Greenhouse-grown susceptible 20-d-old seedlings of Theobroma cacao genotypes Catongo and tolerant genotype SCA6xCatongo were inoculated with a mixture of isolates of Crinipellis perniciosa, the causal agent of witches' broom. The characteristics of chlorophyll a fluorescence emission were monitored during leaf ontogeny using a portable system PAM-2000. In both inoculated and non-inoculated genotypes, significant differences were found for the effective quantum yield values of photosystem (PS) 2 (ΔF/Fm') at the B (7 to 14-d-old), D (21 to 30-d-old), and E (>30-d-old) stages of leaf development, and in quantum yield of the non-cyclic photosynthetic electron transport between PS2 and PS1 [qp(Fv/Fm)] and quencher efficiency [(Fm-Ft)/F0] at the B, C (15 to 20-d-old) and D stages. Intergenotypic differences were found only for the [qp(Fv/Fm)] and [(Fm-Ft)/F0] values at the E stage, and for fluorescence quenching (Fm-Ft) at the B and E stages. Highly significant inter- and intragenotype relationships were found between the rate of photosynthetic electron transport to PS2 (Amax) and maximum fluorescence during actinic irradiation (Fm'). Also, each of the highly significant relationships between (Fm-Ft) and Amax, [(Fm-Ft)/F0] and ΔF/Fm', and between [(Fm-Ft)/F0] and Amax were represented by a general model, independent of treatments. Therefore, alterations in energy distribution in the radiant energy collector complex interior of PS2 and reduction in absorption of photosynthetically active radiation were observed in the infected plants, mainly in the hybrid at the C stage. Also, variations were found in the noncyclic photosynthetic electron transport at the B and C stages in the infected Catongo. and I. C. F. Santos, A.-A. F. de Almeida, R. R. Valle.
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 (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. and F. C. Baligar ... [et al.].