Integrating gas-exchange equipment into a field-based undergraduate experience

Background/Question/Methods Photosynthesis is the main entry point for carbon and energy into most ecological systems, and student understanding of resource and energy cycling is enhanced by a firm understanding of key environmental factors that regulate photosynthesis. Our ecology program has developed a lab exercise designed to facilitate student understanding of the role of several environmental factors (light, CO2, leaf nutrient stoichiometry) in regulating photosynthesis in situ, introduce students to common protocols used to assess photosynthetic responses to these variables, and enable students to test predictions for the effects of these environmental factors on carbon uptake in plants. For the lab exercise, students initially develop a set of relevant hypotheses based on assigned readings. To test these hypotheses, students use gas-exchange equipment to measure net photosynthetic rates at a range of internal CO2 concentrations (A-Ci curves) and irradiances (A-Q curves), and fit these curves using mechanistically-based models. Model outputs are used to estimate maximum rubisco activity, maximum electron-transport mediated carbon uptake capacity, respiration rates in the light, quantum yield, the light compensation point, and the light saturation point, as well as light-saturated net photosynthetic rates. To examine relationships between elemental concentrations and physiological processes, students measure nitrogen and phosphorus concentrations in leaf tissue.

Results/Conclusions Results from several years of exercises indicated that nitrogen was significantly correlated with photosynthetic rates and other physiological processes in our nitrogen-limited system. However, leaf phosphorus concentrations were not significantly correlated with other elements or physiological processes. Students are asked to develop alternate hypotheses to explain these, which are incorporated into the manuscripts students write based on the lab exercise.