2017 ASHS Annual Conference

Improving photosynthetic efficiency, which is dependent on the light spectrum, is important in reducing lighting costs in controlled environment agriculture. Photosynthetic efficiency decreases upon unequal excitation of photosystem I (PSI) and photosystem II (PSII), which operate in series to carry out the light reactions of photosynthesis. Shorter wavelengths, e.g., red and blue light, tend to under-excite PSI relative to PSII. Using lettuce (Lactuca sativa) as a model system, we previously found that adding far-red light (peak at 735 nm) to red/blue or white LED light, the commonly used grow lights in controlled environments, enables plants to use the provided light more efficiently, enhancing photochemistry and photosynthesis. The addition of longer wavelength far-red light helps to balance the excitation of the two photosystems by preferentially exciting PSI. The spectral range of far-red light that enhances photochemistry remains unclear. Our objective was to determine which wavelengths of far-red light increase photosynthetic efficiency of lettuce. Lettuce was grown under red/blue LED light and the quantum yield of PSII (Φ_PSII, electrons moved per absorbed photon) was measured under this red/blue light with wavelengths of far-red added. Far-red light was provided using narrow-band (2 nm peak width) laser diodes with wavelengths from 678 to 752 nm. Adding wavelengths within 678-684 nm decreased the Φ_PSII, indicating that a smaller fraction of absorbed light was used for photochemistry, a typical response when the light intensity is increased. Wavelengths within the 686-731 nm region increased the Φ_PSII of lettuce, indicating that these wavelengths excited PSI more efficiently than PSII, restoring the excitation balance between the two photosystems. More efficient excitation of PSI increases photosynthetic efficiency by accelerating the re-oxidation of the plastoquinone pool, the intermediate electron transporter between PSII and PSI. This in turn leads to faster re-opening of PSII and an increase in Φ_PSII. There were no measurements made within the 732-751 nm range, due to unavailability of laser diodes within this wavelength range. Wavelengths greater than 752 nm did not affect Φ_PSII, likely due to the low leaf absorption at these wavelengths. It is also possible that photons with wavelengths > 752 nm do not contain enough energy to drive the photochemical reactions in PSI. Lettuce plants grown under sunlight and subsequently measured under red/blue LED light with added far-red showed similar responses.