Determination of the Effect of Red and Blue Ratios of LED Light on Plant Photosynthesis

Light emitting diodes (LEDs) are an emerging versatile artificial light source offering many advantages over conventional artificial light sources. This could eventually allow determination of the ideal light emission spectrum for optimal plant growth, allowing for lighting system designs tailored to optimize plant growth while minimizing associated energy costs. A combination of blue and red wavebands can result in higher photosynthetic activity and better photomorphogenetic characteristics than red or blue alone. Red light is important in the development of the photosystem complex and morphogenesis through the mutation of phytochrome apparatus while blue has been identified as affecting chlorophyll concentrations, photomorphogenesis and stomatal openings. The objective of this research is to measure the photosynthetic response of lettuce and petunia to varying red and blue ratios with and without background broadband radiation. In this experiment, the photosynthetic response of lettuce (Lactuca sativa) and petunia (Petunia × hybrida) seedlings to varying 1:10, 1:5, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 20:1, 25:1, 30:1, 50:1, and 100:1 ratios of red (660 nm) and blue (435 nm) (r:b) was measured with and without background broadband HPS radiation. Irradiance and spectral distribution measurements were made using a spectroradiometer (PS-300, Apogee Instruments, Logan, Utah) with a converging lens and optical fiber aperture to minimize sampling error due to critical angle losses. The plants were tested at an LED irradiance level of 24.4 W m^-2 or approximately 115 μmol m^-2 sec^-1. Background broadband radiation was achieved with a standard HPS bulb (Lucalox, 400 W, GE, Fairfield, CT) which provided a consistent irradiance level of 24.4 W m^-2 for the test set-up. Photosynthetic measurements were obtained with a portable LICOR (Licor, Lincoln, NE) LI-6400XT portable photosynthesis system and exposing the seedlings to varying red to blue ratios. With and without background radiation, the optimum photosynthesis range occurred within the r:b range of 5:1- 15:1 except for petunia without background radiation for which the maximum occurred at 50:1. The addition of background radiation slightly altered the ratio at which peak photosynthesis rates occurred for lettuce. For every plant, with the addition of background radiation, the r:b resulted in maximum photosynthetic activity was consistently lower than the r:b without background radiation.