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-season CO₂ enrichment were investigated using a Free-Air CO₂ Enrichment (FACE) apparatus. Spring wheat [Triticum aestivum (L). cv. Yecora Rojo] was grown in two experiments under ambient and elevated atmospheric CO₂ (C(a)) concentrations (approximately 370 μmol mol^-1 and 550 μmol mol^-1, respectively) combined first with two irrigation (Irr) schemes (Wet: 100% and Dry: 50% replacement of evapotranspiration) and then with two levels of nitrogen (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 photosynthesis for the ears was increased by 58% and stomatal conductance (g(s)) was decreased by 26% due to elevated C(a) under ample water and N supply when blowers were applied to both C(a) treatments. The use of blowers in the C(a)-enriched plots only during the C(a) x Irr experiment (blower effect) and Low N supply restricted the enhancement of net photosynthesis of the ear due to higher 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 photosynthesis due to elevated C(a) was higher for ears than for flag leaves and this effect was most pronounced under ample water and N supply. The contribution 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 higher 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 photosynthesis of the glume and delayed the increase of the awn portion of net photosynthesis of the ear during drought. Therefore, the role of awns for maintaining high net photosynthesis of the ear under drought may decrease when C(a) increases.