Daily Light Integral and Light Quality from Sole-source Light-emitting Diodes Affect Seedling Quality and Subsequent Flowering of Long-day Bedding Plant Species

Light-emitting diodes (LEDs) have become an increasingly popular alternative to traditional lighting sources due to their energy efficiency, low output of radiant heat, and ability to target specific wavelengths of light. Previous research has shown that annual bedding plant seedlings can be produced using sole-source (SS) lighting provided by LEDs. However, a delay in flowering for some long-day (LD) species during finishing has been observed when LED SS lighting provided only red and blue wavelengths of light. Thus, the objectives of this research were to: 1) further evaluate the effects of various light qualities and quantities on young bedding plant production; and 2) determine whether far-red wavelengths included in SS light quality would promote earlier flowering of LD plants at finish. Seeds of coreopsis (Coreopsis grandiflora 'Sunfire'), pansy (Viola ×wittrockiana 'Matrix^TM Yellow'), and petunia (Petunia ×hybrida 'Purple Wave') were sown in 288-trays and placed on multi-layer shelves in a walk-in growth chamber. A daily light integral (DLI) of 6, 12, or 18 mol·m^–2·d^–1 was achieved from SS LED arrays under a 16-h photoperiod with light ratios (%) of red:green:blue 74:18:8 (R₇₄:G₁₈:B₈), red:blue 87:13 (R₈₇:B₁₃), or red:far-red:blue 84:7:9 (R₈₄:FR₇:B₉). Seedlings were evaluated for transplant quality two, three, and four weeks after treatment initiation. Additionally, seedlings were transplanted and grown under a target DLI of 10 to 12 mol·m^–2·d^–1 in the greenhouse. Regardless of light quality, as the DLI increased from 6 to 12 mol·m^–2·d^–1, stem caliper, shoot, and root dry mass of seedlings increased for all three species. Similarly, stem length of pansy and petunia seedlings decreased as the DLI increased. Pansy seedlings transplanted after three weeks under a DLI of 18 mol·m^–2·d^–1 from LEDs providing R₈₄:FR₇:B₉ flowered an average of 3 and 5 days earlier than those from the R₈₇:B₁₃ and R₇₄:G₁₈:B₈ treatments, respectively; while those seedlings under R₈₄:FR₇:B₉ for 4 weeks flowered an average of 13 and 8 days earlier. These results provide information regarding the specific light parameters from commercially available LEDs necessary to optimally produce high-quality seedlings under SS lighting. Furthermore, the addition of far-red wavelengths could significantly reduce time to flower after transplant and allow for a faster greenhouse turnover of some LD crops.