Modelling net photosynthesis rate of field grown cocksfoot leaves under different nitrogen, water and temperature regimes

A simple multiplicative model using temperature, foliage nitrogen (N) concentration and water status was developed to predict the maximum photosynthetic rate (Pmax) of field-grown cocksfoot (Dactylis glomerata L.) leaves when none, one, two or all the factors were limiting. The highest Pmax was 27.4 μmol CO2 m-2 s-1 in non-limited conditions, which was defined as the standardized Pmax value dimensionless (Pmaxs = 1). Pmaxs increased 0.058 units per °C from 10°C to the optimum range (19-23°C) (Pmaxs= 1) and then declined 0.077 units of Pmaxs per °C from 23 to 31°C. Pmaxs = 1 was also measured from 59 to 52 g N kg-1 dry matter (DM) foliage N. Pmaxs then decreased at the rate of 0.115 units per 10 g N kg-1 DM from 52 to 26 g N kg-1 DM, and 0.409 units of Pmaxs per 10 g N kg-1 DM from 26 to 15 g N kg-1 DM. For predawn leaf water potential (ψlp), Pmaxs = 1 was measured from -0.1 to -1.2 bar but declined linearly at a rate of 0.078 units per bar of ψlp from -1.2 to -14.0 bar because of a linear decrease in stomatal conductance. An interaction between low N content (≤20 g N kg-1 DM) and high temperature (>23°C) was also detected. Together, this multiplicative model accounted for 0.82 of the variation in Pmaxs.