Improving photosynthetic lighting efficiency in controlled environment agriculture:
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Photosynthetic lighting is often needed to produce high quality crops in controlled environment agriculture but can substantially increase production costs. To improve lighting efficiency and reduce costs, a better understanding of how crops use light for photosynthesis is needed. We used chlorophyll fluorescence and gas exchange measurements to quantify the photosynthetic light responses of three horticultural crops as affected by acclimation to different light intensities, and the effect of far-red light on photosynthesis. Quantum yield of PSII (ΦPSII), a common measure of photochemical efficiency, decreased with increasing light intensity, indicating that supplemental lighting is used less efficiently for photochemistry when provided at high ambient light intensity. Electron transport rate, which is often closely correlated with photosynthetic rate, increased asymptotically with increasing light. The high light-adapted species sweetpotato (Ipomea batatas) used high light more efficiently for electron transport than light-intermediate lettuce (Lactuca sativa) and shade-tolerant pothos (Epipremnum aureum). Plants acclimated to high light (full sun) also tended to have higher ΦPSII than those acclimated to low light (44% or 75% shade). Far-red light increased ΦPSII and net photosynthetic rate of lettuce when added to red/blue or warm-white LED light, which over-excites photosystem II (PSII). The addition of far-red light helps to balance the excitation between the two photosystems, thus increasing photosynthetic efficiency, by preferentially exciting photosystem I (PSI). This indicates that different wavelengths interactively affect photosynthetic efficiency, likely through affecting the excitation balance between PSI and PSII. We also determined which wavelengths of far-red light increase photochemical efficiency of lettuce. Longer wavelengths within the 678-703 nm range were increasingly used more efficiently by PSI than by PSII, as indicated by the increasing ΦPSII when light of longer wavelengths was added to red/blue light. The enhancement of ΦPSII tended to be smaller as wavelengths increased from 721 to 731 nm, probably due to lower leaf light absorption at longer wavelengths. Photons at 752 nm no longer increase ΦPSII, likely because they do not excite PSI. Additional measurements with narrow-band far-red wavelengths that are currently unavailable (especially 732-751 nm) will provide more information on the efficiency of different far-red wavelengths at enhancing photochemistry.