Scaling the response of wheat to elevated CO2: Comparison of photosynthetic acclimation and organ/plant growth

Spring wheat (Triticum aestivum L. cv. Minaret) was exposed to ambient (A-NF) and ambient + 320 ppm CO 2 (A320-NF) in non-filtered (NF) open-top chambers. Wheat plants were grown in pot culture and were supplied with 300 kg N ha -1 . For a diagnosis of photosynthetic acclimation to elevated CO 2 A/C i -curves were measured at youngest fully emerged leaves using a closed loop photosynthesis measurement system. Wheat plants that grew under elevated CO 2 did not show a modification of theA/C i -curves at tillering and stem elongation and thus did not exhibit any photosynthetic acclimation before anthesis. A widely used photosynthesis model was parameterized from the A/C i -curves. Within model inaccuracies the key model parameters carboxylation efficiency CE and maximum e - -transport rate J max were linearly dependent on total leaf N concentration irrespective of the treatment. However, total leaf nitrogen content declined in flag leaves ofA320-NF plants. Using the N-dependence of the key photosynthesis model parameters it was calculated that a photosynthetic acclimation due to N-dilution was likely to occur in flag leaves. Besides, an increase of water-soluble carbohydrate (WSC) content due to CO 2 enrichment occurred exclusively in flag leaves during grain filling. From the flag leaf area of the whole plant, the calculated photosynthetic rate of the unit leaf area and the modeled sink strength of the ears a source-sink imbalance was estimated at the organ level. Direct feedback limitation due to disturbed carbohydrate translocation probably inhibited photosynthesis inA320-NF beyond the N-induced inhibition. Source-sink imbalance primarily was provoked by wheat plants inflating their flag leaf area under elevated CO 2 . At the whole plant level a functional growth analysis was carried out to characterize the growth response of wheat to elevated CO 2 . Above-ground dry weight was not increased due to elevated CO 2 until grain filling began. The CO 2 -induced stimulation of the relative growth rate diminished with time. However, this behaviour was not a consequence of photosynthetic acclimation, since the normally occurring decline of the relative growth rate with increasing plant size wholly accounted for this effect (before anthesis). Absolute growth rate in A320-NF always was above A-NF.