Genotypic variation in photosynthesis in cacao is correlated with stomatal conductance and leaf nitrogen

Abstract

Traits such as transpiration efficiency (TE) that are influenced by plant water use can be used to characterise the adaptability of crops to specific growth environments. TE is defined as the amount of biomass produced per unit of water used, and can ensure continued crop production in drought-prone regions. Where TE is associated with reduced use of soil water during the vegetative growth phase, water availability during grain filling may be greater, which can delay the onset of drought stress and increase grain yield under water-limited conditions. This may become even more pertinent with predicted increases in severity and frequency of droughts with climate change. The aims of this study were to firstly dissect TE into its leaf-level physiological components to better understand the effects of genetic variation in these components on TE in sorghum. Secondly, to examine whether TE responses observed under well-watered conditions were preserved under drought, and whether transpiration response was an adaptive response to drought. Twenty-seven genotypes were screened for TE under well-watered conditions using a fully automated lysimetry platform to obtain accurate plant water use data. To determine whether variation in TE among these genotypes was associated with differences in maximum photosynthesis (Amax) or leaf conductance (g) we measured the net carbon assimilation rate of the second last fully expanded leaf at high light intensity, using an infrared gas analyser, and leaf water flux was measured using a porometer, as a proxy for conductance. Genotypic variation in TE among the sorghum germplasm used was mainly associated with differences in the response of transpiration rates to vapour pressure deficit (VPD). Genotypes with low transpiration rates per unit green leaf area (T/GLA) tended to have high TE. Variation in Amax explained some of the differences in TE that could not be explained by T/GLA and may have been a result of mechanisms associated with differences in biochemical pathways that affect the efficiency of conversion of CO2 into photosynthate. While drought tended to increase TE, genotypic variation in TE was largely conserved. However, the response of transpiration rates to drought stress differed across genotypes, with some genotypes showing reduced T/GLA under drought when VPD was high, whereas others did not. These contrasting responses were associated with differences in stomatal responses to drought stress, such that some genotypes were better able to conserve water under drought stress than others. This adaptive response was not related to TE per se and may have important implications for adaptation to drought stress. Hence, the phenotyping of sorghum lines using the associated physiological traits underpinning TE differences is beneficial in identifying traits that may support growth in certain environments and can optimise grain yield production under water-limited conditions. III

Does Criollo cocoa have the same ecophysiological characteristics than Forastero?

“…The very low average A observed for both cultivars might be associated with low g s . Similarly, A values have been found to range from 0.7 to 6.5 µmol m -2 s -1 with low g s (20 -150 mmol m -2 s -1 ) in different studies Daymond et al 2011;Araque et al 2012;de Almeida et al 2014;Ávila-Lovera et al 2016). Higher instantaneous A and lower E determine a significant higher WUE in Criollo compared to Forastero, suggesting that Criollo cocoa could grow well in regions with limited water availability.…”

“…Cocoa exhibits considerable genetic variability regarding morphological and physiological traits (Daymond et al 2002a;. However, studies of genotypic variation of photosynthetic traits in cocoa are limited (Daymond et al 2011).…”

Pigmentos fotosintéticos y conductancia estomática en ecotipos de copoazú (Theobroma grandi orum Willd. Ex. Spreng K. Schum.).

“…En promedio, independientemente de la hora los ecotipos que presentaron mayores valores de gs fueron UA-67, UA-039 y UA-37, una mayor conductancia estomática con frecuencia resulta en una mayor tasa de fotosíntesis, sin embargo, esto promueve un incremento de la transpiración que repercute negativamente en el estado hídrico de la planta, estas relaciones han sido demostradas por diferentes autores (Daymond et al, 2011;Raja-Harun y Hardwick 1988;Sena-Gomes et al, 1987) para plantas de Theobroma cacao L. Las diferencias entre ecotipos se debió a posibles adaptaciones fisiológicas, tales como mayor densidad estomática y contenidos de clorofila, así ha sido reportado para especies como copoazú, arazá (Eugenia stipitata) (Barrera et al, 2009) y Feijoa (Acca sellowiana Berg) (Naizaque et al, 2014). La importancia ecofisiológica de la gs se debe a la relación con el proceso de difusión de CO 2 al interior de la hoja, a partir de la regulación de la apertura estomática, priorizando la ganancia de carbono en relación a la pérdida de agua (Caplan y Yeakley, 2010).…”

“…De acuerdo con Almeida et al (2014), existe una relación directa entre la transpiración y la conductancia estomática, este complemento se ha evidenciado en diferentes especies del género Theobroma incluida T. grandiflorum. Asimismo, se ha encontrado que una amplia proporción de la variación observada en la tasa de fotosíntesis en clones de cacao, podría ser explicada por la variación en la conductancia estomática (Daymond et al, 2011). La gs presentó una correlación negativa con la temperatura (-0,84; P<0,0001), radiación (-0,91; P<0,0001) y DPV (-0,94; P<0,0001), indicando que un incremento en alguna de estas variables tendría un efecto negativo sobre el intercambio gaseoso del copoazú, en tal sentido diversos autores han encontrado relaciones similares, por ejemplo, Joly y Hahn (1989) encontraron que las altas temperaturas conducen una reducción de la conductancia estomática y una disminución de la fotosíntesis; por otra parte, Balashimha et al (1991), Baligar et al (2008) y Bobich et al (2010), encontraron correlaciones negativas entre DPV y la gs en Theobroma cacao L., y otras especies perennes, planteando que la fotosíntesis decrece significativamente con el aumento del DPV.…”

“…Una tendencia contraria se presentó para la humedad relativa (0,90; P<0,0001), al respecto Daymond et al (2011) al evaluar la variación genotípica en la fotosíntesis del cacao y su relación con la gs, destacaron la influencia de la humedad relativa sobre la fotosíntesis. Las variaciones de las condiciones climáticas durante el día repercuten en procesos fisiológicos, se registró los mayores valores de temperatura y DPV al medio día (Figura 1).…”