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Frontiers in Horticulture ·

Volume 5 - 2026 | https://doi.org/10.3389/fhort.2026.1971896

Preharvest exposure to far-red and UVA light enhances growth and pigment content of oakleaf lettuce in an indoor vertical farming system

  • Most Tahera Naznin1*corresponding author
  • Md Obyedul Kalam Azad1
  1. Department of Agriculture, Veterinary and Rangeland Sciences, University of Nevada, Reno, (UNR), UNR Extension and Agricultural Experimental Station, Las Vegas, NV, United States
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Abstract

This study investigated how preharvest supplemental far-red (FR) and UVA light under white LED light influences growth, photochemical performance, phenolic content, and antioxidant activity in leaf lettuce grown in a vertical plant factory. Lettuce plants were cultivated under a white LED background light with a photosynthetic photon flux density (PPFD) of 100 µmol m −2 s −1. During the final two weeks before harvest, plants received supplemental FR at 10 µmol m −2 s −1 or UVA at 5 µmol m −2 s −1, while white-light plants served as the control. Throughout the cultivation period, plants were maintained under a 16-h light/8-h dark photoperiod, 35–40% relative humidity, and a CO 2 concentration of 350–380 ppm in the plant factory. Research revealed that preharvest FR significantly enhanced plant height, leaf area, fresh weight, and dry weight by 19.4%, 22.7%, 13.1%, and 39.5%, respectively, compared with white light alone. FR improved photochemical performance, resulting in higher F v / F m, performance index (PI), and stomatal conductance. Furthermore, FR increased the contents of chlorophyll a, chlorophyll b, carotenoids, by 39.3%, 50.0%, 50.0%, respectively, and UVA light increased anthocyanin by 84.6%, compared to white light. In contrast, UVA treatment was more effective at enhancing nutritional quality, increasing anthocyanin content by 146.2%, total phenolic content by 34.5%, total flavonoids by 25.4%, amino acid content by 106.2%, DPPH radical-scavenging activity by 7.7%, and FRAP antioxidant capacity by 16.7% compared with white light. However, UVA also enhanced antinutrient nitrate content by 64.0% compared to white. This study shows that short-term preharvest spectral supplementation can be strategically applied to improve either biomass production via FR or nutritional quality via UVA, without increasing baseline light intensity; however, further investigation is needed to reduce nitrate levels.

Keywords

Introduction

Lettuce (Lactuca sativa L. Red Oak Leaf) is one of the most widely cultivated leafy vegetables worldwide and is highly valued for its nutritional quality, fresh consumption, and economic importance (Kim et al., 2016). Among leaf lettuce types, oakleaf lettuce has gained popularity for its attractive leaf morphology, tender texture, pleasant flavor, and high concentration of health-promoting phytochemicals (Rouphael et al., 2018; Shi et al., 2022). In recent years, consumer demand for premium-quality leafy vegetables with enhanced nutritional value has increased substantially, encouraging growers and researchers to optimize cultivation strategies under controlled-environment agriculture (CEA) systems (Kozai et al., 2019; Pennisi et al., 2020).

CEA enables year-round production of leafy vegetables with enhanced yield and quality. Among environmental factors, light quality plays a pivotal role, serving not only as the primary energy source for photosynthesis but also as a key regulator of plant growth, morphology, and secondary metabolite accumulation (Bian et al., 2015; Paradiso and Proietti, 2022).

Preharvest dynamic lighting strategies have emerged as an effective and energy-efficient approach to improve crop quality without substantially increasing production costs. Short-term exposure to specific wavelengths can enhance the accumulation of plant phytochemicals while maintaining acceptable growth. In leafy vegetables such as lettuce, preharvest spectral manipulation can improve growth quality, pigment concentration, antioxidant capacity, and nutritional value.

White light provides a broad spectrum that supports photosynthesis and balanced plant development across the photosynthetically active radiation range (Paradiso and Proietti, 2022). Beyond serving as the primary growth spectrum, supplemental far-red (FR) and ultraviolet A (UVA) light can induce distinct physiological and biochemical responses. FR light (700–750 nm), perceived mainly by phytochromes, regulates shade-avoidance responses, carbon partitioning, and leaf expansion, whereas UVA (315–400 nm) radiation is detected by cryptochromes and other UV-responsive photoreceptors, promoting the synthesis of secondary metabolites and antioxidant compounds (Galvão and Fankhauser, 2015; Rai et al., 2021).

