A canopy photosynthesis model to predict the dry matter production of cocksfoot pastures under varying temperature, nitrogen and water regimes.

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

GRASS AND FORAGE SCIENCE (PRINT)

WILEY-BLACKWELL PUBLISHING, INC

Año: 2003 vol. 58 p. 416 - 430

0142-5242

Daily net canopy photosynthesis (Pn) of cocksfoot (Dactylis glomerata L.) was predicted for combinations of temperature, water status, herbage nitrogen (N) content and light conditions from the integration of leaf photosynthesis models of the light-saturated photosynthetic rate (Pmax), the photosynthetic efficiency (¥á) and the degree of curvature (¥è) of leaf light-response curves. The effect on Pn, maximum Pn (Pnmax) and the optimum LAI (LAI at Pnmax) were examined when any one of these factors was limiting. The range of the optimum Pn (Pnmax= 30.8 - 33.5 g CO2 m-2 d-1) for temperature (19 to 22 ¨¬C) and N (4.0-5.0 g N kg-1 DM) was smaller than for net leaf photosynthesis. Also, Pn fell to zero at a lower level of water stress (pre-dawn leaf water potential, ¥÷lp= -12.5 bar) than for Pmax. When only shade was limiting, Pnmax decreased approximately linearly from 33.4 g CO2 m-2 d-1 to zero as photosynthetic photon flux density (PPFD) fell from full sunlight to 10% of open PPFD in a fluctuating light regime. Also, it was predicted that for a continuous light regime (50% transmissivity) Pnmax was higher than for a fluctuating light regime with the same intensity. The canopy photosynthesis model was then used to compare predicted dry matter (DM) for cocksfoot pastures grown under a diverse range of environmental conditions with field data. To predict DM production leaf area index and leaf canopy angle were included from field measurements. The model explained about 85% of the variation in cocksfoot growth for a range of 6 to 134 kg DM ha-1 d-1. The canopy model overestimated (<10%) growth in most of the validations tested which indicates a further Pmax function for leaves of different ages and a partitioning sub-model may be needed for improvement.