Does nitrogen supply affect the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO2 and O3?

Spring wheat (Triticum aestivum cv. Hanno) was grown at ambient (350 mu mol mol(-1)) or elevated CO2 (700 mu mol mol(-1)) in charcoal/Purafil(R)-filtered air (CFA <5 nmol mol(-1)) or ozone (CFA +75 nmol mol(-1) 7 h d(-1)) at three levels of N supply (1.5, 4 and 14 mM NO3-), to test the hypothesis that the combined impacts of elevated CO2 and O-3 on plant growth and photosynthetic capacity are affected by nitrogen availability. Shifts in foliar N content reflected the level of N supplied, and the growth stimulation induced by elevated CO2 was dependent on the level of N supply. At 60 d after transfer (DAT), elevated CO2 was found to increase total biomass by 44%, 29%, 12% in plants supplied with 14, 4 and 1.5 mM NO3-, respectively, and there was no evidence of photosynthetic acclimation to elevated CO2 across N treatments; the maximum in vivo rate of Rubisco carboxylation (V-cmax) was similar in plants raised at elevated and ambient CO2. At 60 DAT, ozone exposure was found to suppress plant relative growth rate (RGR) and net photosynthesis (A) in plants supplied with 14 and 4 mM NO3-. However, O-3 had no effect on the RGR of plants supplied with 1.5 mM NO and this effect was accompanied by a reduced impact of the pollutant on A. Elevated CO2 counteracted the detrimental effects of O-3 (i.e. the same ozone concentration that depressed RGR and A at ambient CO2 resulted in no significant effects when plants were raised at elevated CO2) at all levels of N supply and the effect was associated with a decline in O-3 uptake at the leaf level.