THE EFFECT OF ARCTIC LIGHT ON LEAF GAS EXCHANGE MODELS: CONNECTING GREENHOUSE, FIELD WORK, AND CLASSROOM

Plant leaf-gas exchange is central to photosynthetic processes that determine carbon fixation and water efficiency in response to climate change. However, the impact of different light intensity and duration on the dynamics of leaf-gas exchange models, such as the Franks model, under the rapid change of Anthropocene is poorly understood. Using three conifer species grown in greenhouse under the same atmospheric CO₂ concentration (360 ppm), Metasequoia glyptostroboides Hu & Cheng and Taxodium distichum L. C. Rich. (both in Cupressaceae) and Larix laricina (Du Roi) K. Koch (Pinaceae), we investigate key Franks model parameters, whole-leaf stomatal density (SD-WL), guard cell dimensions, and bulk-leaf carbon isotope composition (δ¹³C) in leaves exposed to different light conditions. Three months of 24 hours low intensity continuous light (CL) is designed to simulate light conditions at high Arctic (above 75°N) summer. These samples are compared with leaves grown under the same condition but with natural diurnal light (DL) at the 45°N latitude. While guard cell length (GCL) and width (CGW) measured on cleared leaves show little difference between CL and DL leaves, SD-WL in CL leaves tend to be 20-30mm² less dense than that in DL leaves. Our data show that different light irradiations affect various taxa differently. For example, the application of Larix on the Franks model resulted in overall overestimates of CO₂ level for both CL and DL leaves (up to 4 times of the target CO₂ values using CL leaves). In contrary, DL leaves from both Metasequoia and Taxodium yielded Franks model results most closely matching the known CO₂ concentration, while reconstructions using CL leaves overestimate CO₂ by 128-330 ppm. Leaves developed under different light regimes result in different δ¹³C values, which apparently play a key role in driving the Franks model equilibrium dynamics to balance the different photosynthetic rates. Our study bears implications for the application of the Franks model for both past CO₂ reconstruction and future leaf gas exchange under the increasing CO₂ levels during Anthropocene. This project illustrates the integration of greenhouse data, paleobotanical fieldwork, and classroom modeling for innovative teaching and learning under the Anthropocene framework.