Optimizing spectral quality with quantum dots to enhance crop yield in controlled environments

The study addresses how modifying light spectral quality, independent of light quantity, can enhance photosynthetic efficiency and crop yield in controlled environment agriculture (CEA). While photosynthetically active radiation (PAR) quantity (PPFD/DLI) correlates with growth, photosynthetic efficiency varies with wavelength; wavelengths between ~575–675 nm are ~30% more efficient than blue light for many crops, including lettuce. Prior work shows that both light quantity and quality affect morphology and biomass, and blue-heavy spectra can reduce dry mass in several species at high PPFD. CEA is also critical for space applications where mass, energy, and reliability constraints limit conventional lighting. Existing photoselective greenhouse films reduce intensity and may be unsuitable in light-limited settings. The authors propose luminescent quantum dot (QD) films to convert less-efficient UV/blue photons into more-efficient orange/red wavelengths to improve light-use efficiency (LUE) and biomass without increasing energy input, relevant for terrestrial greenhouses and bioregenerative life-support systems for space.

The paper situates its work within: (1) plant photobiology demonstrating wavelength-dependent photosynthetic quantum yield (McCree spectra) and species-specific responses to blue/green light; (2) CEA and space agriculture systems development (LED chambers, deployable greenhouses like the Prototype Mars-Lunar Greenhouse); (3) limitations of conventional photoselective films which reduce total light and prior luminescent films using organic fluorophores or other fluorescent materials that faced issues with narrow emission bands, low conversion efficiency, and/or poor stability; and (4) the versatility of QDs in optoelectronics and photonics. CuInS₂/ZnS (CIS/ZnS) QDs offer tunable, broad, defect-mediated emission with high quantum yield and large Stokes shift, enabling absorption of UV/blue and re-emission in orange/red to align with high-efficiency photosynthetic wavelengths while minimizing re-absorption within PAR.

Experimental design: A custom plant growth test chamber (PGTC; 1.8 × 1.0 × 2.2 m) housed three adjacent light treatment zones separated by reflective/diffusive barriers to prevent cross-illumination. Above, four 400 W metal halide lamps (SolisTek 10K Finisher) in P.L. Light Systems luminaires provided uniform illumination approximating solar spectrum onto a test film frame holding: (1) an orange-emitting QD film (O-QD, peak ~600 nm), (2) a red-emitting QD film (R-QD, peak ~660 nm), and (3) a control polyethylene film (C) without QDs. The QD films were laminated with PET barriers; both films exhibited PL QY ~85% and ~2% haze. Plant material and cultivation: Red romaine lettuce (Lactuca sativa L. cv. "Outredgeous") was grown hydroponically in a recirculating deep-water culture system. Three replicate experiments were conducted; each experiment had N=36 plants (n=12 per treatment) arranged at 12 × 12 cm spacing (55 plants m⁻²). Seeds were germinated in rockwool, thinned to one seedling per cube at 7 DAS. Photoperiod: 14 h (target DLI ~17 mol m⁻² d⁻¹). Environmental control/monitoring: air temperature averaged 24/20 °C (light/dark), CO₂ monitored (361–376 ppm, not controlled), nutrient solution maintained at pH 6.1–6.2 and EC 1.8–1.9 mS cm⁻¹ with continuous recirculation and ~5 min turnover. PPFD and spectral quality were monitored using Apogee SQ-500 quantum sensors and PS-300 spectroradiometer; environmental uniformity across zones was confirmed. Spectral characterization: Spectra under each film were measured pre- and between experiments to ensure stability. Table 1 reported: Control PFD 435 µmol m⁻² s⁻¹ (PPFD 382); O-QD PFD 458 (PPFD 395); R-QD PFD 413 (PPFD 338). Under both QD films, UV and blue decreased while orange/red and far-red increased relative to Control. Due to emission beyond PAR (>700 nm), 4.6% of O-QD and 28.3% of R-QD emission did not contribute to PPFD, explaining ~12% lower measured PPFD under R-QD. Measurements: After 28 DAS, edible fresh mass (FM, g), edible dry mass (DM, g; dried 40 °C for 72 h), and total leaf area (TLA, cm²) were measured (TLA via image analysis with manual corrections). Light-use efficiency (LUE, g mol⁻¹) was calculated as mass produced per DLI. Statistical analysis used a two-factor model (film treatment: O-QD, R-QD, C; experiment replicate: 1–3) with two-tailed Student’s t-tests for pairwise comparisons of least-squares means. Cohen’s d effect sizes were computed for treatment effects. Modeling for space applications: Film absorption spectra were convoluted with AM 1.5 (terrestrial) and AM 1.0 (space) solar spectra to estimate absorbed photon fractions in UV (<400 nm), blue (400–500 nm), and green (500–600 nm). Net PPFD change beneath films was estimated including realistic loss mechanisms: PL QY 85%, ~25% isotropic emission loss away from plants, and emission portions beyond PAR. Net ΔPPFD (µmol m⁻² s⁻¹ and %) was reported for each film under AM 1.0 and AM 1.5.

