Shade tree species affect gas exchange and hydraulic conductivity of cacao cultivars in an agroforestry system.

Shade tolerance is a widespread strategy of rainforest understory plants. Many understory species have green young stems that may assimilate CO2 and contribute to whole-plant carbon balance. Cacao commonly grows in the shaded understory and recent emphasis has been placed on diversifying the types of trees used to shade cacao plants to achieve additional ecosystem services. We studied three agricultural cacao cultivars growing in the shade of four timber species (Cedrela odorata L., Cordia thaisiana Agostini, Swietenia macrophylla King and Tabebuia rosea (Bertol) A.D.C.) in an agroforestry system to (i) evaluate the timber species for their effect on the physiological performance of three cacao cultivars; (ii) assess the role of green stems on the carbon economy of cacao; and (iii) examine coordination between stem hydraulic conductivity and stem photosynthesis in cacao. Green young stem photosynthetic CO2 assimilation rate was positive and double leaf CO2 assimilation rate, indicating a positive contribution of green stems to the carbon economy of cacao; however, green stem area is smaller than leaf area and its relative contribution is low. Timber species showed a significant effect on leaf gas exchange traits and on stomatal conductance of cacao, and stem water-use efficiency varied among cultivars. There were no significant differences in leaf-specific hydraulic conductivity among cacao cultivars, but sapwood-specific hydraulic conductivity varied significantly among cultivars and there was an interactive effect of cacao cultivar × timber species. Hydraulic efficiency was coordinated with stem-stomatal conductance, but not with leaf-stomatal conductance or any measure of photosynthesis. We conclude that different shade regimes determined by timber species and the interaction with cacao cultivar had an important effect on most of the physiological traits and growth variables of three cacao cultivars growing in an agroforestry system. Results suggested that C. odorata is the best timber species to provide partial shade for cacao cultivars in the Barlovento region in Venezuela, regardless of cultivar origin.

Seasonal gas exchange and resource-use efficiency in evergreen versus deciduous species from a tropical dry forest.

Tropical dry forests (TDFs) experience a long dry season in which plant species are subject to several months of water deficit. However, TDFs maintain a diverse group of plant life forms, growth forms and leaf phenology, and it is not clear how they vary in their mechanisms for coping with seasonal drought. We studied seasonal changes in leaf water potential (Ψleaf), gas exchange, photochemical activity and functional traits in evergreen and drought-deciduous species from a TDF to determine if leaf phenology mediates plant responses to drought. We found seasonal decreases in Ψleaf, stomatal conductance (gs) and transpiration rate (E), and increases in both intrinsic and instantaneous water-use efficiency. We did not find seasonal differences in photosynthetic rate (Aarea) and carbon isotope composition (δ13C); however, these traits differed between leaf phenology groups, with drought-deciduous plants having higher Aarea and δ13C than evergreen plants. We also found that plants with high leaf nitrogen concentration (Narea) also had low mass-based photosynthetic rate (Amass), photosynthetic-nitrogen-use efficiency and specific leaf area, contrary to the expected relationships given by the leaf economics spectrum. Despite higher Narea, sclerophyllous leaves maintained lower Amass, and this increased structural toughness of leaves may be imposing a stronger limitation for CO2 diffusion and hence photosynthesis. Overall, we found more water-conservative traits in deciduous than in evergreen plants, contrary to what is known about these two leaf phenology groups in other seasonal sites both at tropical and temperate latitudes.

Phenotypic plasticity to photon flux density of physiological, anatomical and growth traits in a modern Criollo cocoa clone.