FR light promotes leaf expansion, stem elongation, canopy development, and biomass accumulation through phytochrome-regulated responses (Zhen and Bugbee, 2020; Meng et al., 2024). In leafy vegetables such as lettuce, FR supplementation has been shown to increase fresh weight, leaf area, and light interception efficiency, thereby enhancing productivity under low- to moderate-light conditions (Bi et al., 2024; Dou et al., 2024). FR light in particular instance can also improve photosynthetic performance by enhancing light penetration, thereby increasing chlorophyll content, photochemical efficiency, and stomatal conductance (Zhen and Bugbee, 2020; Yan et al., 2023). However, light supplementation may also increase antinutrient nitrate accumulation by altering nitrogen metabolism, potentially negatively affecting nutritional quality (Yan et al., 2023).

UVA represents another important spectral component that can influence plant physiology and metabolism. UVA light stimulates the production of secondary metabolites, including phenolics, flavonoids, anthocyanins, and other antioxidant compounds (Rai et al., 2020; Verdaguer et al., 2017). These metabolites contribute to plant stress tolerance and provide health benefits for humans due to their antioxidant and anti-inflammatory properties (Li and Kubota, 2009). Several studies have demonstrated that UVA supplementation can enhance antioxidant activity and phytochemical accumulation in leafy greens (Brazaitytė et al., 2015; Zhang et al., 2024). Increased anthocyanin and flavonoid biosynthesis under UVA exposure is often linked to activation of phenylpropanoid metabolism and photoprotective mechanisms (Rai et al., 2020; Verdaguer et al., 2017). UVA also improves nutritional quality by increasing total phenolic content and antioxidant capacity (Zhang et al., 2024). However, UVA effects on plant growth are often inconsistent and depend on species, exposure intensity and duration, and interactions with other spectral components (Zandi et al., 2022).

White light was selected as the primary growth spectrum because it provides a broad and balanced distribution of wavelengths that supports photosynthesis, plant development, and crop production without requiring complex multispectral lighting systems (Pattison et al., 2018). In commercial CEA, white LEDs are increasingly adopted for their high energy efficiency, ease of installation, and ability to provide uniform illumination for crop growth (Pattison et al., 2018; Kozai et al., 2019). Furthermore, maintaining a relatively low light intensity (≈100 µmol m−2 s−1) can substantially reduce electricity consumption, which is among the highest operational costs in indoor farming and plant factory systems (Kozai et al., 2019; Graamans et al., 2018). Rather than increasing energy input by raising light intensity, this study investigated whether short-term preharvest supplementation with FR or UVA light could enhance crop quality and phytochemical accumulation while maintaining a low baseline light level. This strategy offers a potentially cost-effective approach to improving nutritional quality and antioxidant properties with minimal additional energy expenditure compared with continuously operating high-intensity lighting systems (Azad et al., 2024).

Although the individual effects of FR and UVA light on plant growth and phytochemical accumulation have been widely reported, information regarding their preharvest application in oakleaf lettuce under controlled-environment conditions remains limited. Therefore, this study evaluated the effects of preharvest supplemental FR and UVA lighting on the growth, physiological responses, pigment composition, antioxidant activity, and nutritional quality of oakleaf lettuce. The findings aim to improve the understanding of lettuce responses to preharvest spectral manipulation and identify effective lighting strategies to enhance crop quality in controlled-environment production systems.

Materials and methods

Plant materials and growing conditions

Oakleaf lettuce (Lactuca sativa var. oakleaf) seeds (Johnny’s Selected Seeds, USA) were germinated in moistened 1.2-inch rockwool cubes (Grodan, Roermond, Netherlands). Three weeks after sowing, uniform seedlings were transplanted into a three-tier growing rack (called radix) equipped with an NFT hydroponic system and supplied with a commercial nutrient solution (Humboldt’s Secret Supplies, California, USA) at 1.6 mL L−1, following the manufacturer’s recommendations. The nutrient solution was maintained at an electrical conductivity (EC) of 1.5–2.0 dS m−1 and a pH of 6.0–6.5, adjusted as needed using a phosphoric acid-based pH-down solution (General Hydroponics, Rosa, CA, USA). The solution was continuously aerated and replaced every two weeks to maintain nutrient balance and prevent salt accumulation. The experiment was conducted twice as independent biological replicates.

