INTERACTING EFFECTS OF CO2 CONCENTRATION, TEMPERATURE AND NITROGEN SUPPLY ON THE PHOTOSYNTHESIS AND COMPOSITION OF WINTER-WHEAT LEAVES

Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO, concentrations (350 and 700 mu mol mol(-1)) two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4 degrees C] and two rates of nitrogen supply (equivalent to 489 kg ha(-1) and 87 kg ha(-1)). Leaves grown at 700 mu mol mol(-1) CO2 had slightly greater photosynthetic capacity (10% mean increase over the experiment) than those grown at ambient CO2 concentration, but there were no differences in carboxylation efficiency or apparent quantum yield. The amounts of chlorophyll, soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area did not change with long-term exposure to elevated CO2 concentration. Thus winter wheat, grown under simulated field conditions, for which total biomass was large compared to normal field production, did not experience loss of components of the photosynthetic system or loss of photosynthetic competence with elevated CO2 concentration. However, nitrogen supply and temperature had large effects on photosynthetic characteristics but did not interact with elevated CO2 concentration. Nitrogen deficiency resulted in decreases in the contents of protein, including Rubisco, and chlorophyll, and decreased photosynthetic capacity and carboxylation efficiency. An increase in temperature also reduced these components and shortened the effective life of the leaves, reducing the duration of high photosynthetic capacity.