Abstract Background: Coffee breeding programs in Ecuador have information on production and disease tolerance in many genotypes; however, they lack physiological information, especially on photosynthetic characteristics and their response to drought. Questions: Whether high genetic variability among coffee genotypes will explains the photosynthetic and production differences expected? Will the physiological response to the dry season (DS) be different between genotypes? Studied species: Coffea arabica L. Study site and dates: Pichincha canton, Manabí province, Ecuador during March-April 2017 (rainy season, RS) and June-July 2017 (DS). Methods: Leaf relative water content (RWC) and gas exchange of 21 coffee genotypes were measured during DS and RS. Coffee production during a period of three years was evaluated. Results: Significant differences were found in RWC, photosynthetic rate (A), stomatal conductance (gs) and water use efficiency (WUE) among genotypes, between seasons, an interaction effect of genotype × season. Drought caused a significant reduction in A and gs of 30 and 44 % respectively, while WUE was not affected. A positive linear relationship was found between A and gs, and a negative relationship between A and the leaf-air vapor pressure gradient (ΔW) and between gs and ΔW. Differences in coffee production were found among genotypes. Conclusions: The high genetic variability of C. arabica genotypes may explain the significant differences in RWC and gas exchange and interaction genotypes x season, suggesting a differential response of each genotype to drought. Eleven of the 21 coffee genotypes were sensitive to drought, but showed different responses, suggesting possible genotypic differences in tolerance.

Resumen Antecedentes: Los programas de mejoramiento genético del café en Ecuador tienen información sobre producción y tolerancia a enfermedades en muchos genotipos; sin embargo, carecen de información sobre características fotosintéticas y su respuesta a la sequía. Preguntas: ¿La alta variabilidad genética entre los genotipos de café explicará las diferencias fotosintéticas y de producción esperadas? ¿Será diferente la respuesta fisiológica a la estación seca (DS) entre los genotipos? Especie de estudio: Coffea arabica L. Sitio de estudio y fechas: Cantón Pichincha, provincia de Manabí, Ecuador durante la temporada de lluvias (RS) y DS del 2017. Métodos: Se midieron el contenido relativo de agua foliar (CRA) y el intercambio gaseoso de 21 genotipos de café durante RS y DS. Se evaluó la producción de café durante un período de tres años. Resultados: Se encontraron diferencias significativas en el CRA, A, gs y la EUA entre genotipos, temporadas, e interacción genotipo × temporada. La sequía causó una reducción en A y gs del 30 y 44 %, mientras que EUA no varió. Se encontró una relación lineal positiva entre A y gs, y negativa entre A y ΔW y entre gs y ΔW. Se encontraron diferencias en la producción de café entre genotipos. Conclusiones: La variabilidad genética de los genotipos de C. arabica explicó las diferencias en CRA, A, gs y la interacción genotipo × temporada, sugirió una respuesta diferencial de cada genotipo a la sequía. Once de los 21 genotipos de café fueron sensibles a la sequía; mostrando posibles diferencias genotípicas en la tolerancia.

Resumen El cacao (Theobroma cacao L.) no se cultiva con frecuencia en ambientes donde existen limitaciones hídricas, sin embargo, se encuentra cultivado en algunas regiones propensas a la sequía, donde el agua del suelo se agota gradualmente en un ciclo anual. En regiones donde el cacao está sujeto a largos períodos sin precipitación, las respuestas fisiológicas en condiciones de campo pueden diferir entre los clones de cacao. Por ello, se requiere la evaluación de estas respuestas para seleccionar clones potenciales que podrían usarse para programas de mejoramiento en el contexto de escenarios climáticos futuros. Con el fin de identificar la respuesta fisiológica y de rendimiento a la sequía de 10 clones de cacao ecuatorianos, se evaluó el contenido relativo de agua foliar (RWC), la tasa de asimilación de CO2 (A), la transpiración (E), la conductancia estomática (gs), la eficiencia en el uso del agua (WUE), el rendimiento cuántico del fotosistemas II (®PSII), el coeficiente de extinción no fotoquímica (NPQ), el contenido de nutrientes en la hoja (N, P K, Ca y Mg) y la producción durante las estaciones seca y lluviosa. En promedio, A y E disminuyeron en un 33 y 22 %, respectivamente, durante la sequía, mientras que el WUE fue mayor durante la temporada de lluvias. Los clones mostraron un aumento de NPQ durante la sequía, lo que sugiere una mayor disipación de energía. Los clones mostraron diferencias en el contenido de macronutrientes y en la mayoría de los casos la sequía provocó una reducción en el contenido de Ca y Mg. Se establecieron dos grupos según rendimiento: uno que mantuvo mayores rendimientos durante la época de lluvias, mientras que el otro tuvo mayores rendimientos durante la sequía. Esto ofrece una alternativa estratégica para mantener un rendimiento relativamente alto durante todo el año con la combinación de clones en las plantaciones.

