부산대학교 도서관

Understanding the high-temperature sensitivity of photosynthesis in the Tropics is crucial for predicting the impacts of climate change on global carbon and energy fluxes. Hence, to first understand whether the photosynthetic process has already reached heat thresholds in tropical trees, responses of photosynthesis to high leaf temperatures were measured on trees in the warmest and most rapidly warming tropical forest region of Amazonia - Nova Xavantina, located in the southern borders. Results from photosystem-II (PSII) chlorophyll fluorescence quenching measurements on heat-treated leaf discs showed that, during the end of dry periods, trees in Nova Xavantina are already experiencing temperatures that negatively impact PSII functioning. Among the seven species studied, Amaioua guianensis, showed remarkable heat tolerance, the highest recorded in the literature for a tropical tree. Instantaneous CO2 assimilation rate (Anet) temperature response measurements on intact leaves, during the end of the dry period, showed the lowest reported Anet at thermal optima in literature, with a high leaf-to-leaf variation. The pan-tropical comparison revealed that, while temperature optima of photosynthesis increased with the maximum air temperature of the forests, the high-temperature CO2 compensation point (Tmax), increased with site temperature but plateaued due to similar Tmax values for both Amazonian forests. In the second part, the thesis introduces 'leaf absorption differential spectroscopy' (LADS) technique, addressing the question of 'whether leaf absorption spectra can indicate photosynthetic changes over short time scales of minutes'. LADS couples radiometric capabilities with a photosynthesis analyser enabling the study of instantaneous changes in leaf absorption spectra on intact photosynthesising leaves. The instrument prototype implementing the concept was tested on soybean, grown under greenhouse conditions. I demonstrate that leaf light absorption is temperature-sensitive. The differential absorption spectra induced by 10°C temperature difference had signatures of chlorophyll, carotenoids and photosystem-I. Pigment and molecular analyses revealed an increase in 1) chlorophyll-b and xanthophyll carotenoid concentration, 2) abundance of two heat-shock proteins and 3) relative abundance of two of the seven Whirly genes - chloroplast genes known to have roles in instantaneous stress responses. The implications of the findings and potential application as rapid phenotyping technique and remote sensing approach are presented.