- Spectral modification: Under QD films, UV and blue photon flux decreased; orange/red and far-red increased versus control. Measured PPFD (400–700 nm): Control 382, O-QD 395 (+3%), R-QD 338 (−12%) µmol m⁻² s⁻¹. Emission beyond PAR: O-QD 4.6%; R-QD 28.3%. - Biomass and morphology (average across three 28-day experiments, n=12 per treatment per experiment): - Edible dry mass (DM): O-QD +13% (p=0.003); R-QD +8.7% (p=0.05) vs Control. - Edible fresh mass (FM): O-QD +11% (p=0.004); R-QD +11% (p=0.005). - Total leaf area (TLA): O-QD +8% (p=0.02); R-QD +13% (p=0.0003). - Light-use efficiency (LUE, g mol⁻¹ vs Control): O-QD DM +5.6%, FM +3.9%; R-QD DM +17.6%, FM +17.2%. - Despite ~12% lower DLI under R-QD (due to emission >700 nm), DM, FM, and TLA still increased significantly, indicating improved photosynthetic efficiency per incident photon. - Considering expanded photosynthetically active range (400–750 nm) including far-red synergy: photon fluxes were 402 (Control), 423 (O-QD, +5.2%), and 378 (R-QD, −6%) µmol m⁻² s⁻¹; O-QD’s increase aligned with its LUE improvement. - Space/terrestrial modeling of net PPFD change due to films (including realistic losses): - AM 1.0: O-QD +62.6 µmol m⁻² s⁻¹ (+2.6%); R-QD −58.3 (−2.4%). - AM 1.5: O-QD −26.8 (−1.6%); R-QD −86.0 (−5.0%). - Estimated additional yield benefit in space due to greater UV conversion: O-QD +3.4%; R-QD +2.1% (beyond spectral-quality effects observed here). - Overall: QD films increased lettuce DM (+13% O-QD; +9% R-QD), FM (+10–11%), and TLA (+8% O-QD; +13% R-QD), demonstrating more efficient growth versus control.

The results confirm the hypothesis that enhancing spectral quality by down-converting UV/blue photons into orange/red wavelengths can improve photosynthetic efficiency and crop productivity without increasing electrical energy or total PPFD. Both O-QD and R-QD films shifted the spectrum toward wavelengths with higher photosynthetic quantum yield for lettuce. Despite a measured 12% reduction in PPFD under the R-QD film (driven by emission >700 nm), plants exhibited significant gains in dry and fresh mass and leaf area, reflecting higher light-use efficiency and indicating that spectral quality can offset lower photon counts within 400–700 nm. The observed benefits are consistent with literature showing that far-red photons (700–750 nm) can act synergistically with PAR to drive photosynthesis; increases in far-red under QD films may have contributed to higher LUE. For terrestrial greenhouses (AM 1.5), the model suggests little to modest net PPFD change (slightly negative when accounting for losses), implying that yield gains will primarily derive from spectral quality rather than increased light quantity. In space or high-UV environments (AM 1.0), additional benefits are anticipated from converting more abundant UV, potentially augmenting PPFD and yield beyond the spectral-quality effect. Collectively, the findings support QD luminescent films as a low-mass, passive spectral control strategy to enhance CEA productivity on Earth and as a promising component of bioregenerative life-support systems for long-duration space missions.

CIS/ZnS quantum dot luminescent films that emit at 600 nm (orange) and 660 nm (red) improved lettuce growth under controlled environments by shifting incident light toward more photosynthetically efficient wavelengths. Across three replicated experiments, the films increased edible dry mass (O-QD +13%; R-QD +9%), fresh mass (both +11%), and total leaf area (O-QD +8%; R-QD +13%), with significant gains in light-use efficiency even when measured PPFD decreased (R-QD). Modeling indicates that in space-like AM 1.0 spectra, O-QD films could modestly increase PPFD and yield via enhanced UV conversion, whereas terrestrial gains will largely stem from spectral-quality improvements. These results demonstrate a passive, lightweight approach to boost CEA productivity, relevant for greenhouse agriculture and bioregenerative systems in space. Future directions include: validating performance across additional crops and cultivars; optimizing QD emission peaks, bandwidths, and concentrations for species-specific responses; long-term durability and stability assessments in operational greenhouses; integrating QD films with natural sunlight and LED systems; quantifying contributions of far-red synergy; and evaluating performance under actual extraterrestrial spectral conditions and in closed-loop life-support systems.

- Species and cultivar specificity: Experiments were conducted on a single lettuce cultivar (Outredgeous), limiting generalizability to other crops. - Controlled indoor setup: Experiments used MH lamps approximating solar spectra in a PGTC, not full sunlight conditions; greenhouse field validation is needed. - PPFD mismatch: R-QD treatment had ~12% lower measured PPFD due to emission beyond 700 nm; intensity was not matched to control to isolate spectral effects, which complicates direct PPFD-equated comparisons. - Environmental factors: CO₂ was monitored but not actively controlled; although conditions were uniform across treatments, minor fluctuations may influence outcomes. - Duration: Growth period was 28 days; longer crop cycles and repeated harvests were not assessed. - Modeling assumptions: Space/terrestrial PPFD modeling incorporated simplified loss assumptions (fixed PL QY, isotropic emission losses, emission beyond PAR) and did not simulate all optical/environmental complexities; results are estimates rather than measured outcomes.