Strategic Light Use Efficiency Optimization of Hydroponic Lettuce Exposed to Different Photosynthetic Photon Flux Densities

Light use efficiency characterizes the ability of a crop to convert radiation into biomass. Determining optimum cultivar-specific photosynthetic photon flux density (PPFD) values from sole-source lighting can be used to optimize leaf expansion, maximize biomass, and shorten the production period. This study evaluated the growth of hydroponic lettuce (Lactuca sativa) ‘Rex’ cultivated under different PPFD levels using sole-source lighting. At lower PPFD levels of 201 to 292 µmol·m−2·s−1, the plant projected canopy size (PCS) and specific leaf area increased to enhance light capture by 36.2% as compared to higher PPFD levels (333 and 413 µmol·m−2·s−1), while plants exhibited 10.3% lower canopy overlap ratio and 27.8% lower shoot dry weights. Both low and high PPFD conditions lead to a similar trend in PCS among plants. Light use efficiency was not a major factor in influencing lettuce growth. Instead, the critical factor was the total incident light the plants received. This study showcased the importance of incident light and PPFD on the growth, morphology, and biomass accumulation in lettuce.