Abstract Cacao (Theobroma cacao L.) is not frequently cropped in water limited environments, however it is cultivated in several drought-prone regions, where soil water is gradually depleted on an annual cycle. In regions where cacao is subjected to long periods without precipitation, the physiological responses under field conditions may differ between cacao clones. Evaluation of these responses are required to select potential clones that could be used for breeding programs in the context of future climatic scenarios. In order to identify physiological and yield responses to drought of 10 Ecuadorian cacao clones, relative leaf water content (RWC), CO2 assimilation rate (A), transpiration (E), stomatal conductance (gs), water use efficiency (WUE), relative quantum yield of photosystems II (®PSII), non-photochemical quenching (NPQ), leaf nutrient content (N, P K, Ca, and Mg) and yield were assessed during the dry (DS) and rainy seasons (RS). On average, A and E decreased by 33 and 22 %, respectively under drought, whereas WUE was higher during RS. Clones exhibited an increase in NPQ during DS, suggesting greater energy dissipation. Clones showed differences in macronutrient content and in most cases drought caused a reduction in Ca and Mg content. Two groups were established according to yield: one which maintained higher yields during RS, while the other had higher yields during drought. This offers strategic alternative in order to maintain a relatively high yield throughout the year with combination of clones in the plantations.

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.

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.

Abstract: Background: The Palma Real Mountain of Venezuela has Criollo and Forastero cocoa trees of over 50 years old. These individuals are considered ancestral representatives of high quality cocoa that have survived for many years in this location without any agronomic management. Question: This study shows some physiological traits of these old trees, in order to determine whether differences exist in photosynthetic capacity and some biochemical parameters between the two types of cocoa growing under natural conditions. Studied species: Theobroma cacao L. Study site and years of study: was done in the mountain Palma Real in the Island of Margarita, Edo. Nueva Esparta, in January 2008 and November 2009. Methods: The individuals present in the area were initially characterized as Criollo or Forastero and gas exchange, chlorophyll fluorescence and the response of the photosynthetic rate (A) to intercellular CO2 concentration (Ci) and photosynthetic photon flux density (PFD), carbon isotope ratio (δ13C), nitrogen isotope ratio (δ15N), leaf N and chlorophyll content were evaluated. Results: The results showed that the gas exchange characteristics of Criollo cocoa differ slightly from those of Forastero. However, similar PFD saturation (~400 μmol m-2 s-1) with values of A at saturating light approaching 4 μmol m-2 s-1, carboxylation efficiency (CE) and photochemical activity, indicated a comparable photosynthetic capacity in both cocoa types. Conclusions: There are apparently no physiological disadvantages in Criollo compared with Forastero trees and therefore both could be recommended as suitable materials for cultivation in similar environments, thereby expanding cultivation of high quality cocoa.

Resumen: Antecedentes: La Montaña Palma Real en Venezuela tiene árboles de cacao Criollo y Forastero de más de 50 años de edad. Estos individuos se consideran representantes ancestrales de cacao de alta calidad que han sobrevivido durante muchos años en este lugar sin ningún tipo de manejo agronómico. Pregunta: Este estudio muestra algunas características fisiológicos de estos árboles, con el fin de determinar si existen diferencias en la capacidad fotosintética y algunos parámetros bioquímicos entre los dos tipos de cacao que crecen en condiciones naturales. Especie estudiada: Theobroma cacao L. Sitio de estudio y años del estudio: se hizo en la montaña Palma Real en la Isla de Margarita, Estado Nueva Esparta, en enero de 2008 y noviembre de 2009. Métodos: Los individuos presentes en la zona, inicialmente se caracterizaron como cacao Criollo o Forastero; se evaluó el intercambio gaseoso, la fluorescencia de la clorofila, la respuesta de la tasa fotosintética (A) a la concentración intercelular de CO2 (Ci) y a la densidad de flujo de fotones (PFD), la relación de isótopos de carbono (δ13C), la proporción de isótopos de nitrógeno (δ15N), y el contenido de clorofila y N foliar. Resultados: Los resultados mostraron que las características de intercambio de gases de cacao Criollo difieren levemente de los de cacao Forastero. Sin embargo, la PFD de saturación fue similar (~ 400 μmol m-2 s-1) con valores de A a luz saturante cerca de 4 μmol m-2 s-1, la eficiencia de carboxilación (CE) y la actividad fotoquímica, indicaron una capacidad fotosintética comparable en ambos tipos de cacao. Conclusiones: Aparentemente no hay desventajas fisiológicas en el cacao Criollo en comparación con los árboles de cacao Forasteropor lo tanto ambos podrían ser recomendados como materiales adecuados para el cultivo en ambientes similares al lugar de estudioaumentando así el cultivo de cacao de alta calidad