Light treatments

Plants were cultivated under cool white LED lighting (correlated color temperature, CCT 6500 K) for two weeks, followed by a two-week preharvest lighting treatment consisting of supplemental far-red (FR, 730 nm) or UVA (320 nm) (LED Panel Grow Light, China) (Figure 1). White LED lighting provided a photosynthetic photon flux density (PPFD) of 100 ± 10 µmol m−2 s−1.

Schematic illustration of the experimental setup and treatment arrangement.
Figure 1. Schematic illustration of the experimental setup and treatment arrangement.

During the preharvest treatment period, plants received supplemental FR and UVA radiation at photon flux densities of 10 and 5 µmol m−2 s−1, respectively. The experiment was conducted in an indoor plant factory maintained at 23 ± 2 °C, 35–40% relative humidity, and a 16 h light/8 h dark photoperiod and CO2 at 350–380 ppm. Photosynthetic photon flux density (PPFD) was measured at the canopy level using a calibrated quantum sensor (LI-250A and LI-180 Spectrometer, LI-COR Biosciences, Lincoln, NE, USA).

Analysis of maximum quantum yield and stomatal conductance

The photosynthetic performance was evaluated at 44 days after transplanting, corresponding to two weeks after the onset of the preharvest FR and UVA treatments. Maximum quantum yield of photosystem II (Fv/Fm) and performance index (PI) were measured using a portable chlorophyll fluorometer (Pocket PEA, Hansatech Instruments, King’s Lynn, UK) after leaves had been dark-adapted for 30 min (Maxwell and Johnson, 2020). Stomatal conductance (gs) was determined with a leaf porometer (SC-1 Leaf Porometer, METER Group Inc., Pullman, WA, USA). We used twelve randomly selected plants from each treatment/replication for all physiological measurements.

Plant morphological measurements

At 44 days after transplanting, corresponding to two weeks after initiation of the preharvest FR and UV-A treatments, we harvested twelve plants from each treatment (24 plants in total) for morphological and biochemical analyses. The recorded morphological parameters included plant height (cm), measured from the base to the shoot apex using a ruler; leaf number, determined by counting fully expanded leaves per plant; leaf area (cm2), measured with a leaf area meter (LI-3100, LI-COR, USA); root length (cm), measured using a ruler; and fresh weight (FW) and dry weight (DW), determined using an electronic balance. We measured dry weight after oven-drying the samples at 55 °C for 36 h until a constant weight was achieved.

Plant polyphenol and flavonoid analysis

Sample preparation

For the determination of total phenolic content (TPC), total flavonoid content (TFC), total free amino acids, nitrate concentration (antinutrient), and antioxidant activities (DPPH and FRAP), twelve plants were collected from each replicate. Plant samples were oven-dried at 60C for 16 hr, ground into a fine powder, and stored in airtight ziplock bags until analysis. For extraction, 0.5 g of the dried powder was mixed with 30 mL of 80% methanol in a 50 mL Falcon tube. The mixture was extracted by continuous shaking at 300 rpm for 12–16 h using an orbital shaker (Thermo Fisher Scientific, Bohemia, NY, USA). The extracts were then filtered through Whatman No. 1 filter paper (0.20 mm, Whatman 5B, Cytiva, Marlborough, MA, USA), and the filtrates were stored at 4 °C before biochemical analyses.

Estimation of total phenolic content

TPC was determined using the Folin–Ciocalteu colorimetric method as previously described (Singleton and Rossi, 1965). Briefly, 1 mL of the methanolic extract was combined with 0.2 mL of 1 N Folin–Ciocalteu reagent, then 1.8 mL of deionized water was added. After vortexing, the mixture was allowed to react for 3 min, then 0.4 mL of 10% (v/v) sodium carbonate (Na2CO3) solution was added, and the final volume was adjusted to 4 mL with 0.6 mL of deionized water. The reaction mixture was incubated at room temperature for 1 h, then the absorbance at 725 nm was measured using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan). Total phenolic content was quantified from a gallic acid standard curve and expressed as milligrams of gallic acid equivalents (mg GAE g−1 DW).

