Wednesday, August 10, 2016: 9:00 AM
Capitol Center Room (Sheraton Hotel Atlanta)
Light emitting diodes (LEDs) are increasingly used for supplemental lighting to increase photosynthesis or as the sole lighting source for indoor production of high value crops. To achieve maximum photosynthetic efficiency, light needs to provide a balanced excitation of photosystems I and II, which have different absorption and action spectra. Using lettuce (Lactuca sativa) as a model system, we found that far-red light (peak at 735 nm) has a synergistic effect on the light reactions of photosynthesis, when combined with red and blue (peaks at 453 and 638 nm) or warm white (peaks at 453and 599 nm) LED light with a photosynthetic photon flux density (PPFD) of 50 to 750 μmol·m-2·s-1. The quantum efficiency of photosystem II (ΦPSII) increased within 10 - 15 s after adding far-red light, with an average increase of 6 - 7% under red/blue and 3 - 4% under warm white LEDs. Similar or greater increases in ΦPSII were observed 20 min after adding far-red light. This longer-term effect of far-red light on ΦPSII was accompanied by a reduction in non-photochemical quenching of chlorophyll fluorescence, indicating that far-red light reduced the dissipation of absorbed light as heat. Net photosynthetic rate (Pn) was also enhanced by far-red light: For each 1% increase in PPFD provided by the far-red light, Pn increased by an average of 4% and 3% under the red/blue and warm white light, respectively. Under shorter wavelength light (< 680 nm), photosystem I (PSI) tends to be under-excited relative to PSII. As light with wavelengths > 680 nm largely excites PSI, but not PSII, the addition of far-red light may enhance photosynthesis by balancing the excitation energy between the two photosystems and ensure that they operate at matching rates. Our findings suggest that adding far-red light to lighting sources containing little or no far-red can improve the efficiency of photosynthetic lighting over a wide range of light intensities.