Nicotiana glauca, a shrub native to southern South America, is widely distributed in the Americas, from Patagonia to the USA, from zero up to 3,700 m; it rapidly invades disturbed environments. In Venezuela, it has been reported from zero up to 2,000 m growing in contrasting conditions of relative humidity, temperature, rainfall, and salinity. In order to gain insight into the extent and mode of resistance to drought and salinity in N. glauca, we studied the effect of these factors on water relations and photosynthesis under both natural and greenhouse conditions. In the field, water potential, photosynthetic rate (A) and stomatal conductance (gs), but not relative water content (RWC) decreased because of drought. Manual removal of epicuticular wax increased excess energy dissipation through non-photochemical quenching without altering the capacity of photochemical quenching. In the greenhouse, water deficit as well as salinity resulted in osmotic adjustment; at the end of the experiment, turgor potential and RWC under water deficit were similar to control and higher under salinity. Water deficit and salinity caused marked decreases in A and gs. There were very few or no changes with natural drought, salinity or experimental water deficit in potential quantum efficiency of PSII, which could be explained partly by an increased non-photochemical quenching. We conclude that the partial tolerance to drought and salinity in plants of N. glauca resides in their ability to achieve water conservation through stomatal closure and osmotic adjustment, reduce absorption of excess radiation through the presence of leaf wax and dissipate it through increased non-photochemical quenching. All these characteristics confer plants of N. glauca advantages to invade disturbed areas, subject to salinity and/or seasonal drought under high irradiance.

Vegetative as well as reproductive phenology of tropical trees frequently occur synchronously at the end of the dry season. It has been repeatedly observed on two individuals of the tropical species Ficus obtusifolia growing in Caracas that they lose their entire foliage in a period of 48 h during the dry season and renew it within the next 72 h. In order to gain knowledge on the mechanisms that govern leaf exchange in F. obtusifolia, we followed in these two individuals the seasonal changes in leaf and fruit phenology, water relations and environmental variables, and in another 25, phenology only. Although foliar exchange was always observed before the rainy season, it also occurred near the end of the rainy season. An increase in leaf turgor potential of mature leaves and a low sap flow velocity coincided with leaf fall. Foliar exchange in all 27 individuals, growing in locations with different elevation and, possibly, access to underground water, was bi-modal and occurred in a period of approximately one month around both equinoxes. The presence of fruits was also bi-modal, occurring after leaf renewal. The sequence of phenological events repeated itself in all the individuals studied, although with inter-annual and individual variations in date of foliar exchange. Leaf fall was independent of leaf water status and significantly correlated with, in decreasing order, day length, minimum air temperature, rainfall, maximum air temperature and total radiation. We conclude that in F. obtusifolia high nocturnal temperature, increasing daytime temperature and radiation, and low cloud cover apparently drove foliar exchange. Day length showed a strong correlation with leaf renewal possibly to due to the significant relationships between day length and environmental variables. Direct effects of day length alone were not evidenced, as foliar exchange took place during a long time-span around the equinoxes.

Las especies del género Lantana son conocidas por el alto valor comercial de sus aceites naturales, pero se sabe poco de su respuesta fotosintética a variables ambientales. El efecto del déficit hídrico sobre la actividad fotosintética se estudió en tres especies de Lantana: L. caracasana, L. canescens y L. camara, con tratamientos de suspensión del riego. L. caracasana tuvo la mayor asimilación de carbono durante el período de medida y el déficit en la disponibilidad de agua ocasionó una disminución del potencial hídrico, de la fotosíntesis y de la conductancia estomática en las tres especies. Los cambios observados en las variables de fluorescencia, en la eficiencia cuántica aparente y en la fotosíntesis a luz saturante, sugieren una regulación descendente de la actividad fotoquímica del fotosistema II durante el déficit hídrico. En plantas control se observaron pocas diferencias entre especies en la respuesta de la fotosíntesis a la concentración intercelular de CO2. La temperatura óptima para la fotosíntesis varió entre 25-26ºC. El efecto del déficit hídrico sobre el intercambio gaseoso de las tres especies fue diferente, siendo L. camara la más afectada.