Determination of total flavonoid content

TFC was determined according to the method described by Ghimeray et al. (2014) with slight modifications. Briefly, 0.5 mL of the sample extract was mixed with 0.1 mL of 10% aluminum nitrate and 0.1 mL of 1 M potassium acetate. The reaction mixture was then brought to a final volume of 4 mL by adding 3.3 mL of 80% methanol, vortexed thoroughly, and incubated at room temperature for 40 min. The absorbance was measured at 415 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan). Total flavonoid content was calculated using a coumarin standard curve and expressed as milligrams of coumarin equivalents per gram of dry weight (mg CE g−1 DW).

Amino acid analysis

Total free amino acid content was determined using the ninhydrin–citrate buffer method described by Moore and Stein (1954). A citrate buffer (pH 5.5) was prepared from citric acid monohydrate and trisodium citrate dihydrate, and the ninhydrin reagent was freshly prepared by dissolving 2 g of ninhydrin in 100 mL of dimethyl sulfoxide (DMSO) before diluting to volume with the citrate buffer. For the assay, 1.0 mL of the sample extract was mixed with 1.0 mL of the ninhydrin reagent in a test tube, thoroughly vortexed, and heated at 80 °C for 10 min to facilitate color development. After cooling to room temperature for 5 min, 2.0 mL of citrate buffer was added to bring the final reaction volume to 4.0 mL. Absorbance was measured at 570 nm against a reagent blank using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan) within 30 min of reaction completion. Total free amino acid content was quantified using a standard calibration curve and expressed as mg g−1 DW.

Total nitrate content analysis

Nitrate concentration was determined using the salicylic acid method described by Cataldo et al. (Cataldo et al., 1975) with slight modifications. Briefly, 0.2 mL of the sample extract was mixed with 0.8 mL of 5% (w/v) salicylic acid prepared in concentrated sulfuric acid (H2SO4) and incubated at room temperature for 20 min. Subsequently, 19 mL of 2 N sodium hydroxide (NaOH) was carefully added to terminate the reaction and promote color development. After the reaction mixture had cooled to room temperature, absorbance was measured at 410 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan). A standard calibration curve was generated using potassium nitrate (KNO3) solutions ranging from 0 to 100 µg mL−1. Nitrate concentration was calculated from the calibration curve and expressed as milligrams of nitrate (NO3−) per gram of dry weight (mg NO3− g−1 DW).

DPPH free radical scavenging capacity

Antioxidant capacity was evaluated by measuring the free radical scavenging capacity against 2,2-diphenyl-1-picrylhydrazyl (DPPH) following the method of Braca et al. (2003). A DPPH working solution was prepared by dissolving 5.914 mg of DPPH in 100 mL of absolute methanol and adjusting the solution to an initial absorbance of 1.1–1.3 at 517 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan). For the assay, 1 mL of the sample extract was mixed with 3 mL of the DPPH solution, whereas the blank consisted of 1 mL of methanol mixed with 3 mL of the DPPH solution. The reaction mixtures were vortexed thoroughly and incubated in the dark at room temperature for 30 min. Absorbance was then measured at 517 nm using the same spectrophotometer. The DPPH radical scavenging activity was expressed as the percentage inhibition relative to the blank and calculated using the following equation:

Inhibition(%)=[(blank sample−extract sample)/blank sample]×100.

Ferric reducing antioxidant power assay (FRAP)

Ferric reducing antioxidant power (FRAP) was determined according to Pulido et al. (2000). Briefly, 1 mL of the plant extract was mixed with 1 mL of 0.2 M phosphate buffer (pH 6.6) and incubated at 50 °C for 20 min. After incubation, 1 mL of trichloroacetic acid (TCA) was added to terminate the reaction, and then the mixture was centrifuged at 3,000 rpm for 10 min. The resulting supernatant was diluted 1:1 (v/v) with distilled water, after which 0.25 mL of 0.1% ferric chloride (FeCl3) solution was added. We measured absorbance at 700 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan). Higher absorbance values indicated greater ferric reducing antioxidant power.

