Far-Red and UVA Light Give Vertical-Farm Lettuce a Two-Week Boost Before Harvest

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The practical appeal of the preharvest lighting strategy described in the study lies in its timing rather than its intensity. Because the supplemental far-red and UVA treatments were applied only during the final two weeks of a production cycle, the additional electrical load was confined to a short window at the end of cultivation, when leafy greens are already close to their harvestable size. In vertical farming economics, electricity is consistently identified as one of the dominant operating expenses, and any technique that shifts quality improvement away from continuous high-intensity lighting toward brief, targeted spectral additions can meaningfully change the cost-benefit calculation. The study’s design deliberately held the baseline photosynthetic photon flux density at a modest 100 µmol m−2 s−1, which is well below the levels often used in commercial leafy-green production, and asked whether spectral composition rather than sheer photon supply could still deliver measurable gains in both biomass and phytochemical content. The affirmative answer suggested by the results has implications for growers operating on thin margins, particularly in markets where premium pricing for nutritionally enhanced produce can offset modest production adjustments.

The far-red response observed in the lettuce plants fits within a well-established body of plant photobiology. Far-red wavelengths in the 700 to 750 nanometer range are absorbed by phytochrome photoreceptors, and an enrichment of far-red relative to red light shifts the phytochrome equilibrium toward its biologically inactive form. Plants interpret this spectral signal as an indication of vegetative shade, as would occur when neighboring foliage filters out red light, and they respond with the classic shade-avoidance syndrome: stem elongation, expanded leaf blades, and altered carbon partitioning toward shoot growth. In a leafy crop such as oakleaf lettuce, this response translates directly into the increases in plant height, leaf area, and fresh and dry weight that the researchers documented. The magnitude of the dry-weight gain, nearly forty percent above the white-light control, is particularly notable because it suggests that far-red supplementation did not merely drive water uptake and cell expansion but supported genuine biomass accumulation. This aligns with the emerging concept in controlled-environment horticulture that far-red photons extend the photosynthetically useful spectrum, contributing to photosynthesis in ways that older definitions of photosynthetically active radiation, which stopped at 700 nanometers, failed to capture.

The improvements in photochemical performance reported under far-red treatment deserve particular attention. The chlorophyll fluorescence parameter known as maximum quantum efficiency of photosystem II, commonly abbreviated as Fv/Fm, is widely used as an indicator of the health and functional integrity of the photosynthetic apparatus, and values near the theoretical optimum of approximately 0.8 indicate that the light-harvesting machinery is operating without significant photoinhibitory stress. The performance index, a more integrative fluorescence metric, captures information about photon absorption, energy trapping, and electron transport beyond photosystem II, so an increase in this parameter alongside higher Fv/Fm suggests a broad enhancement of photosynthetic function rather than an isolated artifact. The accompanying rise in stomatal conductance indicates that the plants were opening their stomata more readily, facilitating carbon dioxide diffusion into the leaf interior and supporting the observed gains in photosynthetic pigment content. The reported increases in chlorophyll a, chlorophyll b, and carotenoids, ranging from roughly thirty-nine to fifty percent, are consistent with this picture of a photosynthetic system that had been upregulated in response to the far-red signal, with more pigment per unit leaf area available to intercept photons and more reaction centers available to process the captured energy.

The UVA results tell a complementary but distinct story. Ultraviolet A radiation, spanning roughly 315 to 400 nanometers, is perceived in plants primarily through cryptochrome photoreceptors and related UV-sensing mechanisms, and it characteristically triggers the phenylpropanoid biosynthetic pathway, the metabolic route responsible for producing phenolic acids, flavonoids, and anthocyanins. These compounds serve the plant as photoprotective agents, absorbing excess radiation and quenching reactive oxygen species that would otherwise damage cellular structures. From a human nutrition standpoint, the same compounds are valued for their antioxidant and anti-inflammatory properties, which is why the substantial increases in anthocyanin content, total phenolics, and total flavonoids under UVA treatment are commercially relevant. The reported 146 percent increase in anthocyanins is especially striking for a red-leafed lettuce cultivar, since anthocyanin accumulation directly influences the visual appeal of the foliage, deepening red and purple tones that consumers associate with freshness and nutritional density. In premium salad markets, appearance and phytochemical content often reinforce one another, so a lighting treatment that enhances both simultaneously addresses two marketing dimensions at once.

The more than doubling of amino acid content under UVA exposure is an intriguing finding that connects to nitrogen metabolism. Ultraviolet exposure can induce mild stress responses that mobilize nitrogen-containing compounds, including free amino acids, as part of the plant’s acclimation machinery. Some of these amino acids serve as precursors for phenylpropanoid and flavonoid synthesis, so elevated amino acid pools may partly underpin the concurrent increases in phenolic compounds. However, the same metabolic territory encompasses nitrate accumulation, and the reported 64 percent increase in nitrate under UVA treatment represents the principal caveat of the study. Nitrate itself is not inherently harmful, and leafy vegetables are a recognized dietary source of nitrate that can be converted to nitric oxide with potential cardiovascular benefits, but regulatory frameworks in several jurisdictions impose limits on nitrate concentrations in lettuce and spinach precisely because excessive intake, particularly in infants and under certain storage conditions, can pose health risks. A treatment that raises antioxidant content while simultaneously raising nitrate content therefore presents a trade-off that growers and regulators would need to weigh carefully. The authors’ acknowledgment that further work is needed to reduce nitrate levels points toward possible solutions, such as adjusting nitrogen nutrition in the final days before harvest, timing the UVA exposure to minimize nitrate buildup, or combining UVA with spectral treatments known to promote nitrate assimilation.

