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

Responses of photosynthesis (A) to intercellular CO2 concentration (C-i) 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 portion of the CO2 response rate that is limited by maximal carboxylation rate (V-cmax) at low-nitrogen supply and at the transition between V-cmax and J(max) 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 (N-a/P-a), by minimizing the mean square error of segmented linear models relating photosynthetic parameters (V-cmax, J(max), T-p) to foliar nitrogen and phosphorus concentrations. A value of N-a/P-a equal to 23 (mole basis) was identified as the threshold separating nitrogen (N-a/P-a <= 23) from phosphorus (N-a/P-a > 23) limitations independent of clones. On an area basis, there were significant positive linear relationships between the parameters, V-cmax, J(max), T-p and N-a and P-a, but only the relationships between Tp and Na and P, differed significantly between clones. These findings suggest that, in genotypes with contrasting growth, the responses of V-cmax and J(max) to nutrient limitation are equivalent. The relationships between the parameters V-cmax, J(max), T-p 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.