Manipulating Light Quality, Light Intensity, and Carbon Dioxide Concentration to Optimize Indoor and Greenhouse Production of Annual Bedding Plant Seedlings
Annual bedding plant seedlings (plugs) are commonly produced in northern latitudes during the late winter and early spring when the natural daily light integral (DLI) in greenhouses is below recommended levels. Greenhouse supplemental lighting (SL) provides a means of increasing the DLI, with high-pressure sodium (HPS) lamps representing the current industry standard. However, low-profile and high-intensity light-emitting diode (LED) fixtures have recently emerged as a possible alternative for greenhouse SL. Additionally, due to the emission of very little radiant heat, LEDs may be used for sole-source lighting (SSL) applications where plants are produced on vertical shelving units in warehouses or shipping containers and in close proximity to the fixtures. Thus, with the development of LEDs for horticultural applications, the possibility of producing seedlings indoors using multi-layered, vertical production systems has become an increasingly realistic possibility. Therefore, the objectives of this research were to 1) compare HPS and LED SL sources in a commercial greenhouse for the propagation and finishing of annual bedding plant seedlings (Expt. 1); 2) evaluate the effects of various LED light qualities and intensities in a SSL environment on the morphology, nutrient uptake, and subsequent flowering of coreopsis (Coreopsis grandiflora ‘Sunfire’), pansy (Viola ×wittrockiana ‘MatrixTM Yellow’), and petunia (Petunia ×hybrida ‘Purple Wave’) seedlings (Expt. 2); and 3) determine the morphological and physiological responses of petunia ‘Dreams Midnight’ seedlings to the interactive effect of light intensity, light quality, and carbon dioxide (CO2) concentration under LED SSL (Expt. 3). In Expt. 1, both seedlings and finished plants produced under LED and HPS SL were comparable in quality, while seedlings produced under no SL were of significantly lower quality. In Expt. 2, light intensity was the dominant factor in determining seedling quality, with higher light intensities generally leading to seedlings that were more compact with greater dry mass accumulation. The inclusion of far-red wavelengths during propagation was also found to reduce the time to flower for pansy ‘MatrixTM Yellow’. In Expt. 3, petunia ‘Dreams Midnight’ seedlings grown under LED SSL with a red:blue light ratio (%) of 90:10 and light intensity of 300 µmol˙m –2˙s–1 had greater dry mass accumulation and leaf area (LA) than those under the light ratio of 50:50 at the same light intensity. However, seedlings produced under a light ratio of 50:50 and light intensity of 300 µmol˙m–2˙s–1 displayed the highest Rubisco efficiency (ϕ), photosynthesis at operating Ci concentration (AOP), electron transport rate (ETR), and maximum net photosynthetic rate (An,max). A trend of increased dry mass accumulation and decreased ϕ for seedlings produced at a CO2 concentration of 900 µmol˙mol-1 was also observed compared to 450 µmol˙mol–1. From results obtained in a commercial greenhouse, low-profile LEDs for greenhouse SL may be used as an alternative to traditional HPS lamps. However, the possibility of spectral manipulation in a greenhouse environment for desired growth responses appears to be limited when the relative contribution of SL from LEDs to DLI is low. For SSL production, while petunia ‘Dreams Midnight’ seedlings showed significantly higher ϕ, AOP, ETR, and A n,max under increased intensities of blue radiation, the increased LA observed under a higher percentage of red radiation ultimately led to increased light interception and greater dry mass accumulation. While the response is highly dependent on species and cultivar, the inclusion of far-red radiation under SSL may also be beneficial if accelerated flowering upon transplant is desired for plants with a long-day photoperiodic response. Additionally, while the CO2-enriched environment led to higher dry mass accumulation, acclimation responses, such as reduced ϕ, may limit potential gains from this input. The present research provides deeper insight into the morphological and physiological responses of bedding plant seedlings to light and CO 2 in controlled environments, and establishes a foundation for future research to investigate how to best optimize these inputs.
Mitchell, Purdue University.
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