Comparing Growth Responses of Bedding Plant Seedlings under Different Red Wavelengths from Light-emitting Diodes

Light-emitting diodes (LEDs) are of increasing interest in controlled environment plant production due to their increasing energy efficiency and narrow bandwidth capability.  Red light is generally the most efficient at eliciting photosynthesis, but little research has been published comparing growth responses under different wavelengths of red, especially on ornamental plants.  We grew four popular bedding plant seedlings under six sole-source LED lighting treatments that each delivered a PPF of 160 μmol·m^-2·s^-1 with an 18-h photoperiod.  All treatments delivered 10% blue (peak = 446 nm) and 10% green (peak = 516 nm) light, with the remaining light quality percentages consisting of orange (peak = 596 nm) – red (peak = 634 nm) – hyper red (peak = 664 nm) of 20–30–30, 0–80–0, 0–60–20, 0–40–40, 0–20–60, and 0–0–80, respectively. Seedlings of impatiens (Impatiens walleriana ‘SuperElfin XP Red’), marigold (Tagetes patula ‘Deep Orange’), petunia (Petunia ×hybrida ‘Wave Pink’), and tomato (Lycopersicum esculentum ‘Early Girl’) were grown after germination for 31 to 45 d at a constant 20 °C.  Growth parameters measured included stem length, shoot fresh and dry weight, leaf area, and leaf number.  Although some statistical significance was observed, there were no prevailing growth response trends among light quality treatments. Therefore, we conclude that varied ratios of orange, red, and hyper red light have similar effects on plant growth when background levels of green and blue light are provided.  The choice of red LEDs used for plant growth can depend on other factors, such as cost, electrical efficiency, and longevity.