Long-term CO2 Enrichment Increases Biomass but Results in Rapid Physiological Acclimation of Petunia and Pansy

Although crops often respond immediately to enriched CO2 concentrations (e.g., increased photosynthesis), this initial response is often not sustained throughout production, thus reducing the benefit of this input. For horticulture species, the timing and extent of these acclimation responses are still widely uncertain. Therefore, the objective of this research was to determine species-specific acclimation responses to elevated CO2 concentrations for pansy (Viola ×wittrockiana ‘Matrix Blue Blotch Improved’) and petunia (Petunia ×hybrida ‘Dreams Midnight’). Seedlings were transplanted to 11.5-cm pots and placed in growth chambers with air temperature, relative humidity, and radiation intensity setpoints of 21 °C, 55%, and 250 μmol⋅m−2⋅s−1, respectively. Carbon dioxide treatments were established using the two growth chambers with setpoints of either 400 (ambient) or 1000 μmol⋅mol−1 (elevated) maintained during a 16-hour photoperiod. In addition to data collected through destructive harvest, the rate of photosynthesis (A) in response to increasing internal leaf CO2 concentration (A-Ci) and at the operating CO2 concentration (A-Ca) were measured weekly with a portable leaf photosynthesis system at saturating [A-Ci (1000 μmol⋅m−2⋅s−1)] or production [A-Ca (250 μmol⋅m−2⋅s−1)] radiation intensities. For both pansy and petunia, elevated CO2 produced greater total shoot dry mass than ambient CO2 after 4 weeks. However, the decreased maximum rate of photosynthetic electron transport, maximum rate of Rubisco carboxylase, and triose phosphate utilization rate of both species were also observed under elevated CO2. Similarly, A measured at 400 and 1000 μmol⋅mol−1 was reduced for both pansy and petunia grown under the elevated compared with ambient CO2 concentration based on A-Ca responses after 7 days, indicating quick physiological acclimation to this input. These results provide information regarding the timing and extent of physiological acclimation in response to elevated CO2 concentrations. However, because of physiological acclimation potentially occurring within 7 days of treatment initiation, additional research is necessary to develop species-specific recommendations for controlled environment production.