Cocoa grows under shade, but some cultivars develop successfully in full sunlight. In order to characterize the response to photosynthetic photon flux density (PPFD) of a Modern Criollo cocoa clone, gas exchange, photochemical activity and leaf traits, and their relation to growth were measured in seedlings growing in a greenhouse at three different values of PPFD, as well as in adults in full sunlight and shade in the field. Plants showed changes in physiological, biochemical, and morpho-anatomical traits in response to the different light conditions, and in the phenotypic plasticity of these variables. Seedlings subjected to high PPFD in the greenhouse showed decreases in photosynthetic rate, apparent quantum yield of CO2 fixation and photochemical quenching, and increases in non-photochemical quenching, suggesting down-regulation of PSII. In contrast, trees under full sunlight in the field showed a marked reduction in maximum quantum yield of PSII, indicating photoinhibition and supporting that cocoa is a shade tolerant crop. Cocoa showed higher plasticity of physiological and biochemical variables than morpho-anatomical variables in response to PPFD. Effects of time under treatment in the greenhouse and plant age (greenhouse vs field) on plasticity were observed. The acclimation observed in some of the variables studied after 6 months in high light did not represent a particular advantage to seedlings, since relative growth rate was lower than in low- and medium-light seedlings.

Respuestas fotosintéticas de cacao cultivado sin sombra a alta y baja radiación en áreas de baja demanda evaporativa / Photosynthetic response to low and high light of cacao growing without shade in an area of low evaporative demand

ABSTRACT Cacao (Theobroma cacao L.) breeding programmes in Ecuador have focused on obtaining high-yield clones with improved disease resistance. Cacao clones should also have photosynthetic characteristics which support increased productivity. Regarding the weather conditions at the coast of Ecuador, where most of the year there are overcasts and low air evaporative demand, there is the possibility to grow cacao without overhead shade. This study focused on the photosynthetic response at two different photosynthetic photon flux densities (PPFD) of Ecuadorian cacao clones. Seven-year old cacao clones were evaluated: eight clones of Nacional type and two commercial clones (CCN 51 and EET 103), used as controls. All clones showed an increase of 35 % on average in net photosynthetic rate (A)with increasing PPFD from the light saturation point for cacao (i.e. 400 μmol m-2 s-1) to high values (1000 μmol 2 s-1). Such light responsiveness in A has not been reported before. Higher A was associated with higher apparent electron transport rate, while high stomatal conductance was maintained under both PPFD conditions. Under high PPFD, low non-photochemical quenching values were found, suggesting low energy dissipation. All clones showed high maximum quantum yields of PSII (Fv/Fm), suggesting the absence of damage of the photochemical system.

RESUMEN Los programas de mejoramiento de cacao (Theobroma cacao L.) en Ecuador se han centrado en la obtención de clones de alto rendimiento con mayor resistencia a las enfermedades. Estos clones también deben tener características fotosintéticas que apoyen una mayor productividad. En las condiciones climáticas en la costa de Ecuador, donde la mayor parte del año hay alta densidad de nubes y baja demanda evaporativa, existe la posibilidad de cultivar cacao sin sombra. Este estudio se centró en la respuesta fotosintética de clones de cacao del Ecuador en dos diferentes densidades de flujo de fotones fotosintéticos (PPFD). Se evaluaron diez clones de cacao de siete años de edad: ocho clones de tipo Nacional recientemente desarrollados por el Instituto Nacional de investigaciones Agropecuarias, y dos clones comerciales utilizados como controles (CCN 51 y EET 103). Todos los clones de cacao mostraron un aumento del 35 % en promedio en la tasa fotosintética neta (A) con el incremento del PPFD desde el punto de saturación de luz para el cacao (400 μmol m -2 s -1) hasta valores altos (1000 μmol m -2 s- 1). Dicha respuesta de A a estas condiciones de luz alta no se ha reportado en cacao. La tasa fotosintética neta se asoció con una mayor velocidad aparente de transporte de electrones (J), mientras que la alta conductancia estomática (gs) se mantuvo en ambas condiciones de PPFD. En condiciones de alto PPFD, se encontraron bajos valores del coeficiente de extinción no fotoquímico (NPQ), lo que sugiere una baja disipación de energía, además de presentarse altos rendimientos cuánticos máximos de PSII (Fv / Fm), indicando la ausencia de daño del sistema fotoquímico.

