Chlorophyll Fluorescence of Three Species with Different Light Requirements: A Tool to Optimize Supplemental Lighting Efficiency?

Supplemental lighting is commonly used in greenhouse production to increase plant photosynthetic rate, thus increasing crop yield and shortening the production cycle.  As electrical consumption of lighting increases the production cost, more energy-efficient lighting sources, such as light-emitting diodes (LEDs), have increased in popularity.  Another way to improve energy use efficiency is to provide plants with the optimal level of photosynthetically active radiation (PAR) at which plants utilize PAR most efficiently to produce biomass. However, this approach has received little attention.  To understand how efficiently different species utilize PAR, mature plants of sweet potato (Ipomoea batatas ‘Desana Lime’, a high light plant), lettuce (Lactuca sativa ‘Green Ice’, a medium light plant), and pothos (Epipremnum aureum, a shade-tolerant plant) were exposed to gradually increasing PAR in a growth chamber equipped with dimmable LEDs.  Chlorophyll fluorescence analysis was used to estimate how the quantum efficiency of photosystem II (Φ_PSII), linear electron transport rate (ETR), and non-photochemical quenching (NPQ, i.e. dissipation of excessive PAR as heat) of the three species change when PAR gradually increases from 0 to 1000 μmol∙m^-2∙s^-1. Φ_PSII of all three species decreased with increasing PAR. In contrast, ETR, which is often strongly positively correlated with carbon fixation, increased with increasing PAR.  All three species had similar Φ_PSII and ETR under relatively low light (PAR < 200 μmol∙m^-2 ∙s^-1).  However, as PAR further increased, sweet potato consistently had higher Φ_PSII and ETR than pothos (intermediate) and lettuce (lowest) when exposed to the same light level, suggesting that sweet potato has a greater light use efficiency under higher light.  There was little increase in the ETR of pothos and lettuce above a PAR > 500 μmol∙m^-2∙s^-1, while ETR of sweet potato was still not light saturated at a PAR of 900 μmol∙m^-2∙s^-1.  NPQ of lettuce increased most rapidly with increasing PAR, indicating inefficient use of the absorbed PAR, with much of the absorbed light energy being converted into heat at higher PAR.  We hope to use chlorophyll fluorescence measurements, especially ETR, as a simple and reliable measure to develop a bio-feedback system which can automatically adjust the intensity of the LED lighting to the level at which plants most efficiently use the PAR to fix carbon.