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Abstract
Crop growth is strongly affected by the amount of light captured by the canopy and light use efficiency (LUE). Increased leaf expansion and cultivar-specified photosynthetic photon flux densities (PPFD) rates from supplemental lighting can maximize biomass production and shorten the production period, essential in decreasing electricity costs in controlled environmental agriculture (CEA). Understanding the effects PPFD has on plant morphology, we conducted a study on lettuce growth under sole-source lighting, examined its impact on six different cultivars with varying leaf anthocyanin content, and investigated how increased potassium (K) concentrations influenced growth with rising daily light integrals (DLI). In the first study, changes in lettuce physiology and morphology were examined throughout a cropping cycle under various PPFDs using sole-source LED lighting. The results indicated that as PPFD increased, there was a corresponding rise in total incident light, leading to an increase in shoot dry weight under higher PPFDs. In the second study, focusing on changes in plant physiology and morphology during the cropping cycle, supplemental PPFD was applied to six lettuce cultivars, each with varying leaf anthocyanin content. Our findings suggest that a higher leaf anthocyanin content may contribute to an increase in shoot dry weight in red cultivars with rising PPFD, although this effect was prominent only in the ‘Cherokee’ cultivar, which exhibited the highest shoot dry weight among all six cultivars, including our sole green cultivar, ‘Rex’. In the final study, the use of K as a supplemental nutrient to increase lettuce growth was determined to not be an effective way to increase growth in hydroponic lettuce. Together, these studies offer further insight into how light influences plant morphology, cultivar-specific responses, and the finding that the addition of excess potassium (K) does not impact growth.