Water relations and photosynthetic capacity of two species of Calotropis in a tropical semi-arid ecosystem.

BACKGROUND AND AIMS: Calotropis procera and Calotropis gigantea, originally from warm parts of Africa and Asia, are now pan-tropical and in ecological terms considered an indicator of overgrazed, disturbed lands; they grow successfully in dry areas. Variations in water relations, morphology and photosynthesis of the two species growing in the same habitat were studied to assess possible mechanisms of tolerance to drought and how these relate to their ecophysiological success. Also the hypothesis that their photosynthetic rate (A) under drought would be affected by stomatal and non-stomatal limitations was tested. METHODS: Water relations, gas exchange, water use efficiency (WUE), fluorescence parameters, pubescence and specific leaf area (SLA) of Calotropis procera and C. gigantea plants growing in the field were evaluated during the wet (WS) and dry (DS) seasons. RESULTS: The xylem water potential (ψ) was similar in both species during the WS and DS; drought caused a 28 % decrease of ψ. In C. procera, A, stomatal conductance (g(s)) and carboxylation efficiency (CE) were higher in the WS with half the values of those during the DS, this species being more affected by drought than C. gigantea. A high δ(13)C of C. gigantea (-26·2 ‰) in the WS indicated a higher integrated WUE, in agreement with its lower g(s). Leaves of C. gigantea were more pubescent than C. procera. Relative stomatal and non-stomatal limitation of A increased with drought in both species; no changes in maximum quantum yield of photosystem II (PSII; F(v)/F(m)) were observed. The decrease in the relative quantum yield of PSII (ϕ(PSII)) and in the photochemical quenching coefficient (q(P)) was more pronounced in C. procera than in C. gigantea. CONCLUSIONS: The photosynthetic capacity of C. procera was higher than that of C. gigantea. During the DS, A was regulated by stomatal and non-stomatal factors in a coordinated manner and drought did not cause chronic photoinhibition. A higher density of trichomes and leaf angle in C. gigantea may contribute to the maintenance of A and confer more efficient protection of photochemical activity in the DS. Ecophysiological traits such as high photosynthetic rate throughout the year even during the DS, and high WUE, highly pubescent leaves and low SLA observed in both species contribute to the establishment and growth of Calotropis in dry conditions.

Relaciones entre anatomía foliar, intercambio de gases y crecimiento en juveniles de cuatro especies forestales / Relations between foliar anatomy, gas exchange and growth in juveniles of four forest species / Relações entre anatomia foliar, intercâmbio de gases e crescimento em juvenis de quatro espécies florestais

Este estudio tuvo por objetivo relacionar la estructura anatómica foliar con las características ecofisiológicas de Cordia thaisiana (Boraginaceae), Cedrela odorata y Swietenia macrophylla (Meliaceae) y Tabebuia rosea (Bignoniaceae) durante la fase de establecimiento (<2 años), cultivadas en una región al Sur del Lago de Maracaibo, Venezuela. Se realizaron cortes transversales de las láminas foliares, tinciones, mediciones de intercambio gaseoso (CO2 y H2O) y mediciones de crecimiento. Todas las especies presentaron hojas hipoestomáticas, con epidermis uniestratificada en ambas caras, mesófilo bifacial, parénquima esponjoso medianamente laxo y estomas anomocíticos. Se detectaron diferencias significativas para las variables número de estomas/mm², espesores de lámina foliar, parénquima en empalizada, parénquima esponjoso y epidermis. La mayor tasa de asimilación (A) de CO2 ocurrió en C. odorata y la menor en C. thaisiana. Esta última especie presentó los mejores rendimientos en crecimiento diametral y longitudinal, y, en consecuencia, es la especie más apta para integrar plantaciones agroforestales en el sitio estudiado.

