Growth of Festuca pallescens in Silvopastoral Systems in Patagonia, Part 2: Parameterization of Models of Stomatal Conductance and Leaf Photosynthesis

A combined model of stomatal conductance and photosynthesis was developed for Festuca pallescens (St. Ives) Parodi, a forage species in Patagonia. Curves showing the relationship between photosynthesis and photosynthetic photon flux density (PPFD) were constructed for plants grown under differing levels of water availability, relative humidity (RH) and air temperature (T). Stomatal conductance (g_s) was related to these variables and pre-dawn leaf water potential (ψ_pd) using an empirical multiplicative submodel. Parameters of the photosynthesis-PPFD curves were related to the average g_s values for each curve to introduce stomatal limitation on photosynthesis. Considering the simplicity of the models, estimated stomatal conductance and photosynthesis agree satisfactorily with independent measured values in the field and in the glasshouse, particularly in the range of low and medium values of both variables (R² = 0.84 and 0.87 for g_s and photosynthesis models, respectively). Photosynthesis–PPFD curves were also determined under field conditions for plants growing under shade and in the open, in a silvopastoral trial in northwestern Patagonia. No significant differences in the photosynthetic light response curves were found between these locations, but slight increases in maximum assimilation rate and quantum yield (light use efficiency) were found for leaves grown under shade. This study of environmental influences on photosynthesis in F. pallescens may help to predict its capacity to grow under trees in silvopastoral systems. In addition, this simple model may be easily parameterised for other species to predict photosynthetic responses under different environmental conditions.