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

item Baligar, Virupax
item Bunce, James
item Elson, Marshall

Submitted to: Photosynthetica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2007
Publication Date: 4/1/2008
Citation: Baligar, V.C., Bunce, J.A., Machado, R.R., Elson, M.K. 2008. Photosynthetic photon flux density, carbon dioxide concentration, and vapor pressure deficit effects on photosynthesis in cacao seedlings. Photosynthetica. 46:216-221.

Interpretive Summary: Cacao is a shade plant, native to understory of the evergreen rainforest of South and Central America and is adapted to low level of light intensities. In cacao growing regions, loss of trees has reduced the amount of shade and moisture content of the air. Globally there is an overall increase in the concentration of carbon dioxide. Information is lacking on effects of climatic variables such as light levels, external carbon dioxide concentrations and moisture content of air on net photosynthesis in cacao genotypes. Greenhouse experiment were conducted on three cacao genotypes to assess the influence of climatic variables on net photosynthesis and its components. Increasing light up to one forth of the full sunlight and doubling the carbon dioxide concentration increased net photosynthesis of all three cacao genotypes. However, high light intensities were detrimental to cacao especially under low humidity conditions. These findings will be useful to cacao farmers for managing shade trees density and pruning of cacao. Such management techniques will improve cacao production potentials.

Technical Abstract: Cacao (Theobroma cacao) is a shade plant, native to the under-story of the evergreen rain forest of the Amazon basin and adapted to low levels of photosynthetic photon flux density (PPFD). The influence of PPFD, leaf to air water vapor pressure deficit (VPD) and external carbon dioxide concentration [CO2] on net photosynthesis (Pn) and its components in cacao genotypes has not been fully explored. In this study independent effects of PPFD of 50 to 400 µmol m-2 s-1, [CO2] of 85 to 850 cm3 m-3, and VPD of 0.9 to 2.2 kPa on Pn, stomatal conductance (Gs), leaf internal CO2 concentration (Ci) 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 Pn by about 50%, but further increases in PPFD up to 1500 µmol m-2 s-1 had no effect on Pn. Increasing external [CO2] significantly increased Pn and Ci while Gs and E decreased more strongly than in most trees that have been studied. In all genotypes, increasing VPD reduced Pn, but the slight decrease in Gs and the slight increase Ci with increasing VPD were non-significant. Increasing VPD significantly increased E and this may have caused the reduction in Pn. The unusually small response of Gs to VPD could limit the ability of cacao to grow where VPD is high. There were no significant differences in gas exchange characteristics (Gs, Ci, E) among the three cacao genotypes used in this study.