The aim of the study was to compare anatomical structure of the leaves and ecophysiological traits of the species Cordia thaisiana (Boraginaceae), Cedrela odorata, Swietenia macrophylla (Meliaceae) and Tabebuia rosea (Bignoniaceae) cultivated in the South of the Maracaibo Lake, Venezuela. Transverse leaf sections were prepared and stained, and gas exchange (CO2 and H2O) and growth were measured. All the species had hypostomatics leaves with monostratified epidermis at both surfaces, bifacial mesophyll moderately lax spongy parenchyma, and anomocytic stomata. Significant differences were found for the number of stomata/mm² and leaf thickness, as well as palisade, spongy parenchyma and epidermis thicknesses. The gas exchange parameters showed that C. odorata has the highest photosynthetic rate (A) and C. thaisiana the lowest rate. The latter species presented better yields, with higher diametrical and length growth, being thus recommended for agroforestry plantations in the studied site.

Este estudo teve por objetivo relacionar a estrutura anatômica foliar com as características ecofisiológicas de Cordia thaisiana (Boraginaceae), Cedrela odorata e Swietenia macrophylla (Meliaceae) e Tabebuia rosea (Bignoniaceae) durante a fase de estabelecimento (<2 anos), cultivadas em uma região ao Sul do Lago de Maracaibo, Venezuela. Realizaram-se cortes transversais das lâminas foliares, tinção, medições de intercâmbio gasoso (CO2 e H2O) e medições de crescimento. Todas as espécies apresentaram folhas hipoestomáticas, com epiderme uniestratificada em ambas caras, mesófilo bifacial, parênquima esponjoso medianamente laxo e estomatos anomocíticos. Detectaram-se diferenças significativas para as variáveis número de estomas/mm², espessores de lâmina foliar, parênquima em empalizada, parênquima esponjoso e epiderme. A maior taxa de assimilação (A) de CO2 ocorreu em C. odorata e a menor em C. thaisiana. Esta última espécie apresentou os melhores rendimentos em crescimento diametral e longitudinal, e, em consequência, é a espécie mais apta para integrar plantações agroflorestais na área estudada.

Causes of decreased photosynthetic rate and metabolic capacity in water-deficient leaf cells: a critical evaluation of mechanisms and integration of processes.

BACKGROUND: Water deficit (WD) decreases photosynthetic rate (A) via decreased stomatal conductance to CO(2) (g(s)) and photosynthetic metabolic potential (A(pot)). The relative importance of g(s) and A(pot), and how they are affected by WD, are reviewed with respect to light intensity and to experimental approaches. SCOPE AND CONCLUSIONS: With progressive WD, A decreases as g(s) falls. Under low light during growth and WD, A is stimulated by elevated CO(2), showing that metabolism (A(pot)) is not impaired, but at high light A is not stimulated, showing inhibition. At a given intercellular CO(2) concentration (C(i)) A decreases, showing impaired metabolism (A(pot)). The C(i) and probably chloroplast CO(2) concentration (C(c)), decreases and then increases, together with the equilibrium CO(2) concentration, with greater WD. Estimation of C(c) and internal (mesophyll) conductance (g(i)) is considered uncertain. Photosystem activity is unaffected until very severe WD, maintaining electron (e(-)) transport (ET) and reductant content. Low A, together with photorespiration (PR), which is maintained or decreased, provides a smaller sink for e(-)(,) causing over-energization of energy transduction. Despite increased non-photochemical quenching (NPQ), excess energy and e(-) result in generation of reactive oxygen species (ROS). Evidence is considered that ROS damages ATP synthase so that ATP content decreases progressively with WD. Decreased ATP limits RuBP production by the Calvin cycle and thus A(pot). Rubisco activity is unlikely to determine A(pot). Sucrose synthesis is limited by lack of substrate and impaired enzyme regulation. With WD, PR decreases relative to light respiration (R(L)), and mitochondria consume reductant and synthesise ATP. With progressing WD at low A, R(L) increases C(i) and C(c). This review emphasises the effects of light intensity, considers techniques, and develops a qualitative model of photosynthetic metabolism under WD that explains many observations: testable hypotheses are suggested.

