Characteristics and changes of thermokarst lakes in Canada, Russia and Sweden

Peat plateaus are widespread at high northern latitudes and are important soil organic carbon reservoirs. A warming climate can cause either increased ground subsidence (thermokarst) resulting in lake formation or increased drainage as the permafrost thaws. A better understanding of spatiotemporal variations in these landforms in relation to climate change is important for predicting the future thawing permafrost carbon feedback. In this study, dynamics in thermokarst lake extent during the last 35-50 years has been quantified through time series analysis of aerial photographs and high-resolution satellite images (IKONOS/QuickBird) in three peat plateau complexes, spread out across the northern circumpolar region along a climatic and permafrost gradient. From the mid-1970s until the mid-2000s there has been an increase in mean annual air temperature, winter precipitation, and ground temperature in all three study areas. The two peat plateaus located in the continuous and discontinuous permafrost zones, respectively, where mean annual air temperatures are below -5��C and ground temperatures are -2��C or colder, have experienced small changes in thermokarst lake extent. In the peat plateau located in the sporadic permafrost zone where the mean annual air temperature is around -3��C, and the ground temperature is close to 0��C, lake drainage and infilling with fen vegetation has been extensive and many new thermokarst lakes have formed. In a future progressively warmer and wetter climate permafrost degradation can cause significant impacts on landscape composition and greenhouse gas exchange also in areas with extensive peat plateaus, which presently still experience stable permafrost conditions.

Simulated Bromoform emission and concentration using the ocean biogeochemistry model MPIOM/HAMOCC forced by 6-hourly NCEP data

Bromoform (CHBr3) is one important precursor of atmospheric reactive bromine species that are involved in ozone depletion in the troposphere and stratosphere. In the open ocean bromoform production is linked to phytoplankton that contains the enzyme bromoperoxidase. Coastal sources of bromoform are higher than open ocean sources. However, open ocean emissions are important because the transfer of tracers into higher altitude in the air, i.e. into the ozone layer, strongly depends on the location of emissions. For example, emissions in the tropics are more rapidly transported into the upper atmosphere than emissions from higher latitudes. Global spatio-temporal features of bromoform emissions are poorly constrained. Here, a global three-dimensional ocean biogeochemistry model (MPIOM-HAMOCC) is used to simulate bromoform cycling in the ocean and emissions into the atmosphere using recently published data of global atmospheric concentrations (Ziska et al., 2013) as upper boundary conditions. Our simulated surface concentrations of CHBr3 match the observations well. Simulated global annual emissions based on monthly mean model output are lower than previous estimates, including the estimate by Ziska et al. (2013), because the gas exchange reverses when less bromoform is produced in non-blooming seasons. This is the case for higher latitudes, i.e. the polar regions and northern North Atlantic. Further model experiments show that future model studies may need to distinguish different bromoform-producing phytoplankton species and reveal that the transport of CHBr3 from the coast considerably alters open ocean bromoform concentrations, in particular in the northern sub-polar and polar regions.

Noble gases sampled from three cruises in the eastern tropical Atlantic in September 2005 and June/July 2006

Upwelling velocities w in the equatorial band are too small to be directly observed. Here, we apply a recently proposed indirect method, using the observed helium isotope (3He or 4He) disequilibria in the mixed layer. The helium data were sampled from three cruises in the eastern tropical Atlantic in September 2005 and June/July 2006. A one-dimensional two-box model was applied, where the helium air-sea gas exchange is balanced by upwelling from 3He-rich water below the mixed layer and by vertical mixing. The mixing coefficients Kv were estimated from microstructure measurements, and on two of the cruises, Kv exceeded 1 x 10**-4 m**2/s, making the vertical mixing term of the same order of magnitude as the gas exchange and the upwelling term. In total, helium disequilibrium was observed on 54 stations. Of the calculated upwelling velocities, 48% were smaller than 1.0 x 10**-5 m/s, 19% were between 1.0 and 2.0 x 10**-5 m/s, 22% were between 2.0 and 4.0 x 10**-5 m/s, and on 11% of upwelling velocities exceeded this limit. The highest upwelling velocities were found in late June 2006. Meridional upwelling distribution indicated an equatorial asymmetry with higher vertical velocities between the equator and 1�� to 2�� south compared to north of the equator, particularly at 10��W. Associated heat flux into the mixed layer could be as high as 138 W/m**2, but this depends strongly on the chosen depths where the upwelled water comes from. By combining upwelling velocities with sea surface temperature and productivity distributions, a mean monthly equatorial upwelling rate of 19 Sv was estimated for June 2006 and a biweekly mean of 24 Sv was estimated for September 2005.