The species of the Lantana genus are known by the high commercial value of its natural oils, but their photosynthetic response to environmental variables is not well known. To characterize the photosynthetic capacity and establish the effect of water deficit on photosynthesis, plants of three Lantana species (L. caracasana, L. canescens and L. camara) were submitted to water deficit by holding watering.The highest diurnal carbon assimilation was observed in L. caracasana and water deficit caused a decline in water potencial, photosynthesis and stomatal conductance of all species. Changes observed in variable fluorescence, apparent quantum yield and photosynthesis at saturated light, suggested a down regulation of photochemical activity of photosystem II during water deficit. In well irrigated plants, slight differences were observed among species in the photosynthetic response to intercellular CO2 concentration. Optimum temperature of photosynthesis was within 25-26ºC. The effect of water deficit on the photosynthetic parameters varied with the species, being L. camara the most affected.

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.

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% en su potencial hídrico. Las mayores tasas de fotosíntesis (A) y conductancia estomática (g s) fueron observadas en lluvia en ambas especies, siendo 40 y 48% más altas en C. procera que en C. gigantea. La g s fue 41% 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.

The Calotropis genus is a common weed native from Asia present in arid ecosystems. In Venezuela, the genus is represented by two shrub species, C. procera and C. gigantea, both living in xerophytic habitats and characterized by a marked resistance to drought. In order to define the ecophysiologycal characteristics of these species, the water status, gas exchange and leaf anatomy of plants growing under natural conditions were measured during rainy and dry seasons in a littoral area of the Vargas state, Venezuela. Water potential was not significantly different between species. However, water potential was 50% lower for both species under drought. Photosynthetic rate (A) and stomatal conductance (g s) were higher during the rainy season in both species. A and g s were 40 and 48% higher in C. procera than in C. gigantea. g s decreased by 60% during drought in C. procera, while it did not change in C. gigantea. The highest values in water use efficiency were obtained in C. procera. Leaves of both species showed amphystomatic, isolateral mesophyll structure with high pubescence, and both species differed significantly in foliar thickness and specific leaf area. The results suggest that C. procera had higher photosynthetic capacity than C. gigantea.

O gênero Calotropis é originário da Ásia e é considerado feno de zonas áridas. Na Venezuela o gênero esta representado por duas espécies arbustivas, C. procera e C. gigantea, que são encontradas em habitat xerofíticos e se caracterizam por ser muito resistentes à seca. Para conhecer e comparar as características ecofisiológicas destas espécies, foi realizado um estudo no Litoral Central onde se avaliou o estado hídrico, o intercâmbio gasoso e a anatomia foliar de ambas as espécies durante as temporadas de chuva e de seca. Não se observaram diferenças significativas no estado hídrico entre ambas as espécies ainda que a seca causasse uma redução de 50% no seu potencial hídrico. As maiores taxas de fotossínteses (A) e condutância estomática (g s) foram observadas na chuva em ambas espécies, sendo 40 e 48% mais altas em C. procera que em C. gigantea. A g s foi 41% maior na chuva que na seca em C. procera, enquanto que em C. gigantea não variou. C. procera apresentou uma maior eficiência no uso da água (EUA). As folhas de ambas as espécies mostraram uma estrutura mesófila isolateral e anfiestomáticas, com alta pubescência, observando-se diferenças significativas na grossura foliar e na área foliar específica. Os resultados mostram que a capacidade fotossintética de C. procera foi maior que a observada em C. gigantea.

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 contrastantes, lluvia y sequía. El potencial hídrico (psi) fue cerca de 60% 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 (<FONT FACE=Symbol>y p</FONT> ) Las elevadas concentraciones de CO2 causaron una mejora en la fotosíntesis (A) y la conductancia estomática (g s) 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% 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 epicuticulares durante la época de sequía.

In order to gain knowledge on the physiological and anatomical responses of plants to long-term growth under elevated CO2 concentrations, measurements of water status, leaf gas exchange and leaf anatomical characteristics were made during the rainy and the dry season in plants of Brownea coccinea and Spatiphylum cannifolium growing along a natural CO2 gradient, from 35000µmol·mol-1 (supra-atmospheric CO2 concentration, SC) to 435µmol·mol-1 (ambient CO2, AC) in the riparian forest of the Santa Ana river, Sucre State, Venezuela. Water potential (psi) was higher during the rainy than the dry season in both species. Growth under SC did not cause changes in psi of B. coccinea, whereas it decreased it in S. cannifolium; both drought and growth under SC decreased leaf sap osmotic potential (<FONT FACE=Symbol>yp</FONT>) in both species, helping maintain turgor pressure in B. coccinea but not in S. cannifolium. Elevated CO2 increased photosynthetic rate (A) and leaf conductance (g s) of B. coccinea under drought but no effect was found in S. cannifolium. Leaves of both species growing under SC were 10% thicker than under AC, owing to a proportional increase in the thickness of the mesophyll tissues; additionally, during the dry season a higher deposition of epicuticular waxes took place on leaves of S. cannifolium growing under SC. Changes in photosynthetic rate, leaf conductance and leaf anatomy suggest acclimation to growth under elevated CO2.