Stomatal and non-stomatal limitations of photosynthesis in trees of a tropical seasonally flooded forest.

Trees in the flooded forest of the Mapire River in Venezuela suffer a decrease in photosynthetic rate (A) when flood begins, which is reverted at maximum flood. Changes in A are accompanied by similar changes in stomatal conductance (g(s)), and the possibility of changes in photosynthetic capacity is not ruled out. In order to understand how relative stomatal and non-stomatal limitations of photosynthesis are affected by flooding, we studied the seasonal changes in A and its response to intercellular CO(2) concentration in trees of Campsiandra laurifolia, Symmeria paniculata, Acosmium nitens and Eschweilera tenuifolia. Flooding caused in trees of C. laurifolia and S. paniculata a reduction in A, g(s), carboxylation efficiency and total soluble protein (TSP), whereas gas exchange in A. nitens and E. tenuifolia was more sensitive to drought. Under flooding, relative stomatal limitation (L(s)) was on average half the highest, and relative non-stomatal limitation (L(ns)) increased from the dry season to flooding. Under full flood, A, g(s) and TSP regained high values. A was positively correlated to light-saturated electron transport rate, suggesting that part of the decrease in A under flooding was due to impairment of photosynthetic capacity. Under flooding, not only stomatal closure but also increased L(ns) causes a reduction in photosynthesis of all four species, and a process of acclimation as flooding progresses allows gas exchange and related variables to regain high values.

Relaciones hídricas, fotosíntesis, y anatomía foliar de dos especies del género Calotropis / Water relations, photosynthesis, and leaf anatomy in two species of Calotropis genus

El género Calotropis es originario de Asia y es considerado una maleza de zonas áridas. En Venezuela el género esta representado por dos especies arbustivas, C. procera y C. gigantea, que se encuentran en hábitats xerofíticos y se caracterizan por ser muy resistentes a la sequía. Para conocer y comparar las características ecofisiológicas de estas especies, se realizó un estudio en el Litoral Central donde se evaluó el estado hídrico, el intercambio gaseoso y la anatomía foliar de ambas especies durante las temporadas de lluvia y de sequía. No se observaron diferencias significativas en el estado hídrico entre ambas especies, aunque la sequía causó una reducción del 50 por ciento en su potencial hídrico. Las mayores tasas de fotosíntesis (A) y conductancia estomática (gs) fueron observadas en lluvia en ambas especies, siendo 40 y 48 por ciento más altas en C. procera que en C. gigantea. La gs fue 41 por ciento mayor en lluvia que en sequía en C. procera, mientras que en C. gigantea no varió. C. procera presentó una mayor eficiencia de uso de agua (EUA). Las hojas de ambas especies mostraron una estructura mesofilar isolateral y anfiestomáticas, con alta pubescencia, observándose diferencias significativas en el grosor foliar y en el área foliar específica. Los resultados muestran que la capacidad fotosintética de C. procera fue mayor que la observada en C. gigantea.

Seasonal changes in water relations photosynthesis and leaf anatomy of two species growing along a natural CO2 gradient

Se realizaron medidas del estado hídrico, intercambio gaseoso y características anatómicas foliares en dos especies (Brownea coccinea y Spatiphylum cannifolium) que crecen naturalmente a lo largo de un gradiente natural de CO2, desde una concentración supra-atmosférica (CS) de 35000µmol.mol-¹ a una concentración ambiental (CA) de 435µmol-mol-¹, en el bosque ribereño del río Santa Ana (estado Sucre, Venezuela). Las medidas se realizaron en dos épocas contrastes, lluvia y sequía. El potencial hídrico (psy) fue cerca de 60 por ciento mayor en lluvia que en sequía en ambas especies. El crecimiento en CS no ocasionó cambios en el (psi) de B. coccinea, mientras que en S. cannifolium causó un descenso, estos cambios estuvieron acompañados en ambas especies con disminuciones en el potencial osmótico (psi pi). Las elevadas concentraciones de CO2 causaron una mejora en la fotosíntesis (A) y la conductancia estomática (gs) de B. coccinea, pero en S. cannifolium no se encontraron diferencias significativas. En las plantas de ambas especies que crecen en CS las hojas eran 10 por ciento más gruesas debido a un incremento del grosor de los tejidos del mesófilo en la misma proporción; además, las plantas de S. cannifolium que crecen en CS tiene lugar una mayor deposición de ceras espicuticulares durante la época de sequía