Seawater carbonate chemistry and buffer capacity of the coelomic fluid in echinoderms in a laboratory experiment

The increase in atmospheric CO2 due to anthropogenic activity results in an acidification of the surface waters of the oceans. The impact of these chemical changes depends on the considered organisms. In particular, it depends on the ability of the organism to control the pH of its inner fluids. Among echinoderms, this ability seems to differ significantly according to species or taxa. In the present paper, we investigated the buffer capacity of the coelomic fluid in different echinoderm taxa as well as factors modifying this capacity. Euechinoidea (sea urchins except Cidaroidea) present a very high buffer capacity of the coelomic fluid (from 0.8 to 1.8 mmol/kg SW above that of seawater), while Cidaroidea (other sea urchins), starfish and holothurians have a significantly lower one (from -0.1 to 0.4 mmol/kg SW compared to seawater). We hypothesize that this is linked to the more efficient gas exchange structures present in the three last taxa, whereas Euechinoidea evolved specific buffer systems to compensate lower gas exchange abilities. The constituents of the buffer capacity and the factors influencing it were investigated in the sea urchin Paracentrotus lividus and the starfish Asterias rubens. Buffer capacity is primarily due to the bicarbonate buffer system of seawater (representing about 63% for sea urchins and 92% for starfish). It is also partly due to coelomocytes present in the coelomic fluid (around 8% for both) and, in P. lividus only, a compound of an apparent size larger than 3 kDa is involved (about 15%). Feeding increased the buffer capacity in P. lividus (to a difference with seawater of about 2.3 mmol/kg SW compared to unfed ones who showed a difference of about 0.5 mmol/kg SW) but not in A. rubens (difference with seawater of about 0.2 for both conditions). In P. lividus, decreased seawater pH induced an increase of the buffer capacity of individuals maintained at pH 7.7 to about twice that of the control individuals and, for those at pH 7.4, about three times. This allowed a partial compensation of the coelomic fluid pH for individuals maintained at pH 7.7 but not for those at pH 7.4.

Bulk properties and isotopic data of sediment cores from the Bounty Trough and Tasman Sea off New Zealand

Glacial/interglacial changes in Southern Ocean's air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate- to deep-water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943 - 2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

Sea-suface microlayer (SML) and underlying water (ULW) samples focusing on CDOM spectral characteristics during METEOR cruise M91

