The addition of photosynthetically active radiation (PAR, 400–700 nm) with a specific quantity of far-red photons (FR, 700–750 nm) has been demonstrated to positively influence biomass accumulation and nutritional quality in greenhouse lettuce. However, current relevant studies seldom consider comprehensive and systematic comparisons of the efficacy of different approaches: substitution versus supplementation. The present work aimed to compare the two aforementioned strategies, evaluate how they impact plant growth, development and metabolic processes, and analyse the light use efficiency. In this study, loose-leaf lettuce (cv. ‘Dasusheng’) grown in a glass Venlo-type greenhouse was exposed to six supplementary light treatments, including white-red (WR) light-emitting diodes (LEDs), FR LEDs, and WR plus FR LEDs [WR130 + FR30 (the number was the photon flux density provided by WR or FR LEDs, respectively), WR130 + FR50, WR100 + FR30, and WR80 + FR50]. Lettuce that was grown only under natural light (NL) conditions was considered the control. According to the results of the present study, supplementary light increased biomass accumulation, and the contents of ascorbic acid, total soluble sugar, and starch relative to the control. Lettuce plants treated with WR130 + FR50 treatment presented the highest shoot and root fresh/dry weights, the highest total chlorophyll content, and the best nutritional quality, whereas the lettuce weight did not differ between the WR130 + FR30 and WR100 + FR30 treatments. Compared with that of NL, the stacking of thylakoids increased most intensely in response to the WR130 + FR50 and WR100 + FR30 treatments. Biomass accumulation, nutritional quality, stomatal area, chloroplast area, and expression of photosynthesis-related genes (LHCb, PsbA, rbcL, and rbcS) in lettuce plants, as well as light use efficiency, presented increasing-to-decreasing trends as the FR fraction increased. In conclusion, partially substituting PAR with FR photons coincidentally aligns with the supplementation of FR photons, and a supplementary FR fraction of 0.50 to 0.56 is suitable for greenhouse-grown lettuce under weak light conditions because of the increased photochemical efficiency, biomass accumulation, and carbohydrate content.

Insufficient light in autumn–winter may prolong the production periods and reduce the quality of plug seedlings grown in greenhouses. Additionally, there is no optimal protocol for supplementary light strategies when providing the same amount of light for plug seedling production. This study was conducted to determine the influences of combinations of supplementary light intensity and light duration with the same daily light integral (DLI) on the morphological and physiological properties of cucumber seedlings (Cucumis sativus L. cv. Tianjiao No. 5) grown in a greenhouse. A supplementary light with the same DLI of 6.0 mol m⁻² d⁻¹ was applied with the light duration set to 6, 8, 10, or 12 h d⁻¹ provided by light-emitting diodes (LEDs), and cucumber seedlings grown with sunlight only were set as the control. The results indicated that increasing DLI using supplementary light promoted the growth and development of cucumber seedlings over those grown without supplementary light; however, opposite trends were observed in the superoxide dismutase (SOD) and catalase (CAT) activities. Under equal DLI, increasing the supplementary light duration from 6 to 10 h d⁻¹ increased the root surface area (66.8%), shoot dry weight (24.0%), seedling quality index (237.0%), root activity (60.0%), and stem firmness (27.2%) of the cucumber seedlings. The specific leaf area of the cucumber seedlings decreased quadratically with an increase in supplementary light duration, and an opposite trend was exhibited for the stem diameter of the cucumber seedlings. In summary, increased DLI or longer light duration combined with lower light intensity with equal DLI provided by supplementary light in insufficient sunlight seasons improved the quality of the cucumber seedlings through the modification of the root architecture and stem firmness, increasing the mechanical strength of the cucumber seedlings for transplanting.

Optimizing light conditions for vegetable seedling production in a closed transplant production system is critical for plant growth and seedling production. Additionally, energy use efficiency should be considered by growers when managing the light environment. In the present study, cucumber seedlings (Cucumis sativus L. cv. Tianjiao No. 5) were grown under six different daily light integrals (DLIs) at 8.64, 11.52, 14.40, 17.28, 23.04, and 28.80 mol m⁻² d⁻¹ created by two levels of photosynthetic photon flux density (PPFD) of 200 and 400 μmol m⁻² s⁻¹ combined with photoperiod of 12, 16 and 20 h d⁻¹ provided by white light-emitting diodes (LEDs) in a closed transplant production system for 21 days. Results indicated that quadratic functions were observed between fresh and dry weights of cucumber seedlings and DLI at 6, 11, 16, and 21 days after sowing. Generally, higher DLI resulted in longer root length, bigger root volume and root surface area accompanied with shorter plant height and hypocotyl length; however, no significant differences were observed in root length, root volume, and root surface area as DLI increased from 14.40 to 28.80 mol m⁻² d⁻¹. Photon yield based on fresh and dry weights decreased with increasing DLI. In conclusion, increased DLI resulted in compact and vigorous morphology but reduced photon yield of cucumber seedlings produced in a closed transplant production system. In terms of plant growth and energy use efficiency, DLI at 14.40–23.04 mol m⁻² d⁻¹ was suggested for cucumber seedling production in the closed production system. Additionally, different control strategies should be applied at different growth stages of cucumber seedlings.

