Development of Photosynthetic Response Curves and Their Integration into a Decision-support Tool for Floriculture Growers

Irradiance, CO₂, and temperature are critical inputs for photosynthesis and crop growth.  They are also environmental parameters which growers can control in protected horticulture production systems.  We evaluated the photosynthetic response of 13 herbaceous ornamentals (Begonia × hiemalis, Begonia semperflorens, Catharanthus roseus, Cyclamen persicum, Euphorbia pulcherrima, Gerbera jamesonii, Impatiens hawkerii, Impatiens wallerana, Osteospermum × hybrida, Pelargonium x hortorum, Petunia × hybrida, Verbena × hybrida, and Viola × wittrockiana) to irradiance, temperature, and CO₂.  Initial growth chamber conditions were 20 °C air temperature, 300 µmol∙m^-2∙s^-1 irradiance, and a 14 h photoperiod.  Single leaf photosynthetic measurements were measured on one fully mature, non-shaded leaf per plant (5 to 7 plants per species).  Cuvette set points matched chamber conditions, except for the variable of interest.  Photosynthetic irradiance and CO₂ response curves were fit to the data using a non-rectangular hyperbola, and temperature response curves were fit using an asymmetric (Landsberg) function.  Light compensation points varied between 24 and 52 µmol∙m^-2∙s^-1, light saturation values ranged from 156 to 597 µmol∙m^-2∙s^-1, and apparent quantum efficiency varied between 0.033 and 0.087.  Maximum rates of net photosynthesis (A_max) at saturating irradiance varied between 5.0 and 10.5 µmol CO₂∙m^-2∙s^-1, and they were 5% to 90% greater than net photosynthetic rates (A_n) at ambient irradiance.  At saturating CO₂ concentrations, A_max ranged between 8.2 and 17.2 µmol CO₂∙m^-2∙s^-1.  CO₂ compensation points ranged from 6 to 146 µmol CO₂∙mol^-1 air, and CO₂ saturation values ranged from 538 to 2219 µmol∙mol^-1.  Rates of A_n were 31% to 153% greater at twice ambient CO₂ concentrations.  The optimal temperature (T_opt), corresponding to A_max, ranged from 11.1 °C to 30.8 °C.  Simulation software has been developed for these species, providing growers and greenhouse managers with a decision-support tool that allows them to estimate the impacts altering irradiance, temperature, or CO₂ will have on photosynthetic rate.