Characterization of Photosynthetic Responses of Thirteen Herbaceous Ornamentals to Irradiance and CO2

The impact of irradiance (25 to 800 μmol·m^-2·s^-1) and CO₂ (100 to 1000 ppm) on photosynthesis of thirteen herbaceous ornamentals (Begonia x hiemalis Fotsch, B. semperflorens Hook. ‘Cocktail Vodka’, Catharanthus roseus G. Don. ‘Sunstorm Red’, Cyclamen persicum Mill., Euphorbia pulcherrima Wild. Ed Klotzsch ‘Prestige Early Red’, Gerbera jamesonii Adlam ‘Deep Royal Eye Orange’, Impatiens hawkeri Bull ‘Infinity Blushing Lilac’, Impatiens wallerana Hook. f. ‘Accent Orange’, Pelargonium x hortorum Bailey ‘Daredevil Salmon’, Petunia x hybrida Hort. ex E. Vilm. ‘Glow Forest Fire’, Osteospermum x hybrida L. ‘Soprano Purple’, Verbena x hybrida Hort. ex Vilm. ‘Superbena Burgundy’, and Viola x wittrockiana Gams.‘Delta Premium Pure White’) was characterized.  A single-leaf photosynthetic measurement was collected from each of five to seven plants per species at each level of the factor of interest.  Cuvette constants were 20 °C, 300 μmol·m^-2·s^-1 irradiance, 370 ppm CO₂, and 70% relative humidity; irradiance or CO₂ was varied while other factors were held constant.  Maximum net photosynthetic rates in response to irradiance varied from 2.1 μmol CO₂·m^-2·s^-1 (B. x hiemalis) to 6.7 and 7.1 μmol CO₂·m^-2·s^-1 (I. wallerana and P. x hybrida, respectively).  Light saturation levels varied from 200 μmol·m^-2·s^-1 in B. x hiemalis to 600 μmol·m^-2·s^-1 in C. roseus.  Maximum net photosynthetic rates in response to increasing CO₂ ranged from 7.4 μmol CO₂·m^-2·s^-1 in B. x hiemalis to 13.0 μmol CO₂·m^-2·s^-1 in I. wallerana.  CO₂ saturation levels varied from approximately 600 ppm in B. semperflorens, P. x hybrida, and V. x hybrida to approximately 1000 ppm in B. x hiemalis, I. wallerana, and O. x hybrida.  Based on this data, optimal irradiance and CO₂ levels to maximize photosynthesis differ with species.