The contrast between the two treatments highlights a broader principle in controlled-environment lighting: different wavelengths serve different end goals, and the optimal spectrum depends on whether the objective is yield, nutritional quality, visual quality, or some weighted combination. Far-red light in this study functioned primarily as a growth and photosynthesis enhancer, while UVA functioned primarily as a quality and phytochemistry enhancer. This dichotomy is not absolute, since far-red also influenced anthocyanin content and UVA can affect growth under some conditions, but the dominant responses were clearly separated. For a commercial operator, this separation is actually advantageous, because it opens the possibility of sequential or combined treatments: a far-red phase to drive biomass during the bulk of the production cycle, followed by a UVA phase in the final days to enrich phytochemical content before harvest. Such staged lighting recipes would require only modest additional hardware, since both far-red and UVA LEDs are commercially available and can be integrated into existing white-light fixtures or supplemental bars.

The choice of white light as the background spectrum also merits consideration. White LEDs, which typically combine blue, green, and red emissions to produce a broad spectrum, have become the default in many vertical farms because they simplify installation, provide uniform illumination that aids crop inspection, and support balanced plant development. By layering far-red or UVA on top of a white baseline rather than replacing it, the study reflected a realistic scenario for growers who already operate white LED systems and wish to add spectral functionality without redesigning their lighting infrastructure. The low baseline intensity of 100 µmol m−2 s−1 is also worth noting in the context of energy economics. Many commercial leafy-green operations run substantially higher intensities, often in the range of 200 to 300 µmol m−2 s−1 or more, so the study’s results demonstrate that meaningful quality improvements are achievable even at modest photon supply, provided the spectral composition is strategically manipulated. This finding could be especially relevant for operations in regions with high electricity costs or limited grid capacity, where the marginal value of each additional photon is scrutinized closely.

The environmental conditions maintained throughout the experiment, including a sixteen-hour photoperiod, moderate relative humidity between 35 and 40 percent, and ambient carbon dioxide concentrations of roughly 350 to 380 parts per million, represent a relatively conservative growing environment. Carbon dioxide enrichment, which is common in commercial vertical farms, was not applied, and the carbon dioxide level reported is close to current atmospheric concentrations. This suggests that the observed responses to far-red and UVA supplementation were obtained under conditions where carbon availability, rather than elevated carbon dioxide, was the norm. Whether the same spectral treatments would produce similar or amplified responses under carbon dioxide enrichment remains an open question, since photosynthetic rates and secondary metabolism can both shift when carbon supply increases. Similarly, the interaction between spectral supplementation and other environmental variables, such as temperature, nutrient concentration, and root-zone conditions, would need to be characterized before the findings could be generalized across the wide range of operating conditions found in commercial facilities.

The cultivar used in the study, a red oakleaf lettuce, is notable for its naturally elevated anthocyanin content and attractive leaf morphology, which may have predisposed it to strong responses to UVA-induced anthocyanin accumulation. Green-leafed cultivars, which generally contain lower baseline levels of anthocyanins, might respond differently, and the magnitude of phytochemical enhancement could vary across genetic backgrounds. Cultivar-specific lighting recipes are an active area of research in controlled-environment agriculture, and the present findings would need to be validated across a range of lettuce types, including butterhead, romaine, and green leaf varieties, before broad recommendations could be made. The hydroponic nutrient film technique system used in the study is also representative of common commercial practice, which strengthens the relevance of the findings to industry, but differences in nutrient formulation, root-zone temperature, and water management could all modulate the observed responses.

From a food safety and regulatory perspective, the nitrate finding warrants the most careful follow-up. Nitrate accumulation in leafy vegetables is influenced by light intensity, nitrogen availability, photoperiod, and harvest timing, and low-light conditions are classically associated with elevated nitrate because reduced photosynthetic activity limits nitrate assimilation. The fact that UVA supplementation increased nitrate in this study, despite the treatments being applied under continuous illumination for two weeks, suggests that the spectral quality itself influenced nitrogen metabolism in ways that favored nitrate accumulation. Understanding the mechanism, whether through altered nitrate reductase activity, changed nitrogen uptake dynamics, or shifts in carbon-nitrogen partitioning, would be a logical next step. If the mechanism can be identified, targeted interventions, such as brief dark periods before harvest, adjusted nitrogen supply, or combined spectral treatments, could potentially retain the phytochemical benefits of UVA while mitigating the nitrate increase.

The broader significance of this research lies in its demonstration that preharvest spectral supplementation is a viable, energy-efficient lever for tailoring crop attributes in vertical farming. Rather than treating light as a uniform input whose only variable is intensity, the study reinforces the view of light as a programmable signal that can be tuned to elicit specific plant responses at specific developmental stages. As the vertical farming industry continues to mature and faces pressure to reduce energy costs while differentiating products on quality and nutrition, strategies of this kind, which extract additional value from modest spectral adjustments rather than brute-force increases in lighting power, are likely to attract growing attention. The findings also contribute to the scientific literature on photoreceptor-mediated regulation of growth and secondary metabolism, providing quantitative data on how two-week exposures to far-red and UVA under a white LED baseline affect a commercially important leafy crop grown under realistic hydroponic conditions.

Subject of Research: …

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

Article References: 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, 5, Article 1971896. https://doi.org/10.3389/fhort.2026.1971896

Image Credits: AI Generated

DOI: 10.3389/fhort.2026.1971896

Keywords: Preharvest, exposure, far-red, light, enhances, growth, pigment, content, oakleaf, lettuce, indoor, vertical