The coastal upwelling system off the coast of Peru is characterized by high biological activity and a pronounced subsurface oxygen minimum zone, as well as associated emissions of atmospheric trace gases such as N2O, CH4 and CO2. From 3 to 23 December 2012, R/V Meteor (M91) cruise took place in the Peruvian upwelling system between 4.59 and 15.4��S, and 82.0 to 77.5��W. During M91 we investigated the composition of the sea-surface microlayer (SML), the oceanic uppermost boundary directly subject to high solar radiation, often enriched in specific organic compounds of biological origin like chromophoric dissolved organic matter (CDOM) and marine gels. In the SML, the continuous photochemical and microbial recycling of organic matter may strongly influence gas exchange between marine systems and the atmosphere. We analyzed SML and underlying water (ULW) samples at 38 stations focusing on CDOM spectral characteristics as indicator of photochemical and microbial alteration processes. CDOM composition was characterized by spectral slope (S) values and excitation-emission matrix fluorescence (EEMs), which allow us to track changes in molecular weight (MW) of DOM, and to determine potential DOM sources and sinks. Spectral slope S varied between 0.012 to 0.043 1 nm-1 and was quite similar between SML and ULW, with no significant differences between the two compartments. Higher S values were observed in the ULW of the southern stations below 15��S. By EEMs, we identified five fluorescent components (F1-5) of the CDOM pool, of which two had excitation/emission characteristics of amino-acid-like fluorophores (F1, F4) and were highly enriched in the SML, with a median ratio SML : ULW of 1.5 for both fluorophores. In the study region, values for CDOM absorption ranged from 0.07 to 1.47 m-1. CDOM was generally highly concentrated in the SML, with a median enrichment with respect to the ULW of 1.2. CDOM composition and changes in spectral slope properties suggested a local microbial release of DOM directly in the SML as a response to light exposure in this extreme environment. In a conceptual model of the sources and modifications of optically active DOM in the SML and underlying seawater (ULW), we describe processes we think may take place (Fig. 1); the production of CDOM of higher MW by microbial release through growth, exudation and lysis in the euphotic zone, includes the identified fluorophores (F1, F2, F3, F4, F5). Specific amino-acid-like fluorophores (F1, F4) accumulate in the SML with respect to the ULW, as photochemistry may enhance microbial CDOM release by (a) photoprotection mechanisms and (b) cell-lysis processes. Microbial and photochemical degradation are potential sinks of the amino-acid-like fluorophores (F1, F4), and potential sources of reworked and more refractory humic-like components (F2, F3, F5). In the highly productive upwelling region along the Peruvian coast, the interplay of microbial and photochemical processes controls the enrichment of amino-acid-like CDOM in the SML. We discuss potential implications for air-sea gas exchange in this area.

Biogenic composition of the sea-surface microlayer from a mesocosm study (Bergen 2011)

The sea-surface microlayer (SML) is the ocean's uppermost boundary to the atmosphere and in control of climate relevant processes like gas exchange and emission of marine primary organic aerosols (POA). The SML represents a complex surface film including organic components like polysaccharides, pro- teins, and marine gel particles, and harbors diverse microbial communities. Despite the potential relevance of the SML in ocean-atmosphere interactions, still little is known about its structural characteristics and sen- sitivity to a changing environment such as increased oceanic uptake of anthropogenic CO2. Here we report results of a large-scale mesocosm study, indicating that ocean acidification can affect the abundance and activity of microorganisms during phytoplankton blooms, resulting in changes in composition and dynam- ics of organic matter in the SML. Our results reveal a potential coupling between anthropogenic CO2 emis- sions and the biogenic properties of the SML, pointing to a hitherto disregarded feedback process between ocean and atmosphere under climate change.

Upwelling and associated heat flux in the equatorial Atlantic inferred from helium isotope disequilibrium

Sea-to-air and diapycnal fluxes of nitrous oxide (N2O) into the mixed layer were determined during three cruises to the upwelling region off Mauritania. Sea-to-air fluxes as well as diapycnal fluxes were elevated close to the shelf break, but elevated sea-to-air fluxes reached further offshore as a result of the offshore transport of upwelled water masses. To calculate a mixed layer budget for N2O we compared the regionally averaged sea-to-air and diapycnal fluxes and estimated the potential contribution of other processes, such as vertical advection and biological N2O production in the mixed layer. Using common parameterizations for the gas transfer velocity, the comparison of the average sea-toair and diapycnal N2O fluxes indicated that the mean sea-toair flux is about three to four times larger than the diapycnal flux. Neither vertical and horizontal advection nor biological production were found sufficient to close the mixed layer budget. Instead, the sea-to-air flux, calculated using a parameterization that takes into account the attenuating effect of surfactants on gas exchange, is in the same range as the diapycnal flux. From our observations we conclude that common parameterizations for the gas transfer velocity likely overestimate the air-sea gas exchange within highly productive upwelling zones.

Dissecting Arabidopsis G�� signal transduction on the protein surface.

The heterotrimeric G-protein complex provides signal amplification and target specificity. The Arabidopsis (Arabidopsis thaliana) G?-subunit of this complex (AGB1) interacts with and modulates the activity of target cytoplasmic proteins. This specificity resides in the structure of the interface between AGB1 and its targets. Important surface residues of AGB1, which were deduced from a comparative evolutionary approach, were mutated to dissect AGB1-dependent physiological functions. Analysis of the capacity of these mutants to complement well-established phenotypes of G?-null mutants revealed AGB1 residues critical for specific AGB1-mediated biological processes, including growth architecture, pathogen resistance, stomata-mediated leaf-air gas exchange, and possibly photosynthesis. These findings provide promising new avenues to direct the finely tuned engineering of crop yield and traits.

