Extension of a Farquhar model for limitations of leaf photosynthesis induced by light environment, phenology and leaf age in grapevines ( <i>Vitis</i> <i>vinifera</i> L. cvv. White Riesling and Zinfandel)

Measurements of gas exchange and stomatal conductance were made on potted and field-grown grapevines (Vitis vinifera L.) on leaves from different light environments (sun and shade) at different phenological stages during the season to parameterise the Farquhar model. The model parameters for Rubisco activity (V_cmax), maximum electron transport rate (J_max), and triose-phosphate utilisation (TPU) were estimated on the basis of a large data set (n = 105) of CO₂ assimilation (A) versus internal CO₂ pressure (C_i) curves. Leaf age was described with the leaf plastochron index (LPI). Stomatal coupling to photosynthesis was modelled with the Ball-Woodrow-Berry empirical model of stomatal conductance. Mature shade leaves had 35-40% lower values of V_cmax, J_max and TPU than sun leaves. The difference between leaf types decreased at the end of the season. The ratio J_max / V_cmax and values of day respiration (R_d) and CO₂ compensation point in the absence of mitochondrial respiration (Γ*) varied little during the season and were independent of LPI. Validation of the model with independent diurnal data sets of measurements of gas exchange and stomatal conductance at ambient CO₂ concentrations for three days between June and October, covering a large range of environmental conditions, showed good agreement between measured and simulated values.