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 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.