The interacting effects of nitrogen supply and CO2 concentration on photosynthetic capacities in leaves of spring wheat.

Triticum aestivum L. cv. Minaret was grown as a crop stand at two CO2 concentrations (355, 690 umol mol-1 CO2) with 41, 90 and 236 kg/ha-1 nitrogen application regimes. The response of photosynthetic rate to CO2 concentration (A/Ci) was determined in leaf four and through development in flag leaves. Nitrogen treatment had no significant effect on Amax or the carboxylation efficiency in leaf four. For flag leaves, reduction in both parameters was much more rapid at the lowest nitrogen treatment (P<0.001). The Amax for leaf four was not affected by growth at 690 umol mol-1 CO2, but did decline faster in flag leaves grown at 690 umol mol-1 CO2 compared to those grown at 355 umol mol-1 CO2. For flag leaves grown at 690 umol mol-1 CO2, values for carboxylation efficiency became increasingly different, being 27% less than those grown at 355 umol mol-1 CO2 (P