Obtenci��n de clones de alcornoque (Quercus suber L.) mediante embriog��nesis som��tica: aplicaci��n de medios l��quidos y biorreactores en la sincronizaci��n de los procesos de desarrollo y maduraci��n de los embriones

El alcornoque tiene un gran valor ambiental, como integrante de los ecosistemas forestales mediterr��neos, e inter��s comercial por el valor de la bellota (alimentaci��n del cerdo ib��rico), el carb��n, la madera y sobre todo por las aplicaciones industriales del corcho. Las posibilidades de mejora gen��tica del alcornoque, como las de otras especies forestales, est��n limitadas por sus largos ciclos reproductivos y porque su propagaci��n vegetativa mediante estaquillado solo es posible en estados muy juveniles. Por ello este sistema de propagaci��n tiene muy poca, o ninguna, utilidad pr��ctica en la mejora gen��tica. La embriog��nesis som��tica es la v��a m��s apropiada para la clonaci��n de muchas especies forestales y ha hecho posible el desarrollo a gran escala de plantaciones multivarietales de con��feras. En alcornoque es posible la regeneraci��n completa de ��rboles adultos mediante embriog��nesis som��tica. Con los protocolos actuales (en medio semis��lido), los embriones se generan formando ac��mulos y en la fase de multiplicaci��n conviven embriones en distintos estados de desarrollo. Es un sistema asincr��nico, con baja eficacia para la propagaci��n en masa, que no elimina completamente las dificultades para el desarrollo de programas de mejora gen��tica del alcornoque. En otras especies la utilizaci��n de medios l��quidos ha mejorado: la sincronizaci��n, productividad de los cultivos, el manejo y reducido los costes de producci��n. Por ello el desarrollo de suspensiones embriog��nicas de alcornoque se plantea como una v��a para aumentar la eficacia de la propagaci��n clonal a gran escala. En la presente tesis se desarrollan cultivos embriog��nicos de alcornoque en medio l��quido. El cap��tulo 3 aborda el establecimiento y mantenimiento de suspensiones, el cap��tulo 4 el desarrollo de una fase de proliferaci��n en medio l��quido y el cap��tulo 5 la utilizaci��n de sistemas de cultivo en medio l��quido, estacionarios y de inmersi��n temporal, como v��a para favorecer la maduraci��n de los embriones som��ticos. Para iniciar los cultivos en medio l��quido se emplearon agregados de embriones tomados de la fase de proliferaci��n en medio semis��lido. Cuando estos agregados se inocularon directamente en medio l��quido no se logr�� el establecimiento de las suspensiones. El establecimiento se consigui�� empleando como in��culo las c��lulas y Resumen peque��os agregados embriog��nicos, de tama��o comprendido entre 41 y 800 ��m, desprendidas por agitaci��n breve de los agregados de embriones. El mantenimiento se logr�� inoculando en baja densidad masas embriog��nicas compactas de tama��o comprendido entre 0,8 y 1,2 mm. Estas suspensiones, muy heterog��neas, mantuvieron su capacidad de proliferaci��n y de regeneraci��n de embriones al menos durante diez subcultivos consecutivos. El protocolo de iniciaci��n y mantenimiento, desarrollado inicialmente con un solo genotipo, fue eficaz cuando se prob�� sobre otros 11 genotipos de alcornoque. En la fase de proliferaci��n se ensayaron tres tipos de envase y tres velocidades de agitaci��n. La combinaci��n envase �� velocidad determin�� el intercambio gaseoso, la disponibilidad de ox��geno y el estr��s hidrodin��mico. Los agregados embriog��nicos de alcornoque crecieron incluso en condiciones de hipoxia no siendo la disponibilidad de ox��geno un factor limitante del crecimiento para tasas de trasferencia de ox��geno comprendidas entre 0,11 h-1 y 1,47 h-1. Por otra parte la producci��n de biomasa creci�� con el estr��s hidrodin��mico para valores de ��ndice de cizalladura inferiores a 5 x 10-3 cm min-1. La mayor producci��n de biomasa se obtuvo con matraces Erlenmeyer de 100 ml y alta velocidad de agitaci��n (160 rpm) mientras que la diferenciaci��n de embriones se vio favorecida por bajas velocidades de agitaci��n (60 rpm) asociadas con bajas disponibilidades de ox��geno. La posibilidad de madurar embriones de alcornoque en medio l��quido se estudi�� utilizando sistemas de inmersi��n permanente y sistemas de inmersi��n temporal. En inmersi��n permanente no se diferenciaron embriones cotiledonares (posiblemente por hiperhidricidad). Los sistemas de inmersi��n temporal permitieron obtener embriones maduros en estado cotiledonar y capaces de regenerar plantas in vitro. Concentraciones de sacarosa superiores a 60 g l-1 y frecuencias de inmersi��n iguales o inferiores a una diaria, tuvieron efectos negativos para el desarrollo de los embriones som��ticos. En los sistemas de inmersi��n