Abstract Long-term exposure of plants to elevated CO2 often leads to downward photosynthetic acclimation. Nitrogen (N) deficiency could potentially exacerbate this response by reducing growth rate and the sink for photosynthates, but this has not always been observed. Experimentally, the interpretation of N effects on CO2 responses can be confounded by increasing severity of tissue N deficiency over time when N supply is not adjusted as demand increases. In this study, N supply ranged from sub- to supra-optimal (20–540 kgN ha–l equivalent), and relatively stable levels of tissue N concentration were obtained in all treatments by varying twice-weekly application rates in proportion to plant growth. The effects of N on photosynthesis and growth of beans (Phaseolus vulgaris L.) raised at ambient (35 Pa) and three elevated (70, 105, 140 Pa) CO2 partial pressures (pCO2) were evaluated. Averaging across N treatments, leaf total non-structural carbohydrates (TNC) were 2.5- to 3-fold higher and leaf N concentrations were 31–35% lower at elevated compared to ambient pCO2. Light-saturated net CO2 assimilation rates measured at growth pCO2 (Asatg) were significantly higher (26–40% depending on N supply) in plants grown at elevated compared to ambient pCO2. When measured at a common pCO2 of 35 Pa, the Asat of plants grown at elevated CO2 was 15–29% less than that of plants grown at 35 Pa, indicative of downward photosynthetic acclimation. The magnitude of downward photosynthetic acclimation to elevated CO2 was greater in plants grown at high (180 and 540 kgN ha–l) compared to low (20 and 60 kgN ha–l) N supply, and this was associated with a higher Asat at growth pCO2, higher leaf area ratio (leaf area/total biomass), and higher TNC in leaves of high-N plants. Our results indicate that the effect of N on acclimation to CO2 will depend on the balance between supply and demand for N during the growing period, and the effect this has on biomass allocation and source-sink C balance at the whole-plant level.
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