Photosynthesis and conductance of spring wheat ears: field response to free-air CO2 enrichment and limitations in water and nitrogen supply

The mid-day responses of wheat ear CO2 and water vapour exchange to full-se ason CO2 enrichment were investigated using a Free-Air CO2. Enrichment (FAC E) apparatus. Spring wheat [Triticum aestivum (L). cv. Yecora Rojo] was gro wn in two experiments under ambient and elevated atmospheric CO2 (C-a) conc entrations (approximately 370 mu mol mol(-1) and 550 mu mol mol(-1), respec tively) combined first with two irrigation (Irr) schemes (Wet: 100% and Dry : 50% replacement of evapotranspiration) and then with two levels of nitrog en (N) fertilization (High: 350, Low: 70 kg ha(-1)N). Blowers were used for C-a enrichment, Ambient C-a plots were exposed to blower induced winds as well the C-a x N but not in the C-a x Irr experiment. The net photosynthesi s for the ears was increased by 58% and stomatal conductance (g(s)) was dec reased by 26% due to elevated C-a under ample water and N supply when blowe rs were applied to both C-a treatments. The use of blowers in the C-a-enric hed plots only during the C-a x Irr experiment (blower effect) and Low N su pply restricted the enhancement of net photosynthesis of the ear due to hig her C-a. In the latter case, the increase of net photosynthesis of the ear amounted to 26%. The decrease in g(s) caused by higher C-a was not affected by the blower effect and N treatment. The mid-day enhancement of net photo synthesis due to elevated C-a was higher for ears than for flag leaves and this effect was most pronounced under ample wafer and N supply, The contrib ution of ears to grain filling is therefore likely to increase in higher C- a environments in the future. In the comparison between Wet and Dry, the hi gher C-a did not alter the response of net photosynthesis of the ear and g( s) to Irr. However, C-a enrichment increased the drought tolerance of net p hotosynthesis of the glume and delayed the increase of the awn portion of n et photosynthesis of the ear during drought. Therefore, the role of awns fo r maintaining high net photosynthesis of the ear under drought may decrease when C-a increases.