Realizaram-se medidas do estado hídrico, intercambio gasoso e características anatômicas foliares em duas espécies (Brownea coccinea e Spatiphylum cannifolium) que crescem naturalmente ao longo de um gradiente natural de CO2, desde uma concentração supra-atmosférica (CS) de 35.000mmol·mol-1 a uma concentração ambiental (CA) de 435mmol·mol-1, no bosque marginal do rio Santa Ana (Estado Sucre, Venezuela). As medições se realizaram em duas épocas contrastantes, chuva e seca. O potencial hídrico (Y) foi aproximadamente 60% maior na chuva que na seca em ambas espécies. O crescimento em CS não ocasionou mudanças no Y de B. coccinea, enquanto que no S. cannifolium causou um descenso, estas mudanças estiveram acompanhadas em ambas espécies com diminuições no potencial osmótico (Yð?) As elevadas concentrações de CO2 causaram uma melhora na fotossínteses (A) e a condutância estomática (g s) de B. cocinea, mas em S. cannifolium não se encontraram diferenças significativas. Nas plantas de ambas espécies que crescem em CS as folhas eram 10% mais grossas devido a um incremento na espessura dos tecidos do mesófilo na mesma proporção; além disso, nas plantas de S. cannifolium que crescem em CS acontece uma maior deposição de ceras epicuticulares durante a época da seca.

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 déficits 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 desaparece 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 raíz) 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.

The large amount of information on the effects of an experimental increase in [CO2] on the physiology of plants in temperate zones is in contrast with the scant information available on tropical plants. In view of this, the photosynthetic responses to elevated [CO2] of plants growing in Venezuela were studied. In order to elucidate the effect of elevated [CO2] on the photosynthesis and water use of species that are normally subjected to water deficit, plants of four xerophytic species were grown under elevated [CO2]. Photosynthesis increased approximately 3.5 times and elevated [CO2] delayed the decrease of photosynthetic rate under drought. Since in many studies the initial promotion of photosynthetic rate caused by elevated [CO2] has been observed to disappear in time due to limitations imposed by the sinks, we studied the effect of elevated [CO2] on the photosynthesis of cassava plants grown without soil limitations. Photosynthetic rate did not decrease at any time during the growth cycle of the crop. The occurrence of a large sink (the root) together with a sufficiently large volume of substrate permitted the maintenance of a higher photosynthetic rate due to elevated [CO2]. The effect of very high [CO2] on photosynthesis was evaluated in wild plants growing near natural sources; such high [CO2] did not inhibit the photosynthetic rate and promoted it well above that of controls.

A grande quantidade de informação sobre os efeitos de um incremento experimental do [CO2] de cultivo sobre a fisiologia de plantas de zonas temperadas contrasta com a informação reduzida disponível para plantas tropicais. Em atenção a isso, estudaram-se as respostas fotosintéticas ao [CO2] elevados de plantas que crescem na Venezuela. Foram cultivadas plantas de quatro espécies xerófitas sob um [CO2] elevado, para elucidar o efeito desta sobre a fotosíntese e o uso de água de espécies que normalmente enfrentam déficits hídricos. A fotosíntese aumentou ao redor de 3,5 vezes e o [CO2] elevado atrasou a diminuição da taxa fotosintética em seca. Dado que em muitos estudos foi encontrado que o estímulo inicial da fotosíntese por um [CO2] elevado desaparece no tempo devido às limitações de sumideiro de assimilados, fizemos um estudo do efeito do [CO2] elevado sobre a fotosíntese de plantas de mandioca cultivadas sem limitações de solo, encontrando que a taxa fotosintética não diminuiu durante todo o ciclo de cultivo. A existência de um sumideiro grande (a raiz) unida a uma grande disponibilidade de substrato, permitiu que a estimulação da taxa fotosintética pelo [CO2] elevado continuasse no tempo. Foi avaliado o efeito de [CO2] muito altos sobre a fotosíntese de plantas silvestres que crescem ao redor de emanações naturais, descobrindo-se que tais concentrações não só não inibem a taxa fotosintética, senão que a promovem muito além dos controles.