Adding far-red (FR, 700–800 nm) light to photosynthetic active radiation (400–700 nm) proved to be a possible approach to increasing plant biomass accumulation for lettuce production in indoor vertical farms with artificial lighting as a sole-source lighting. However, how FR light addition influences plant growth, development, and metabolic processes and the optimal value of FR photon flux density for greenhouse-grown lettuce under sunlight are still unclear. This work aims to quantify the value of supplementary FR light with different intensities on lettuce morphological and physiological characteristics in a greenhouse. Lettuce ‘Dasusheng’ (Lactuca sativa L.) was grown in a greenhouse under seven light treatments, including white plus red LEDs with FR photon flux density at 0, 10, 30, 50, 70, and 90 µmol m⁻² s⁻¹ (WR, WR + FR10, WR + FR30, WR + FR50, WR + FR70, and WR + FR90, respectively), and lettuce grown with sunlight only was marked as natural light (NL). FR light addition improved the electron transport flux per cross section and performance index (PI_abs, PI_total) and decreased the changes in relative variable fluorescence of lettuce leaves compared to plants under NL. Specifically, the PI_abs of lettuce leaves were 41%, 41%, 38%, 33%, 26%, and 25% lower under control than in plants under treatments WR + FR90, WR + FR70, WR + FR50, WR + FR30, WR + FR10, and WR, respectively. Leaf number, leaf area, and biomass accumulation of lettuce followed a quadratic function with increasing FR light intensity and were the highest under treatment WR + FR50. The shoot fresh weight and dry weight of lettuce were increased by 111% and 275%, respectively, under treatment WR + FR50 compared to NL. The contents of vitamin C, reducing sugar, total soluble sugar, and starch in lettuce showed a similar trend with biomass accumulation. In conclusion, with commonly used photosynthetic photon flux density (PPFD, 400–700 nm) around 200 μmol m⁻² s⁻¹, supplementary FR light intensity of 30~50 μmol m⁻² s⁻¹ was suggested to enhance the photochemistry efficiency, biomass accumulation, and carbohydrates’ contents in greenhouse-grown lettuce.

The relatively low light intensity during autumn–winter or early spring and inclement weather such as rain or fog may lead to extended production periods and decreased quality of greenhouse-grown tomato seedlings. To produce high-quality tomato seedlings rapidly, the influences of supplementary lights with different spectra on the morphological and physiological traits of tomato seedlings were measured in a greenhouse. Supplemental lighting with the same daily light integrals (DLI) of 3.6 mol m⁻²d⁻¹ was provided by white (W) light-emitting diodes (LEDs), white plus red (WR) LEDs, and red plus blue (RB) LEDs, respectively, and tomato seedlings grown under only sunlight irradiation were regarded as the control. Our results demonstrate that raised DLI by supplementary light improved the growth and development of greenhouse-grown tomato seedlings, regardless of the spectral composition. Under conditions with the equal DLI, the tomato seedlings grown under supplementary WR LEDs with a red to blue light ratio (R:B ratio) of 1.3 obtained the highest values of the shoot and root fresh weights, net photosynthetic rate, and total chlorophyll content. The best root growth and highest root activity of tomato seedlings were also found under the supplementary WR LEDs. Supplementary WR LEDs remarkably increased the stem firmness of the greenhouse-grown tomato seedlings, and increased the starch content in the leaves of greenhouse-grown tomato seedlings compared to the control. However, statistically significant differences did not occur in the sucrose, carotenoid contents, superoxide dismutase (SOD), and catalase (CAT) activities among the different supplemental lighting treatments. In conclusion, supplemental LED lighting could promote the growth and development of greenhouse-grown tomato seedlings grown under insufficient sunlight conditions. In addition, WR LEDs could obtain tomato seedlings with a higher net photosynthetic rate, higher root activity, and higher starch content compared with other treatments, which could be applied as supplementary lights in greenhouse-grown tomato seedlings grown in seasons with insufficient light.

Light is a crucial environmental signal and a form of photosynthetic energy for plant growth, development, and nutrient formation. To explore the effects of light quality on the growth and nutritional qualities of greenhouse-grown lettuce (Lactuca sativa L.), lettuce was cultivated under supplementary white (W) light-emitting diodes (LEDs); white plus ultraviolet A LEDs (W+UV); white plus far-red LEDs (W+FR); and the combination of white, far-red, and UV-A LEDs (W+FR+UV) for 25 days, with lettuce grown under natural sunlight used as the control. The results indicate that the leaf length and leaf width values for lettuce grown under the W+FR+UV treatment were significantly higher than those of lettuce grown under other supplementary light treatments. The highest values of shoot fresh weight, shoot dry weight, root fresh weight, and root dry weight were recorded under the W+FR treatment (4.0, 6.0, 8.0, and 12.4 times higher than those under the control treatment, respectively). Lettuce grown under the W+FR treatment exhibited the highest total chlorophyll content (39.1%, 24.6%, and 16.2% higher than that under the W, W+UV, and W+FR+UV treatments, respectively). The carotenoid content of lettuce grown under the W+FR treatment was the highest among all treatments. However, the root activity of greenhouse-grown lettuce was the highest under the W+FR+UV treatment. Soluble sugar content, cellulose content, and starch content in the lettuce responded differently to the light treatments and were highest under the W+UV treatment. In summary, supplementary light promoted growth and nutrient accumulation in lettuce. Specifically, white plus far-red light promoted lettuce growth, and white plus UV increased some specific compounds in greenhouse-grown lettuce. Our findings provide valuable references for the application of light-supplementation strategies to greenhouse lettuce production.