temporal los par��metros f��sico-qu��micos del medio de cultivo se mantuvieron estables y no se observ�� ninguna limitaci��n de nutrientes. No obstante, estos sistemas se vieron afectados por la evaporaci��n que gener�� el flujo de aire necesario para desplazar el l��quido en cada periodo de inmersi��n. Abstract ABSTRACT Cork oak is one of the most important tree species of the Mediterranean ecosystem. Besides its high environmental value has a great economic interest due to the sustainable production of acorns (to feed the Iberian pig) charcoal, timber and cork, which is a renewable natural product with various technological applications. As happens with other forest species, cork oak genetic improvement programs are limited by their long life cycles and because vegetative propagation by cuttings it��s only possible in very juvenile plants. Hence this propagation system is useless or has little practical use for breeding cork oak. Plant regeneration by somatic embryogenesis is the most suitable way for cloning many forest species, and it is the enabling technology which has allowed the establishment of large-scale conifer multi-varietal plantations. Clonal plant regeneration of mature cork oak trees can be achieved through somatic embryogenesis. Somatic embryos at different stages of development and forming clusters are produced during the multiplication phase with current protocols (using semisolid medium). This is an asynchronous low-efficient process not suitable for mass propagation, and therefore it does not solve the difficulties presented by cork oak breeding programs. Culture in liquid medium has been used with other species to improve: synchronization, yield, handling, and to reduce production costs. Thus the development of cork oak embryogenic suspension cultures is envisaged as a way to increase the efficiency of large scale clonal propagation. The thesis herein develops cork oak embryogenic cultures in liquid medium. In chapter 3 establishment and maintenance of suspension cultures are developed, chapter 4 studies proliferation phase in liquid medium and chapter 5 considers the use of different systems of culture in liquid medium, both stationary and temporary immersion, as a way to promote somatic embryos maturation. Clusters of embryos taken from proliferating cultures on semisolid medium were used to initiate the cultures in liquid medium. When these clusters were inoculated directly in liquid medium establishment of suspension cultures was not executed. However using, as initial inoculum, cells and cell aggregates with a size between 41 and 800 ��m detached from these clusters of embryos, subjected to a brief shaking, suspension cultures could be established. Suspension maintenance was achieved by inoculating compact embryogenic Abstract clumps with a size between 0.8 and 1.2 mm at low density. The suspension cultures, very heterogeneous, retained both their proliferation and embryo regeneration capacity for at least ten consecutive subcultures. The initiation and maintenance protocol, initially developed with a single genotype, was effective when tested on 11 additional genotypes of cork oak. In proliferation phase three types of vessels and three different levels of agitation were assayed. The combination vessel �� orbiting speed determined gas exchange, oxygen availability and hydrodynamic stress. Cork oak embryogenic aggregates grew even under hypoxia conditions; oxygen availability at transfer rates between 0.11 and 1.47 h-1 was not a limiting factor for growth. Furthermore the biomass production was increased with hydrodynamic stress when shear rate values were of less than 5 x 10-3 cm min-1. The highest biomass production was obtained with 100 ml Erlenmeyer flask and high stirring speed (160 rpm) while the differentiation of embryos was favored by low agitation speeds (60 rpm) associated with low oxygen availability. The possibility to mature cork oak somatic embryos in liquid medium was studied using both permanent immersion systems and temporary immersion systems. Cotyledonary embryos did not differentiate in permanent immersion conditions (probably due to hyperhydricity). Temporary immersion systems allowed obtaining mature cotyledonary embryos, which were able to regenerate plants in vitro. Sucrose concentrations above 60 g l-1 and immersion frequencies equal to or lower than one each 24 h had negative effects on somatic embryo development. Physicochemical parameters of the culture medium in temporary immersion systems were stable and showed no limitation of nutrients. However, these systems were affected by the evaporation generated by the airflow necessary to relocate the medium at each immersion period.

