Physiologically-Based Estimates of Transpiration Under Well-Watered and Water-Stressed Conditions: Seasonal Trends in Photosynthetic Parameters and Stomatal Conductance of Zea Mays and Helianthus Annuus.

Plant leaves are key factors in the global water exchange cycle - leaves affect the land surface water and energy budgets by controlling the passage of water vapor and carbon dioxide through the aperture or conductance of stomata. A common approach for estimating fluxes of CO₂ and water in leaf and canopy models is to couple a mechanistic model of photosynthesis to a semi-empirical model of stomatal conductance (g_s) by utilizing an observed linear relationship between photosynthesis (P_n) and g_s.  Successful modeling of P_n and transpiration on a seasonal basis requires an understanding of how model parameters change across the season. Additionally, modeling the impacts of water stress adds another layer of complexity - the ways current models simulate the effects of water stress differ among modelling schemes, and it is often unclear whether water stress should be accounted for by imposing stomatal limitations, biochemical limitations, or both. We collected key parameters for the models of P_n  and the Ball-Berry model of g_s across the growing season for sunflower and maize under well-watered and water stressed conditions. We also analyzed leaf tissue across the season under well-watered and water stressed conditions to determine the strength of the correlation between V_max  (the maximum rate of carboxylation), nitrogen (N), and chlorophyll (Chl) content in order to examine the validity of using Chl or N as a proxy for photosynthetic capacity. The results are used to examine the interaction of model parameters seasonally and under water stress and to compare and contrast the response patterns of sunflower versus maize.