Chlorophyll (Chl) and carotenoid (Car) determination

Photosynthetic pigments were analyzed using fresh leaf tissue collected from twelve plants per treatment. Briefly, 50 mg of fresh leaf tissue was mixed with 10 mL of 80% acetone and incubated at room temperature for 30 min. The extract was then transferred to centrifuge tubes and centrifuged at 4,000 rpm for 10 min. The absorbance of the supernatant was measured at 663, 647, and 470 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan). Chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll, and total carotenoid contents were calculated according to the equations of Lichtenthaler (1987) and expressed as mg g−1 fresh weight (FW).

Chl a=12.25×A663−2.79×A647

Chl b=21.50×A647−5.10×A663

Total Chl=7.15×A663+18.71×A647

Car=(1000×A470−1.82×Chl a−85.02×Chl b)/198

The non-destructive total chlorophyll, flavonoid, and anthocyanin content of fully expanded leaves was assessed using an MPM-100 Multiple Wavelength Pigment Meter (Opti-Sciences Inc., Hudson, NH, USA). The device estimates pigment indices based on leaf optical properties (light absorbance and reflectance at specific wavelengths).

Anthocyanin determination

Total anthocyanin content was determined using the pH differential method described by Revilla et al. (1998) with slight modifications. Briefly, 30 mg of fresh leaf tissue was extracted with 5 mL of 2% (v/v) HCl in methanol and incubated at 4 °C for 36 h. The extract was then centrifuged at 1,446 × g for 15 min, and the supernatant was collected for analysis. An aliquot of 400 µL of the extract was separately mixed with 2.0 mL of potassium chloride buffer (0.025 M, pH 1.0) and sodium acetate buffer (0.4 M, pH 4.5). After equilibration for 15 min at room temperature, we measured the absorbance of each solution at 520 and 700 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu Corporation, Kyoto, Japan). Anthocyanin concentration was calculated using the pH differential method and expressed as cyanidin-3-glucoside equivalents (mg C3G g−1 FW) according to the following equation:

Anthocyanin(mg L−1)=A×MW×DF×1000ϵ×l

where the corrected absorbance (A) was calculated as:

A=(A520−A700)pH1.0−(A520−A700)pH4.5

  • where: A520=absorbance measured at 520 nm, corresponding primarily to anthocyanin pigments A700=absorbancemeasured at 700 nm to correct for haze, turbidity, and light scattering.
  • MW = 449.2 g mol−1 (cyanidin-3-glucoside).
  • DF = dilution factor.
  • ϵ = 26,900 L mol−1 cm−1.
  • l = path length (1 cm).
  • 1000= conversion factor from grams to milligrams.

Statistical analysis

Data was analyzed using a one-way ANOVA in Minitab, LLC (version 22.4.0, State College, PA, USA). All data were expressed as mean ± SD of multiple measurements. Results among the light treatments (W, FR, and UV-A) were compared using one-way analysis of variance (ANOVA), followed by Fisher’s least significant difference (LSD) test at the 5% significance level (p<0.05).

Result

The study showed that preharvest FR and UVA lighting influenced the growth and biomass accumulation of oakleaf lettuce compared with plants grown under white light alone (Table 1). FR supplementation enhanced most growth parameters, resulting in greater plant height (34.5 cm), leaf number (79.1), leaf area (35.2 cm2), root length (41.5 cm), fresh weight (54.8 g), and dry weight (4.2 g) compared with white light. The increases were 19.4%, 3.5%, 22.7%, 12.2%, 13.1%, and 39.5% compared to white light, respectively. UVA treatment also improved several growth attributes relative to white light, but it was not significantly different from white light, although the improvements were generally smaller than under FR lighting.

Table 1. Growth characteristics of Oak leaf lettuce under preharvest light treatment.

TreatmentPlant height (cm)Leaf numberLeaf areaRoot length (cm)Fresh wt (g)Dry wt (g)
W28.9 ± 1.7b76.4 ± 6.3b28.7 ± 1.2b37 ± 8.3b48.4 ± 9.8b3.01 ± 0.62c
W+FR34.5 ± 1.9a79.1 ± 6.6a35.21 ± 1.9a41.5 ± 5.0a54.75 ± 8.3a4.2 ± 0.74a
W+UVA30.2 ± 1.2b75.3 ± 4.5b30 ± 1.6b38 ± 6.7b50.7 ± 9.4b3.8 ± 0.73b

Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.

