The tropics play a disproportionately large roll in global CO2 exchange relative to their land coverage. Within the next 20 years, climate change will likely cause unprecedented warming in the tropics. Warmer temperatures may have negative effects on plant photosynthesis, an important component of the carbon cycle. One possible mode of heat stress on photosynthesis is through Photosystem II (PSII) damage. Our research assessed possible effects of in situ leaf-level canopy warming on PSII integrity. Leaf-level warming was implemented using a canopy access tower located in Puerto Rico. We heated individual leaves of two tropical species, Ocotea sintensis and Guarea guidonia, 3oC warmer than a nearby control leaf for 25 days. Both species were accessible in the understory but O. sintensis and G. guidonia were not present on all canopy access heights. O. sintensis was present slightly higher in the canopy than G. guidonia, though there was overlap between the species. PSII stress was assessed by measuring dark-adapted chlorophyll fluorescence, in particular the maximum quantum yield of PSII. We hypothesized that maximum quantum yield would decrease with both warming and height for both species, showing evidence of stress-related decreases in PSII function.
Results/Conclusions
Results showed that there was no significant difference in O. sintensis maximum quantum yield with height or between treatments. We also found no warming treatment effect for G. guidonia treatments; however, maximum quantum yield increased with increasing canopy height for this species. These findings suggest stress-related decreases in PSII function higher in the canopy. The lack of a warming treatment effect in both of our study species suggests that temperatures experienced by these canopy leaves were not high enough to impact maximum efficiency of PSII. The temperature threshold of PSII is relatively higher than other photosynthetic mechanisms, suggesting that any negative effects of warming on net photosynthesis were not due to reduced PSII function. This has important implications for carbon cycling research, as it gives us a better understanding of underlying mechanisms behind tropical plant photosynthetic response to climate warming.