Sole-source light-emitting diode (LED) lighting is indispensable in controlled environments without sunlight, including growth chambers, indoor vertical farms, and spaceflight conditions. Crops like lettuce (Lactuca sativa) are typically grown under fixed light over the entire growing cycle. There is potential to improve light use efficiency through dynamic lighting using dimmable LEDs, but a knowledge gap exists in plant responses to changing light intensities. In an indoor experiment, we characterized how temporal light intensity alternations influenced growth, morphology, and pigmentation of red-leaf lettuce ‘Rouxai’. From day 0 to 28, we germinated and grew lettuce hydroponically at 21-23 °C air temperature and 18%-27% relative humidity under six lighting treatments, including two fixed photosynthetic photon flux densities (PPFDs) of 150 and 350 µmol∙m–2∙s–1 and four temporal PPFD alternations with increasing PPFDs (150→250→250, 150→350→350, 250→250→350, and 250→350→350 µmol∙m–2∙s–1) over three lettuce growth phases [lag phase (days 0-11)→exponential phase (days 11-25)→finish phase (days 25-28)]. All treatments had the same light spectrum (50% warm white 50% red) and 24-hour photoperiod. We collected seedling data and transplanted additional seedlings into Kratky-style hydroponic units on day 11 and collected mature plant data on day 28. Increasing the fixed PPFD from 150 to 350 µmol∙m–2∙s–1 increased seedling shoot fresh and dry mass by 69% and 84%, respectively, leaf number from 4 to 5, leaf width by 22%, chlorophyll concentration index by 15%, and red coloration, while decreasing leaf length by 11%. Similarly, for mature plants, increasing the fixed PPFD from 150 to 350 µmol∙m–2∙s–1 increased shoot fresh and dry mass by 66% and 70%, respectively, leaf number by 23%, leaf width by 11%, and chlorophyll concentration index by 37%, while decreasing light use efficiency (biomass per unit light) by 27%-29%. Compared to the fixed 350 treatment, the 250→250→350 and 250→350→350 alternations resulted in similar biomass, morphology, and pigmentation. However, the 250→250→350 alternation had 23%-31% higher light use efficiency than the fixed 350 treatment. With all treatments taken together, increasing the total light integral from 363 to 847 mol∙m–2 increased shoot fresh and dry mass but decreased light use efficiency. In conclusion, a temporal light intensity alternation produces comparably high biomass in lettuce more efficiently than fixed high light.
