Photosynthesis-nitrogen relationships: interpretation of different patterns between Pseudotsuga menziesii and Populus x euroamericana in a mini-stand experiment

We compared photosynthesis–nitrogen relationships of one broad-leaved (poplar; Populus × euroamericana (Dole) Guinier) and one conifer (Douglas-fir; Pseudotsuga menziesii (Mirb.) Franco) species. Plants were grown in large pots to allow free root development and were kept well watered. We determined effects of low, intermediate and high nitrogen supply rates on area-based leaf nitrogen (Na) and chlorophyll concentrations, leaf mass per area (LMA), light-saturated photosynthesis (Amax), maximum carboxylation (Vcmax) and electron transport rate (Jmax), photosynthetic nitrogen-use efficiency (PNUE), and proportions of leaf N in active Rubisco (PR), bioenergetic pools (PB) and the light-harvesting complex (PLH). Nitrogen supply significantly affected leaf Na. Leaf mass per area did not differ between species and was unaffected by the N treatments. In both species, there was a positive correlation between leaf Na and chlorophyll concentration, and between leaf Na and the photosynthetic parameters Amax, Jmax and Vcmax. At comparable leaf Na, however, poplar showed twofold higher PNUE and a threefold steeper slope of the Amax– nitrogen relationship than Douglas-fir. Leaf Na was negatively correlated with PNUE in Douglas-fir but not in poplar. Leaf Na was also negatively correlated with PR, PB and PLH in Douglas-fir, whereas in poplar, a negative correlation was found only for PLH. Parameter PR was significantly higher in poplar than in Douglas-fir. The ratio of CO2 concentration in the intercellular space to that in ambient air was higher in poplar than in Douglas-fir. Overall, our data suggest that differences in the photosynthesis–nitrogen relationship and PNUE between Douglas-fir and poplar primarily reflect a different investment of N to active Rubisco, and possibly a different constraint to CO2 diffusion.