Photosynthetic thermal tolerance
Measured Fv /Fm ratios (average
of 0.69 ± 0.02 arbitrary unit, n = 35) were stable with
increasing temperatures up to a maximal temperature point and declined
thereafter to near zero levels. The breakpoint temperatureT5 averaged at 43.5 ± 1.7 °C across species
during the end of the dry period and 41.9 ± 0.9 °C during the wet period
(Figure 3A). The paired difference of 1.06 °C over the seasons was not
statistically significant. A. guianensis recorded the highestT5 at 53.9 ± 1.3 °C (\(P\) <.0001,\(t_{3}\) = 10.7) during the end of dry period while the rest of the
species recorded ~ 13.4°C lowerT5 at 40.6 ± 3.3°C. However, during the wet
period T5 for A. guianensis was
statistically indistinguishable from that of the other species.
Furthermore, T5 for A. guianensis was
higher during the dry period compared to wet period – while on the
contrary, the remaining species showed lower or indifferentT5 during the dry period. The mixed effects model
to explain T5 variation showed a weak species
effect (F = 3.48, P =0.01) but no influence of season was
found. The species difference was mainly due to the response of A.
guianensis which was significantly different from C. cognatum(difference of 11.6 °C, P = 0.005) and M. apiranga(difference of 8.7 °C, P = 0.029).