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

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Independent short-term effects of photosynthetic photon flux density (PPFD) of 50–400 μmol m−2 s−1, external CO2 concentration (Ca) of 85–850 cm3 m−3, and vapor pressure deficit (VPD) of 0.9–2.2 kPa on net photosynthetic rate (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 1 500 μmol m−2 s−1 had no effect on PN. Increasing Ca 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 in 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 under any measurement conditions.

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