Determining the response of leaf photosynthetic carbon metabolism and energy balance to changes in temperature, carbon dioxide and nitrogen form

Understanding how photosynthesis responds to temperature, CO2, and nitrogen form is essential for predicting plant response to climate change. Ribulose (1,5) bisphosphate Carboxylase-Oxygenase (Rubisco) explains much of this response by reacting with CO2 to initiate carbon fixation, but also with O2 initiating photorespiration and a stiochiometric loss of CO2. The temperature responses of Rubisco kinetics and mesophyll conductance to CO2 (gm) govern rates of CO 2 and O2 reaction and quantify the impact of temperature on net carbon assimilation, but have only been measured in Nicotiana tabacum. These kinetics also determine the total ATP and NADPH demand of primary metabolism. Cyclic electron flux around photosystem I (CEF) is proposed to balance energy supply with this changing demand from primary metabolism, but is not always initiated as expected.;This dissertation investigates the temperature response of photorespiration, Rubisco kinetics, and gm. It also tests the interaction between CEF and NO3- assimilation in balancing energy status under changing rates of CO2 fixation and photorespiration. Measurements of the CO2 compensation point indicate that the stoichiometry of CO2 released from photorespiration may increase with temperature in Arabidopsis thaliana. The temperature response of Rubisco kinetics determined on A. thaliana and N. tabacum plants revealed no significant differences in individual parameters, but their cumulative use resulted in significantly different modeled photosynthesis. Additionally, the response of gm determined from 13CO 2 isotope discrimination increased with temperature in N. tabacum but not in A. thaliana. These kinetic and g m values were used to model demand for CEF which was then compared to measured CEF determined from the dark interval relaxation kinetics of the electrochromic shift. CEF did not respond to changing ATP and NADPH demand under low light, but did under high light. Furthermore, nitrogen form had no significant effect on CEF in mature, fully-developed A. thaliana .;These findings improve understanding of the photosynthetic response to the environment. Photorespiration may become less efficient at elevated temperatures and the temperature response of Rubisco kinetics and gm can vary between species. Plants also possess a robust ability to balance energy supply despite daily changes in demand.