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

PERI P.L.; MOOT D.J.; MCNEIL D.L.; LUCAS R.J.

NEW ZEALAND JOURNAL OF AGRICULTURAL RESEARCH

RSNZ PUBLISHING

Año: 2003 vol. 46 p. 105 - 115

0028-8233

Maximum light-saturated photosynthetic rate (Pmax) of field grown cocksfoot (Dactylis glomerata L.) leaves in a temperate, sub-humid environment (Canterbury, New Zealand) was measured to derive an individual function for Pmax against regrowth-duration when other factors were non-limiting. Pmax of successive newly expanded leaves decreased with regrowth-duration as 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) but 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 obtained for regrowth-duration was incorporated as an additional factor into a multiplicative model to predict Pmax when temperature, nitrogen and water status, expressed as pre-dawn leaf water potential (ylp), were limiting. The only interaction detected was for the limited conditions of ylp> -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 photosythesis model for the prediction of cocksfoot pasture production.