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

Independent short-term effects of photosynthetic photon flux density (PPFD) of 50-400 μmol m⁻² s⁻¹, external CO₂ concentration (C a) of 85-850 cm³ 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 CO₂ 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.