Photosynthesis of Helianthus annuus does not acclimate to elevated CO2 regardless of N supply

A number of studies have shown that relatively long-term exposure to elevat ed levels of CO2 can lead to the downward acclimation of photosynthesis. Al though the exact mechanisms are not clearly understood, it has been suggest ed that such a downward adjustment may be more common under limited N avail ability. Here we examined the effect of N supply on the photosynthetic accl imation 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 evalu ated by parameters derived from the A-C, data (Rubisco carboxylation capaci ty, V-c,V-max; electron transport capacity, J(max;) and capacity for triose phosphate utilization, TPU) showed no acclimation to elevated CO2. The lea f nitrogen concentration, [N], and total non-structural carbohydrates, [TNC ], were also comparable between ambient- and devated-CO2-grown plants. Howe ver, 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] a nd [TNC] did not significantly differ between ambient- and elevated-CO2, pl ants. These results suggest that H. annuus plants maintained their photosyn thetic capacity during long-term exposure to elevated CO2 because of their capacity to maintain leaf N-status. It is further suggested that plant capa city to maintain the balance between C and N acquisition rather than simply N-supply level, may determine whether photosynthetic acclimation in respon se to elevated CO2 occurs or not. (C) 2000 Editions scientifiques et medica les Elsevier SAS.