Visual evaluation (Figure 2) revealed that lettuce grown under white light exhibited less uniform plant architecture, whereas supplemental FR light promoted a more uniform canopy structure. FR-treated plants displayed greater canopy expansion and broader leaves, which corresponded with the increased leaf area observed quantitatively. In contrast, plants exposed to UVA light showed less uniform growth and a more compact morphology. UVA-supplemented lettuce exhibited reduced canopy size, shorter roots, and lower plant height compared with FR-treated plants.

Effects of preharvest light treatments on the visual appearance and canopy architecture of oakleaf lettuce. W, white; FR, far red; UVA, ultraviolet A.
Figure 2. Effects of preharvest light treatments on the visual appearance and canopy architecture of oakleaf lettuce. W, white; FR, far red; UVA, ultraviolet A.

Preharvest FR and UVA light treatments significantly influenced the photochemical performance (Figure 3) and stomatal conductance (Figure 4) of oakleaf lettuce. The maximum quantum yield of PSII (Fv/Fm) increased to 0.80 under FR treatment, representing an 11.1% increase, while UVA increased Fv/Fm by 9.7% compared to white light. Similarly, the performance index (PI) markedly increased under FR treatment by 100%, rising from 0.40 in the control to 0.80. UVA also improved PI by 50.0% compared with white light. Stomatal conductance (gs) was enhanced by both supplemental light treatments. FR treatment increased gs by 24.2%, from 240 to 298 mmol m−2 s−1, whereas UVA increased it by 10.8% to 266 mmol m−2 s−1 relative to the control.

Photochemical index of lettuce grown under preharvest light treatment. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.
Figure 3. Photochemical index of lettuce grown under preharvest light treatment. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.
Stomatal conductance of lettuce grown under preharvest light treatment. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.
Figure 4. Stomatal conductance of lettuce grown under preharvest light treatment. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.

Preharvest FR and UVA light treatments significantly influenced pigment accumulation in oakleaf lettuce (Table 2; Figure 5). Compared with white light, FR treatment markedly increased chl a, chl b, total chl, carotenoids, and anthocyanins. Chl a increased by 39.3%, chl b by 50.0%, total chl by 42.5%, carotenoids by 50.0%, and anthocyanins by 84.6% relative to the white. Notably, UVA induced the greatest anthocyanin accumulation, increasing by 146.2% over white light. FR supplementation increased chlorophyll content by 33.3% and UVA increased flavonoid, and anthocyanin indices by 40.0%, and 72.0%, respectively, compared with white light.

Table 2. Pigment content of leaf lettuce under preharvest light treatment.

TreatmentChl a(mg g-1 fw)Chl b(mg g-1 fw)Total Chl(mg g-1 fw)Carotenoid(mg g-1 fw)Anthocyanin(mg g-1 fw)
W0.28 ± 0.05b0.12 ± 0.32b0.40 ± 0.06b0.08 ± 0.07b0.052 ± 0.06b
W+FR0.39 ± 0.04a0.18 ± 0.12a0.57 ± 0.04a0.2 ± 0.22a0.096 ± 0.08b
W+UVA0.30 ± 0.04b0.13 ± 0.35b0.43 ± 0.05b0.09 ± 0.09b0.128 ± 0.09a

Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.

Non-destructive pigment content (total chlorophyll, flavonoid and anthocyanin) of lettuce grown under preharvest light treatment. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.
Figure 5. Non-destructive pigment content (total chlorophyll, flavonoid and anthocyanin) of lettuce grown under preharvest light treatment. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.

FR and UVA preharvest light significantly influenced the phenolic composition of oakleaf lettuce (Table 3). Compared with white light, FR treatment increased total phenolic content by 9.4%, total flavonoids by 11.3%, amino acid concentration by 51.4%, and nitrate accumulation by 44.0%. The enhanced amino acid content under FR suggests improved metabolic activity and nitrogen assimilation, although the higher nitrate accumulation may indicate reduced nitrate reduction efficiency under FR light. Total phenolic content increased by 34.5%, total flavonoids by 25.4%, amino acid concentration by 106.2%, and nitrate accumulation by 64.0% under UVA compared with the white.

