Partititioning concurrent influences of nitrogen and phosphorus supply on photosynthetic model parameters of Pinus radiata

Responses of photosynthesis (A) to intercellular CO2 concentration (Ci) were measured in a fast- and a slow-growing clone of Pinus radiata D. Don cultivated in a greenhouse with a factorial combination of nitrogen and phosphorus supply. Stomatal limitations scaled with nitrogen and phosphorus supply as a fixed proportion of the light-saturated photosynthetic rate (18.5%) independent of clone. Photosynthetic rates at ambient CO2 concentration were mainly in the Vcmax-limited portion of the CO2 response curve at low-nitrogen supply and at the transition between Vcmax and Jmax at high-nitrogen supply. Nutrient limitations to photosynthesis were partitioned based on the ratio of foliage nitrogen to phosphorus expressed on a leaf area basis (Na/Pa), by minimizing the mean square error of segmented linear models relating photosynthetic parameters (Vcmax, Jmax, Tp) to foliar nitrogen and phosphorus concentrations. A value of Na/Pa equal to 23 (mole basis) was identified as the threshold separating nitrogen (Na/Pa greater than or equal to 23) from phosphorus (Na/Pa > 23) limitations independent of clones. On an area basis, there were significant positive linear relationships between the parameters, Vcmax, Jmax, Tp and Na and Pa, but only the relationships between Tp and Na and Pa differed significantly between clones. These findings suggest that, in genotypes with contrasting growth, the responses of Vcmax and Jmax to nutrient limitation are equivalent. The relationships between the parameters Vcmax, Jmax, Tp and foliage nutrient concentration on a mass basis were unaffected by clone, because the slow-growing clone had a significantly greater leaf area to mass ratio than the fast-growing clone. These results may be useful in discriminating nitrogen-limited photosynthesis from phosphorus-limited photosynthesis.