Conductancia del mes��filo en especies forestales: implicaciones en la respuesta fotosint��tica a la disponibilidad de agua y luz

La fotos��ntesis es el proceso biol��gico que permite la producci��n primaria y, por tanto, la vida en nuestro planeta. La tasa fotosint��tica viene determinada por la ���maquinaria��� bioqu��mica y las resistencias difusivas al paso del CO2 desde la atm��sfera hasta su fijaci��n en el interior de los cloroplastos. Hist��ricamente la mayor resistencia difusiva se ha atribuido al cierre estom��tico, sin embargo ahora sabemos, debido a las mejoras en las t��cnicas experimentales, que existe tambi��n una resistencia grande que se opone a la difusi��n del CO2 desde los espacios intercelulares a los lugares de carboxilaci��n. Esta resistencia, llamada normalmente por su inversa: la conductancia del mes��filo (gm), puede ser igual o incluso superior a la resistencia debida por el cierre estom��tico. En la presente tesis doctoral he caracterizado la limitaci��n que ejerce la resistencia del mes��filo a la fijaci��n de CO2 en diversas especies forestales y en distintos momentos de su ciclo biol��gico. En la fase de regenerado, hemos estudiado tres situaciones ambientales relevantes en el mayor ��xito de su supervivencia, que son: el d��ficit h��drico, su interacci��n con la irradiancia y el paso del crecimiento en la sombra a mayor irradiancia, como puede suceder tras la apertura de un hueco en el dosel forestal. En la fase de arbolado adulto se ha caracterizado el estado h��drico y el intercambio gaseoso en hojas desarrolladas a distinta irradiancia dentro del dosel vegetal durante tres a��os contrastados en pluviometr��a. Para cada tipo de estudio se han empleado las t��cnicas ecofisiol��gicas m��s pertinentes para evaluar el estado h��drico y el intercambio gaseoso. Por su complejidad y la falta de un m��todo que permita su cuantificaci��n directa, la gm ha sido evaluada por los m��todos m��s usados, que son: la discriminaci��n isot��pica del carbono 13, el m��todo de la J variable, el m��todo de la J constante y el m��todo de la curvatura. Los resultados m��s significativos permiten concluir que la limitaci��n relativa a la fotos��ntesis por la conductancia estom��tica, del mes��filo y bioqu��mica es dependiente de la localizaci��n de la hoja en el dosel forestal. Por primera vez se ha documentado que bajo estr��s h��drico las hojas desarrolladas a la sombra estuvieron m��s limitadas por una reducci��n en la gm, mientras que las hojas desarrolladas a pleno sol estuvieron m��s limitadas por reducci��n mayor de la conductancia estom��tica (gsw). Encontramos buena conexi��n entre el aparato fotosint��tico foliar y el sistema h��drico debido al alto grado de correlaci��n entre la conductancia hidr��ulica foliar aparente y la concentraci��n de CO2 en los cloroplastos en distintas especies forestales. Adem��s, hemos mostrado diferentes pautas de regulaci��n del intercambio gaseoso seg��n las particularidades ecol��gicas de las especies estudiadas. Tanto en brinzales crecidos de forma natural y en el arbolado adulto como en pl��ntulas cultivadas en el invernadero la ontogenia afect�� a las limitaciones de la fotos��ntesis producidas por estr��s h��drico, resultando que las limitaciones estom��ticas fueron dominantes en hojas m��s j��venes mientras que las no estom��ticas en hojas m��s maduras. La puesta en luz supuso un gran descenso en la gm durante los d��as siguientes a la transferencia, siendo este efecto mayor seg��n el grado de sombreo previo en el que se han desarrollado las hojas. La aclimataci��n de las hojas a la alta irradiancia estuvo ligada a las modificaciones anat��micas foliares y al estado de desarrollo de la hoja. El ratio entre la gm/gsw determin�� la mayor eficiencia en el uso del agua y un menor estado oxidativo durante la fase de estr��s h��drico y su posterior rehidrataci��n, lo cual sugiere el uso de este ratio en los programas de mejora gen��tica frente al estr��s h��drico. Debido a que la mayor��a de modelos de estimaci��n de la producci��n primaria bruta (GPP) de un ecosistema no incluye la gm, los mismos est��n incurriendo en una