Table 3. Phenolic content of Oak leaf lettuce under preharvest light treatment.

TreatmentTotal phenolics (mg g-1 dw)Total flavonoid (mg g-1 dw)Amino acid(mg g-1 dw)Total nitrate(mg g-1 dw)
W31.64 ± 4.0c6.3 ± 1.0b12.56 ± 3.1c2.5 ± 0.13c
W+FR34.6 ± 4.0b7.01 ± 1.2a19.01 ± 5.7b3.6 ± 0.39b
W+UVA42.54 ± 7.9a7.9 ± 0.73a25.9 ± 4.8a4.1 ± 0.53a

Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.

Preharvest light significantly enhanced the antioxidant capacity of oakleaf lettuce compared with white light (Figure 6). FR treatment increased DPPH free radical scavenging activity by 3.3%, from 88.24% under white light to 91.14%, while UVA produced the highest DPPH capacity, increasing by 7.7% to 95.04%. UVA treatment markedly enhanced FRAP capacity by 16.7%, reaching 7408 µM Fe2+ compared with the white light.

Antioxidant capacity of oak leaf lettuce grown under pre-harvest supplemental light. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.
Figure 6. Antioxidant capacity of oak leaf lettuce grown under pre-harvest supplemental light. Values are presented as means ± standard deviation (SD). Different letters within each column indicate significant differences among treatments according to Fisher’s least significant difference (LSD) test at P ≤ 0.05. W, white; FR, far red; UVA, ultraviolet A.

Discussion

Plant growth and photochemical parameters

Preharvest FR supplementation significantly improved oakleaf lettuce growth, resulting in greater plant height, leaf area, root length, fresh weight, and dry weight compared with white light alone. Research showed that FR promotes phytochrome-mediated leaf expansion and canopy development, thereby enhancing light interception and biomass accumulation (Jeong et al., 2024; Carotti et al., 2024). The higher fresh and dry biomass observed under FR treatment suggests improved carbon assimilation through increased canopy photon capture rather than changes in leaf-level photosynthesis (Kelly and Runkle, 2024). UVA is primarily perceived by cryptochrome and UVR8 photoreceptors, which stimulate photoprotective and secondary metabolic pathways rather than vegetative growth (Qin et al., 2023).

Application of FR and UVA lighting improved the photochemical performance of oakleaf lettuce. The higher Fv/Fm and PI values under FR indicate improved PSII efficiency and electron transport, suggesting enhanced photosynthetic capacity. FR photons are photosynthetically active and can increase canopy photosynthesis by improving the excitation balance between PSI and PSII and enhancing light-use efficiency (Zhen and Bugbee, 2020). The increased stomatal conductance observed under FR supports greater CO2 uptake and carbon assimilation, contributing to improved biomass production (Jeong et al., 2024; Meng et al., 2024).

UVA supplementation also increased Fv/Fm, PI, and stomatal conductance compared with the white. Moderate UVA exposure can improve photochemical stability through cryptochrome- and UVR8-mediated regulation of photoprotection and antioxidant defense systems (Qin et al., 2023). However, the smaller response compared with FR suggests that UVA primarily promotes acclimation and stress-protective mechanisms rather than maximizing photosynthetic productivity (Pacak et al., 2024).

Pigment and biochemical compounds

Oakleaf lettuce exposed to supplemental FR exhibited significantly greater pigment accumulation than plants grown under white light alone. FR treatment substantially increased chl a, chl b, and carotenoids, while UVA light increased anthocyanin. Previous studies show that FR can stimulate chlorophyll biosynthesis and chloroplast development via phytochrome-mediated signaling, improving light capture and photosynthetic performance (Meng et al., 2024; Jeong et al., 2024). The concurrent increase in carotenoids further suggests enhanced photoprotection and antioxidant capacity, which are essential for maintaining photosystem stability (Carotti et al., 2024). In contrast, UVA induced the greatest anthocyanin accumulation and markedly increased flavonoid indices. Cryptochrome perceives UVA, activating the phenylpropanoid pathway and upregulating key enzymes involved in flavonoid and anthocyanin biosynthesis, including phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) (Qin et al., 2023; Pacak et al., 2024). Anthocyanins function as photoprotective compounds by absorbing excess radiation and scavenging reactive oxygen species, explaining their strong accumulation under UVA exposure.

