activité photosynthétique (diminution de l'assimilation nette de CO2 et augmentation des teneurs en chlorophylles). Q rubra a présenté une réponse très différente : alors que la croissance et les teneurs en chlorophylle présentaient les mêmes modifications que dans les autres espèces, l'assimilation nette de CO2 était plus importante à l'ombre qu'en pleine lumière, et la capacité photosynthétique fortement augmentée. Cette réponse anormale révèle que ces semis de Q rubra étaient soumis à un stress lumineux. La fertilisation apportée en deuxième année a permis d'augmenter la photosynthèse, sans toutefois l'amener au niveau des deux autres espèces et sans éliminer la saturation à de faibles éclairements. L'ennoyage a provoqué d'importantes perturbations dans la photosynthèse : l'assimilation nette de CO2 et la conductance stomatique ont été fortement réduites, la capacité photosynthétique et les teneurs en chlorophylles ont baissé ; des limitations de croissance ont également été constatées. De plus, l'efficience photochimique mesurée en fin de nuit, qui est habituellement proche du niveau optimal d'environ 0,83, a baissé significativement. Les perturbations les plus marquées ont été observées sur les semis de Q rubra, et les plus limitées sur ceux de Q robur, ce qui confirme la bonne corrélation existant entre perturbations à court terme de la photosynthèse, et sensibilité globale à l'ennoyage. L'ombrage a permis de limiter et de retarder, dans une certaine mesure, l'apparition de ces perturbations. Enfin, le comportement des semis pendant la phase de ressuyage a permis de différencier fortement les espèces. Aucune n'a retrouvé un fonctionnement optimal après 3 semaines de ressuyage. Q robur a manifesté la meilleure capacité de récupération, alors que Q petraea a présenté de progressives nécroses foliaires, un potentiel hydrique décroissant et des niveaux de photosynthèse très faibles pendant les 3 semaines. Enfin, Q rubra a subi une très forte sénescence foliaire et une mortalité non négligeable au cours du ressuyage.

We assessed the effects of doubling atmospheric CO 2 concentration, [CO 2 ], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual 13 CO 2 and 15 NO 3 - labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l -1 [CO 2 ] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l -1 [CO 2 ] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO 2 assimilation rate (+37% on a leaf dry weight basis ; +71% on a leaf area basis), and a decrease in both above- and below-ground CO 2 respiration rates (-32 and -26%, respectively. on a dry mass basis) under elevated [CO 2 ]. 13 C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO 2 ] both after the 12-h 13 CO 2 pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO 2 (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO 2 ] on fine root-specific 15 N uptake (amount of recently assimilated 15 N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO 2 ] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO 2 ]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h 13 C chase phase, leaves off the first and second growth flushes were almost completely depleted in recent 13 C under ambient [CO 2 ], whereas these leaves retained important amounts of recently assimilated 13 C (carbohydrate reserves ?) under elevated [CO 2 ].

[1] We present a two-criteria inverse modeling approach to analyze the effects of seasonal drought on ecosystem gas exchange at three Mediterranean sites. The three sites include two nearly monospecific Quercus ilex L. forests, one on karstic limestone (Puechabon), the other on fluvial sand with access to groundwater (Castelporziano), and a typical multispecies shrubland on limestone (Arca di Noe). A canopy gas exchange model Process Pixel Net Ecosystem Exchange (PROXELNEE), which contains the Farquhar photosynthesis model coupled to stomatal conductance via the Ball-Berry model, was inverted in order to estimate the seasonal time course of canopy parameters from hourly values of ecosystem gross carbon uptake and transpiration. It was shown that an inverse estimation of leaf-level parameters was impossible when optimizing against ecosystem H2O or CO2 fluxes alone (unidentifiable parameters). In contrast, a criterion that constrained the optimization against both H2O and CO2 fluxes yielded stable estimates of leaf-level parameters. Two separate model inversions were implemented to test two alternative hypotheses about the response to drought: a reduction in active leaf area as a result of patchy stomatal closure or a change in photosynthetic capacities. In contrast to a previously tested hypothesis of classical (uniform) stomatal control, both hypotheses were equally able to describe the seasonality of carbon uptake and transpiration on all three sites, with a decline during the drought and recovery after autumn rainfall. Large reductions of up to 80%, in either active leaf area or photosynthetic capacities, were necessary to describe the observed carbon and water fluxes at the end of the drought period. With a threshold-type relationship, soil water content was an excellent predictor of these changes. With the drought-dependent parameter changes included, the canopy model explains 80-90% of the variance of hourly gross CO2 uptake (root mean squared error (RMSE): 1.1-2.6 mumol m(-2) s(-1)) and 70-80% of the variance of hourly transpiration (RMSE: 0.02-0.03 mm h(-1)) at all sites. In addition to drought effects, changes in leaf photosynthetic activity not related to water availability, i.e., high spring activity, were detected through the inverse modeling approach. Moreover, our study exemplifies a kind of multiconstraint inverse modeling that can be profitably used for calibrating ecosystem models that are meant for global applications with ecosystem flux data.

