Photosynthetic photon flux density, carbon dioxide concentration, and vapor pressure deficit effects on photosynthesis in cacao seedlings

Abstract

Independent short-term effects of photosynthetic photon flux density (PPFD) of 50-400 µmol m -2 s -1 , external CO 2 concentration (C a ) of 85-850 cm 3 m -3 , and vapor pressure deficit (VPD) of 0.9-2.2 kPa on net photosynthetic rate (P N ), stomatal conductance (g s ), leaf internal CO 2 concentration (C i ), and transpiration rates (E) were investigated in three cacao genotypes. In all these genotypes, increasing PPFD from 50 to 400 µmol m -2 s -1 increased P N by about 50 %, but further increases in PPFD up to 1 500 µmol m -2 s -1 had no effect on P N . Increasing C a significantly increased P N and C i while g s and E decreased more strongly than in most trees that have been studied. In all genotypes, increasing VPD reduced P N , but the slight decrease in g s and the slight increase in C i with increasing VPD were non-significant. Increasing VPD significantly increased E and this may have caused the reduction in P N . The unusually small response of g s to VPD could limit the ability of cacao to grow where VPD is high. There were no significant differences in gas exchange characteristics (g s , C i , E) among the three cacao genotypes under any measurement conditions.

Photosynthetic response to low and high light of cacao growing without shade in an area of low evaporative demand

“…The A of these new clones were similar to those obtained in clone CCN 51. However, lower A have been reported (<3 µmol CO 2 m -2 s -1 ) in six months old CCN 51 grown in pots where root growth limitations were unconsidered (Baligar et al, 2008). Variations in A have been reported in the Theobroma genus, with the maximum value found in Theobroma speciosum ) than the ones reported in the present study, but A was measured at saturating light and CO 2 conditions.…”

“…In studies with other cacao cultivars, the g s values were lower than 150 mmol H 2 O m -2 s -1 (Baligar et al, 2008;Almeida et al, 2014;Avila-Lovera et al, 2016;Tezara et al, 2016). High values of g s result in a greater availability of CO 2 for carboxylation by RUBISCO during the Calvin cycle, and therefore, higher A and carbohydrate production.…”

“…Cacao is a shade tolerant species (Muller and Valle, 2012), with net photosynthetic rates (A) that saturate at low photosynthetic photon flux densities (PPFD) between 400 and 600 μmol photons m -2 s -1 (Baligar et al, 2008;Almeida et al, 2014;Avila-Lovera et al, 2016;Tezara et al, 2016). The A and stomatal conductance (g s ) reported for different cacao cultivars are low and range from 3 to 7 µmol CO 2 m -2 s -1 and 50 to 120 mmol H 2 O m -2 s -1 , respectively (Joly and Hahn, 1989;Baligar et al, 2008;Daymond et al, 2011;Araque et al, 2012;Acheampong et al, 2013).…”

“…The A and stomatal conductance (g s ) reported for different cacao cultivars are low and range from 3 to 7 µmol CO 2 m -2 s -1 and 50 to 120 mmol H 2 O m -2 s -1 , respectively (Joly and Hahn, 1989;Baligar et al, 2008;Daymond et al, 2011;Araque et al, 2012;Acheampong et al, 2013). These characteristics explain why juvenile and mature cacao plants are able to grow under the shade of other trees in regions with high PPFD during most of the year (Somarriba and Beer, 2011;Jaimez et al, 2013).…”

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

“…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.…”