UVA induced substantially greater increases in phenolics, flavonoids, and amino acids than FR, indicating strong activation of secondary metabolism. UVA radiation is known to stimulate cryptochrome mediated signaling pathways, leading to upregulation of the phenylpropanoid pathway and increased synthesis of antioxidant compounds such as phenolics and flavonoids (Qin et al., 2023; Pacak et al., 2024). The marked increase in amino acid concentration may also reflect enhanced production of stress-related metabolites involved in photoprotection and reactive oxygen species scavenging. The increased nitrate accumulation under UVA may reflect enhanced nitrogen uptake and storage, although the simultaneous accumulation of phenolic compounds suggests that a substantial portion of assimilated nitrogen was redirected toward synthesizing protective metabolites.

Preharvest UVA lighting enhanced the antioxidant capacity of oakleaf lettuce, as evidenced by increased DPPH radical scavenging activity and FRAP under this treatment. The improvement with FR treatment likely reflects increased accumulation of carotenoids, phenolics, and flavonoids. FR light has been reported to enhance photosynthetic efficiency and carbon assimilation, thereby providing additional substrates for antioxidant synthesis (Carotti et al., 2024). UVA supplementation produced the greatest antioxidant activity, coinciding with the substantial increases in phenolic, flavonoid, and anthocyanin concentrations. UVA activates cryptochrome-mediated signaling pathways, stimulating the phenylpropanoid biosynthetic pathway and increasing antioxidant metabolite production (Qin et al., 2023; Pacak et al., 2024). Phenolic compounds and flavonoids are major contributors to DPPH and FRAP activities due to their strong electron-donating and free-radical-scavenging properties (Azad et al., 2018). The higher antioxidant capacity under UVA likely reflects enhanced synthesis of these protective secondary metabolites.

Conclusion

Preharvest supplementation with FR and UVA light effectively improved the growth, physiological performance, and nutritional quality of oakleaf lettuce grown under white LED lighting. Far-red light primarily enhanced vegetative growth by increasing plant height, leaf area, biomass accumulation, chlorophyll content, photochemical efficiency, and stomatal conductance, indicating improved photosynthetic performance and carbon assimilation. In contrast, UVA supplementation strongly stimulated the accumulation of anthocyanins, phenolics, flavonoids, amino acids, and antioxidant capacity, demonstrating its effectiveness in enhancing the nutritional and functional quality of lettuce. Overall, the results suggest that preharvest spectral manipulation offers a practical, energy-efficient strategy for tailoring lettuce production to specific market objectives. Far-red supplementation is recommended when maximizing yield and biomass is the primary goal, whereas UVA supplementation is more suitable for producing nutrient-dense lettuce with enhanced antioxidant properties. Future studies should optimize FR and UVA intensities and exposure durations, as well as their combined effects on yield, phytochemical composition, and postharvest quality.

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Footnotes

Edited by: Xiao Yang, Chinese Academy of Agricultural Sciences, China

Reviewed by: Christopher P. Levine, The University of Tokyo, Japan

Aleksandra Rakonjac, Institute for Vegetable Crops Ltd, Serbia

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was supported by USDA Hatch funding through the University of Nevada, Reno Agricultural Experiment Station under the WERA1053 project, Urban Agriculture and Food Systems (2024), and USDA Multistate Research Project NE2335, Resource Optimization in Controlled Environment Agriculture.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

MN: Resources, Project administration, Supervision, Writing – original draft, Writing – review & editing, Funding acquisition. MA: Writing – review & editing, Writing – original draft, Formal Analysis.

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Citation

Naznin, M. T. & Azad, M. O. K. (2026). Preharvest exposure to far-red and UVA light enhances growth and pigment content of oakleaf lettuce in an indoor vertical farming system. Frontiers in Horticulture, Volume 5. https://doi.org/
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5 - 2026

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Copyright © 2026 Naznin and Azad.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Most Tahera Naznin

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