ON THE SUNNY SIDE OF THE TREE: THE IMPACT OF LEAF ORIENTATION ON THE FRANKS MODEL FOR RECONSTRUCTING ANCIENT CO2

The estimation of ancient atmospheric carbon dioxide (CO₂) has been a critical endeavor for paleobotanists to explore different fossil leaf based proxies over the past decades, including the leaf-gas exchange models such as the Franks model. One of the untested aspect of the Franks model is the impact of model dynamics due to leaf orientations (sun versus shade leaves). We examined the key model parameters in sun and shade leaves of Metasequoia glyptostroboides Hu & Cheng 1948 (Cupressaceae s.l.) using a modified cleared leaf preparation method. While guard cell length (CGL, ~0.0487mm on average) and guard cell width (GCW, ~0.0159mm on average) are similar between sun and shade leaves, the stomatal density of whole leaf (SD-WL) in sun leaves averages ~20mm² more denser than that in shade leaves. Previous studies indicate that the bulk-leaf carbon isotope composition (δ¹³C) in sun leaves is ~1‰ more positive than that in shade leaves at middle latitudes. Franks model simulations suggest that the combined effects of different SD-WL and δ¹³C values in sun and shade leaves would result in ~50-60ppm difference in reconstructed CO₂. In order to compensate such a degree of leaf conductance difference, the photosynthetic rate (A) has to adjust by 3-4 µmolm^-2s^-1 to keep the Franks model in balance between sun and shade leaves under the same CO₂ environment. As a result, it appears that the change of A in leaves with different orientations plays as a key factor in the accuracy of estimating CO₂. A comparison with behaviors of Franks model parameters in leaves grown under normal and continuous light regimes in a greenhouse indicates that sun leaves behave like foliage grown under normal low latitudinal light while the shade leaves are similar to foliage developed under high latitudinal continuous lights. Our new data shed light on the impact of the Franks model by different light irradiation and have direct implications for the application of the Franks model in the fossil record for ancient CO₂ reconstruction, especially for fossils from high latitudes where the offsets of δ¹³C values between sun and shade leaves tend to increase. More data on real-time leaf photosynthetic rates are needed to quantitatively determine the compensation dynamics of the Franks model with leaves of different orientations.