Photosynthesis of Helianthus annuus does not acclimate toelevated CO2 regardless of N supply

A number of studies have shown that relatively long-term exposure to elevated levels of CO2 can lead to the downward acclimation of photosynthesis. Although the exact mechanisms are not clearly understood, it has been suggested that such a downward adjustment may be more common under limited N availability. Here we examined the effect of N supply on the photosynthetic acclimation response of Helianthus annuus L. cv. Teddy Bear plants to elevated CO2 at three growth stages – 18, 38 and 56 d after emergence corresponding to vegetative, pre-flowering and flowering stages. Plants were grown at CO2 partial pressures of 37 or 70 Pa, and supplied with 0.5, 2.5 or 5 mol·m–3 N. After 18 d of treatment, photosynthetic capacity of H. annuus as evaluated by parameters derived from the A-Ci data (Rubisco carboxylation capacity, Vc,max; electron transport capacity, Jmax; and capacity for triose phosphate utilization, TPU) showed no acclimation to elevated CO2. The leaf nitrogen concentration, [N], and total non-structural carbohydrates, [TNC], were also comparable between ambient- and elevated-CO2-grown plants. However, all these photosynthetic parameters as well as leaf [N], but not [TNC], significantly increased in response to N supply. Similarly, after 38 and 56 d of exposure to CO2 treatments, photosynthetic capacities, foliar [N] and [TNC] did not significantly differ between ambient- and elevated-CO2 plants. These results suggest that H. annuus plants maintained their photosynthetic capacity during long-term exposure to elevated CO2 because of their capacity to maintain leaf N-status. It is further suggested that plant capacity to maintain the balance between C and N acquisition rather than simply N-supply level, may determine whether photosynthetic acclimation in response to elevated CO2 occurs or not.