2017 ASHS Annual Conference

Indoor production of bedding plant seedlings (plugs) using sole-source lighting (SSL) may present value in increasing plug uniformity and consistency compared to greenhouse production. However, there is currently limited information on physiological responses of seedlings to varying light intensities, light qualities, and CO₂ concentrations under SSL. Seeds of petunia (Petunia ×hybrida ‘Dreams Midnight’) were sown in 128-cell plug trays and placed on multi-layer shelves in walk-in growth chambers. Lighting treatments were established using light-emitting diode (LED) arrays providing red:blue light ratios (%) of 50:50 or 90:10 and light intensities of 150 or 300 µmol·m^–2·s^–1. Carbon dioxide (CO₂) treatments were conducted using two growth chambers with set points of 450 or 900 µmol·mol^-1. Morphological measurements such as extension growth, leaf area (LA), and dry weight (DW) were measured weekly. Additionally, photosynthesis (A) response to increasing light (A-PPF) and leaf internal CO₂ concentration (A-Ci) was measured using a portable leaf photosynthesis system. Results indicated no differences in seedling growth between the two CO₂ treatments. Regardless of CO₂ concentration, seedlings grown under a light intensity of 300 µmol·m^–2·s^–1 possessed a higher total DW (TDW) and lower specific leaf area (SLA; LA/TDW) compared to those grown at an intensity of 150 µmol·m^–2·s^–1. This increased SLA in response to lower incident light was expected, as plants will acclimate to increase light interception under these conditions by developing thinner and wider leaves. Seedlings grown under the light ratio of 90:10 and light intensity of 300 µmol·m^–2·s^–1 produced greater TDW and LA than those grown under the light ratio of 50:50 at the same light intensity. However, A-Ci and A-PPF responses of seedlings grown under the light ratio of 50:50 displayed the highest Rubisco efficiency, photosynthesis at operating C_i concentration (A_OP), electron transport rate (ETR), and light saturation point (LSP). Even though photosynthesis per unit area was highest for seedlings produced under the light ratio of 50:50, the increase in leaf area observed under the light ratio of 90:10 ultimately led to greater biomass accumulation at a light intensity of 300 µmol·m^–2·s^–1. While biomass was greatest under an increased percentage of red light, desirable qualities in a bedding plant seedling also include compactness and stress tolerance during transplant. Seedlings grown under the light ratio of 50:50 were generally more compact (reduced LA and extension growth) and exhibited a higher LSP, indicating potential increases in these desirable qualities.