Modelling net photosynthetic rate of field-grown cocksfoot leaves to account for regrowth duration

Maximum light-saturated photosynthetic rate (Pmax) of field-grown cocksfoot (Dactylis glomerata L.) leaves was measured in a temperate, sub-humid environment (Canterbury, New Zealand). The aim was to derive an individual function for Pmax of newly expanded leaves against regrowth duration when other environmental factors were non-limiting. The decrease in Pmax with regrowth duration was described by a quadratic function. From 20 to 25 days regrowth, Pmax per unit of leaf was constant and maximal (27.4 μmol CO2 m-2 s -1). It then decreased by 0.42 μmol CO2 m-2 s-1 per day of regrowth. The decline in Pmax was attributed to (i) differences in chronological age of the youngest expanded leaf as shown by changes in tiller morphology over time, and (ii) shading within the canopy during leaf expansion. These factors affected Pmax by decreasing the leaf nitrogen and chlorophyll content, and stomatal conductance. The function for regrowth duration was an additional factor included in a multiplicative model to predict Pmax with different levels of temperature, nitrogen, and water status, expressed as pre-dawn leaf water potential (ψ1p). The only interaction detected was when water stress increased (ψ1p < -1.2 bar) and leaves had grown for 40-60 days. In this limited situation, stomatal closure at 40-60 days was greater than expected from the non-limiting condition. The inclusion of this function into a simple multiplicative model enabled 80% of the variation in Pmax for individual cocksfoot leaves to be explained by their temperature, nitrogen, water, and regrowth status. These functions could then be used to develop a canopy photosynthesis model for the prediction of cocksfoot pasture production.