Photosynthetic responses of the tropical spiny shrub Lycium nodosum (Solanaceae) to drought, soil salinity and saline spray.

Water relations and photosynthetic characteristics of plants of Lycium nodosum grown under increasing water deficit (WD), saline spray (SS) or saline irrigation (SI) were studied. Plants of this perennial, deciduous shrub growing in the coastal thorn scrubs of Venezuela show succulent leaves which persist for approx. 1 month after the beginning of the dry season; leaf succulence is higher in populations closer to the sea. These observations suggested that L. nodosum is tolerant both to WD and salinity. In the glasshouse, WD caused a marked decrease in the xylem water potential (psi), leaf osmotic potential (psi(s)) and relative water content (RWC) after 21 d; additionally, photosynthetic rate (A), carboxylation efficiency (CE) and stomatal conductance (gs) decreased by more than 90 %. In contrast, in plants treated for 21 d with a foliar spray with 35 per thousand NaCl or irrigation with a 10 % NaCl solution, psi and RWC remained nearly constant, while psi(s) decreased by 30 %, and A, CE and gs decreased by more than 80 %. An osmotic adjustment of 0.60 (SS) and 0.94 MPa (SI) was measured. Relative stomatal and mesophyll limitations to A increased with both WD and SS, but were not determined for SI-treated plants. No evidence of chronic photoinhibition due to any treatment was observed, since maximum quantum yield of PSII, Fv/Fm, did not change with either drought in the field or water or salinity stress in the glasshouse. Nevertheless, WD and SI treatments caused a decrease in the photochemical (qP) and an increase in the non-photochemical (qN) quenching coefficients relative to controls; qN was unaffected by the SS treatment. The occurrence of co-limitation of A by stomatal and non-stomatal factors in plants of L. nodosum may be associated with the extended leaf duration under water or saline stress. Additionally, osmotic adjustment may partly explain the relative maintenance of A and gs in the SS and SI treatments and the tolerance to salinity of plants of this species in coastal habitats.

Operation of the xanthophyll cycle and degradation of D1 protein in the inducible CAM plant, Talinum triangulare, under water deficit.

Changes in photochemical activity induced by water deficit were investigated in Talinum triangulare, an inducible CAM plant. The aim was to analyse the interactions between C3 photosynthesis, induction and activity of CAM, photosynthetic energy regulation and the mechanisms responsible for photoprotection and photoinhibition under water stress. Gas exchange, chlorophyll a fluorescence, titratable acidity, carotenoid composition and relative contents of the PSII reaction centre protein (D1) were measured. A decrease in xylem tension (psi) from -0.14 to -0.2 MPa substantially decreased daytime net CO2 assimilation and daily carbon gain, and induced CAM, as shown by CO2 assimilation during the night and changes in titratable acidity; a further decrease in psi decreased nocturnal acid accumulation by 60%. Non-photochemical quenching of chlorophyll a fluorescence (NPQ) increased with water deficit, but decreased with a more severe drought (psi below -0.2 MPa), when CAM activity was low. NPQ was lower at 0900 h (during maximum decarboxylation rates) than at 1400 h, when malate pools were depleted. Down-regulation of PSII activity related to the rise in NPQ was indicated by a smaller quantum yield of PSII photochemistry (phiPSII) in droughted compared with watered plants. However, phiPSII was larger at 0900 h than at 1400 h. The de-epoxidation state of the xanthophyll cycle increased with drought and was linearly related to NPQ. Intrinsic quantum yield of PSII (FV/FM) measured at dusk was also lower in severely stressed plants than in controls. Under maximum photosynthetic photon flux and high decarboxylation rates of organic acids, the D1 content in leaves of droughted plants showing maximal CAM activity was identical to the controls; increased drought decreased D1 content by more than 30%. Predawn samples had D1 contents similar to leaves sampled at peak irradiance, with no signs of recovery after 12 h of darkness. It is concluded that under mild water stress, early induction of CAM, together with an increased energy dissipation by the xanthophyll cycle, prevents net degradation of D1 protein; when water deficit is more severe, CAM and xanthophyll cycle capacities for energy dissipation decline, and net degradation of D1 proceeds.

