Growth Responses of Ornamental Annual Seedlings Under Different Wavelengths of Red Light Provided by Light-emitting Diodes

Light-emitting diodes (LEDs) are of increasing interest in controlled environment plant production because of their increasing energy efficiency, long lifetime, and colors can be combined to elicit desirable plant responses. Red light (600-700 nm) is considered the most efficient wavelength for photosynthesis, but little research has compared growth responses under different wavelengths of red. We grew seedlings of impatiens (Impatiens walleriana), petunia (Petunia X hybrida), tomato (Solanum lyeopersicum), and marigold (Tagetes patula) or salvia (Salvia splendens) at 20 C under six sole-source LED lighting treatments. In the first experiment, a photosynthetic photon flux (PPF) of 160 mu mol.m(-2).s(-1) was provided for 18 h.d(-1) by 10% blue (B; peak = 446 nm) and 10% green (G; peak = 516 nm) lights, with the remaining percentages consisting of orange (0; peak = 596 nm)-red (R; peak = 634 nm)-hyper red (HR; peak = 664 nm) of 20-30-30, 0-80-0, 0-60-20, 0-40-40, 0-20-60, and 0-0-80, respectively. There were no consistent effects of lighting treatment across species on any of the growth characteristics measured including leaf area, plant height, or shoot fresh weight. In a second experiment, seedlings were grown under two light intensities (low, 125 mu mol.m(-2).s(-1) and high, 250 mu mol.m(-2).s(-1)) consisting of 10% B and 10% G light and the following percentages of RHR: 0-80, 40-40,80-0. Shoot fresh weight was similar in all light treatments, whereas shoot dry weight was often greater under the higher light intensity, especially under the 40-40 treatments. Leaf chlorophyll concentration under 40-40(low), 80-0(low), or both was often greater than that in plants under the high light treatments, indicating that plants acclimated to the lower light intensity to better use photons available for photosynthesis. We conclude that O, R, and HR light have generally similar effects on plant growth at the intensities tested when background G and B lights are provided and thus, selection of red LEDs for horticultural applications could be based on other factors such as economics and durability.