Analysis of differences in photosynthetic nitrogen-use efficiency between four contrasting species

Causes of differences in photosynthetic nitrogen-use efficiency (PNUE), the rate of photosynthesis per unit leaf N, were investigated in four species. These were in order of decreasing PNUE; the two herbs Galinsoga ciliata and Origanum vulgare , and the two trees Populus nigra and Quercus robur . Plants were grown in pots outdoors at three levels of nutrient availability. The light- and CO₂ response of gas exchange of leaves were measured, and their nitrogen and chlorophyll contents were determined. Furthermore, the internal conductance for CO₂ diffusion was estimated. Nutrients did not have a large effect on PNUE except in Galinsoga . Leaf mass per unit area was negatively correlated with PNUE_max, which is likely to be partly caused by N present in cell wall proteins among other non-photosynthetic N compounds. The trees had a larger fraction of photosynthetic N in light harvesting components compared to the herbs. This contributed also substantially to the difference in PNUE at light saturation (PNUE_max) between the two groups, but not for PNUE calculated for an overcast day. Intercellular CO₂ concentration was high in Galinsoga and Populus , which contributed significantly to their higher PNUE_max, particularly at low nutrient availability. The large gradient in CO₂ concentration between intercellular spaces and chloroplasts was another factor that explained a substantial part of the differences in PNUE_max between Quercus and the other species that had smaller gradients. Stomatal and internal conductances for CO₂ explained most of the difference in PNUE_max between Quercus and Populus at high nutrient availability for which these data were available.