Lack of downregulation of photosynthesis in a tropical root crop, cassava, grown under an elevated CO2 concentration.

We evaluated the effects of an elevated [CO2] on photosynthesis and growth of cassava plants grown in open-top chambers with an adequate supply of water and N and a sufficient rooting volume. Cassava plants (Manihot esculenta Crantz. cv. Motilona) showed higher photosynthetic rates (Pn) when grown and measured at elevated [CO2] (680 µmol mol-1) than when grown and measured at ambient [CO2] (480 µmol mol-1). No downregulation of photosynthesis due to elevated [CO2] was found, since carboxylation efficiency increased after 220 d in spite of a decrease in leaf soluble protein, Rubisco, and leaf N content. Soluble sugar and starch contents decreased with time under elevated [CO2], the decrease in starch content coinciding with the beginning of the increase in root mass. Canopy Pn by leaf area decreased with time under elevated [CO2] but, when canopy Pn was expressed by ground area, higher and constant rates were observed, suggesting a higher productivity in plants grown at elevated [CO2]. The absence of differences between growth [CO2] in root : shoot ratio observed suggests that elevated [CO2], while causing increases in the shoot as well as the root, did not affect the pattern of biomass allocation. Acclimation responses of gas exchange parameters changed during the experiment. The absence of downregulation of photosynthesis was associated with a decrease in leaf sugar and starch contents of plants grown at elevated [CO2], which suggests a favourable source/sink relationship.

Efecto de la concentración elevada de CO² sobre la fotosíntesis en especies tropicales / Effect of high CO² concentration upon photosyntesis in tropical species

La gran cantidad de información acerca de los efectos de un incremento experimental de la [CO²] de cultivo sobre la fisiología de plantas de zonas templadas contrasta con la reducida información disponible para plantas tropicales. En atención a ello, se estudiaron las respuestas fotosintéticas a [CO²] elevadas de plantas que crecen en Venezuela. Se cultivaron plantas de cuatro especies xerófitas bajo una [CO²] elevada, para elucidar el efecto de ésta sobre la fotosíntesis y el uso de agua de especies que normalmente enfrentan déficit hídricos. La fotosíntesis aumentó alrededor de 3,5 veces y la [CO²] elevada retrasó la disminución de la tasa fotosintética en sequía. Dado que en muchos estudios se ha encontrado que el estímulo inicial de la fotosíntesis por una [CO²] elevada desaparace en el tiempo debido a las limitaciones de sumidero de asimilados, hicimos un estudio del efecto de la [CO²] elevada sobre la fotosíntesis de plantas de yuca cultivadas sin limitaciones de suelo, encontrando que la tasa fotosintética no disminuyó durante todo el ciclo de cultivo. La existencia de un sumidero grande (la raiz) unida a una gran disponibilidad de sustrato, permitió que la estimulación de la tasa fotosintética por la [CO²] elevada continuara en el tiempo. Se evaluó el efecto de [CO²] muy altas sobre la fotosíntesis de plantas silvestres que crecen alrededor de emanaciones naturales, encontrándose que tales concentraciones no sólo no inhiben la tasa fotosintética, sino que la promueven muy por encima de la de los controles