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 mu mol m(-2) s(-1), external CO2 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 CO2 concentration (C-i), and transpiration rates (E) were investigated in three cacao genotypes. In all these genotypes, increasing PPFD from 50 to 400 mu mol m(-2) s(-1) increased P-N by about 50 %, but further increases in PPFD up to 1 500 mu mol 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.