sobreestimaci��n del GPP particularmente bajo condiciones de estr��s h��drico, porque m��s de la mitad de la reducci��n en fotos��ntesis en hojas desarrolladas a la sombra se debe a la reducci��n en gm. Finalmente se presenta un an��lisis de la importancia en las estimas de la gm bajo estr��s h��drico de la refijaci��n del CO2 emitido en la mitocondria a consecuencia de la fotorrespiraci��n y la respiraci��n mitocondrial en luz. ABSTRACT Photosynthesis is the biological process that supports primary production and, therefore, life on our planet. Rates of photosynthesis are determined by biochemical ���machinery��� and the diffusive resistance to the transfer of CO2 from the atmosphere to the place of fixation within the chloroplasts. Historically the largest diffusive resistance was attributed to the stomata, although we now know via improvements in experimental techniques that there is also a large resistance from sub-stomatal cavities to sites of carboxylation. This resistance, commonly quantified as mesophyll conductance (gm), can be as large or even larger than that due to stomatal resistance. In the present PhD I have characterized the limitation exerted by the mesophyll resistance to CO2 fixation in different forest species at different stages of their life cycle. In seedlings, we studied three environmental conditions that affect plant fitness, namely, water deficit, the interaction of water deficit with irradiance, and the transfer of plants grown in the shade to higher irradiance as can occur when a gap opens in the forest canopy. At the stage of mature trees we characterized water status and gas exchange in leaves developed at different irradiance within the canopy over the course of three years that had contrasting rainfall. For each study we used the most relevant ecophysiological techniques to quantify water relations and gas exchange. Due to its complexity and the lack of a method that allows direct quantification, gm was estimated by the most commonly used methods which are: carbon isotope discrimination, the J-variable, constant J and the curvature method The most significant results suggest that the relative limitation of photosynthesis by stomata, mesophyll and biochemistry depending on the position of the leaf within the canopy. For the first time it was documented that under water stress shaded leaves were more limited by a reduction in gm, while the sun-adapted leaves were more limited by stomatal conductance (gsw). The connection between leaf photosynthetic apparatus and the hydraulic system was shown by the good correlations found between the apparent leaf hydraulic conductance and the CO2 concentration in the chloroplasts in shade- and sun-adapted leaves of several tree species. In addition, we have revealed different patterns of gas exchange regulation according to the functional ecology of the species studied. In field grown trees and greenhouse-grown seedlings ontogeny affected limitations of photosynthesis due to water stress with stomatal limitations dominating in young leaves and nonstomatal limitations in older leaves. The transfer to high light resulted in major decrease of gm during the days following the transfer and this effect was greater as higher was the shade which leaves were developed. Acclimation to high light was linked to the leaf anatomical changes and the state of leaf development. The ratio between the gm/gsw determined the greater efficiency in water use and reduced the oxidative stress during the water stress and subsequent rehydration, suggesting the use of this ratio in breeding programs aiming to increase avoidance of water stress. Because most models to estimate gross primary production (GPP) of an ecosystem do not include gm, they are incurring an overestimation of GPP particularly under conditions of water stress because more than half of An decrease in shade-developed leaves may be due to reduction in gm. Finally, we present an analysis of the importance of how estimates of gm under water stress are affected by the refixation of CO2 that is emitted from mitochondria via photorespiration and mitochondrial respiration in light.