52 ref.; International audience; Stomatal opening, which controls gas exchanges between plants and the atmosphere, results from an increase in turgor of the two guard cells that surround the pore of the stoma. KAT1 was the only inward K+ channel shown to be expressed in Arabidopsis guard cells, where it was proposed to mediate a K+ influx that enables stomatal opening. We report that another Arabidopsis K+channel, KAT2, is expressed in guard cells. More than KAT1, KAT2 displays functional features resembling those of native inward K+ channels in guard cells. Coexpression inXenopus oocytes and two-hybrid experiments indicated that KAT1 and KAT2 can form heteromultimeric channels. The data indicate that KAT2 plays a crucial role in the stomatal opening machinery.

La composition de l’atmosphere terrestre a beaucoup varie au cours des temps geologiques. Avant l’apparition de la vie, l’atmosphere etait sans doute dominee par le gaz carbonique produit par les eruptions volcaniques, comme c’est encore le cas pour les planetes sans vie qui nous entourent, Venus et Mars. Au cours des temps geologiques, l’absorption du CO2 par la photosynthese, et l’enfouissement de grandes quantites de carbone dans les roches sedimentaires ont progressivement reduit la teneur en CO2 et eleve la teneur en oxygene de l’air. A la fin du Secondaire, au moment de l’extinction des dinosaures, il y avait encore environ deux fois plus de CO2 qu’en 1800, avant l’ere industrielle (Berner, 1994). La composition actuelle de l’atmosphere resulte d’un equilibre entre des processus biologiques comme la photosynthese et la respiration, et des processus physico-chimiques comme l’absorption du CO2 par les eaux froides des oceans, sous-saturees en CO2, et son rejet par les eaux chaudes, sur-saturees en CO2. On sait, par l’analyse des bulles d’air piegees dans les glaces de l’Antarctique et du Groenland, que la teneur en CO2 de l’air a varie relativement recemment en fonction de la temperature de la Terre, d’environ 200 ppmv (parties par million en volume) pendant les periodes glaciaires, a environ 280 ppmv pendant les periodes plus chaudes, comme celle que nous connaissons depuis pres de 10 000 ans (Barnola et al., 1991).

International audience; Temperature effects on photosynthesis were studied in seedlings of evergreen Mediterranean cork oak (Quercus suber L.). Responses to changes in temperature and the temperature optima of maximal carboxylation rate (V-cmax) and maximal light-driven electron flux (J(max)) were estimated from gas exchange measurements and a leaf-level photosynthesis model. The estimated temperature optima were approximately 34 and 33 degreesC for V-cmax and J(max), respectively, which fall within the lower range of temperature optima previously observed in deciduous tree species. The thermostability of the photosynthetic apparatus was estimated according to the temperature at which basal chlorophyll a fluorescence begins to increase (T,). The T, was highly variable, increasing from 42 to 51 degreesC when ambient temperature rose from 10 to 40 degreesC, and increasing from 44 to 54 degreesC with decreasing soil water availability while net CO2 assimilation rate dropped to almost zero. When a heat shock was imposed, an additional small increase in T-c was observed in drought-stressed and control seedlings. Maximal T-c values following heat shock were about 56 degreesC, which, to our knowledge, are the highest values that have been observed in tree species. In conclusion, the intrinsic temperature responses of cork oak did not differ from those of other species (similar T-c under ambient temperature and water availability, and relatively low thermal optima for photosynthetic capacity in seedlings grown at cool temperatures). However, the large ability of cork oak to acclimate to drought and elevated temperature may be an important factor in the tolerance of this evergreen Mediterranean species to summer drought and high temperatures.

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