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

Anthropogenic climate change reduces arable land, driving interest in controlled environment agriculture (CEA) for increased resource use efficiency and yield. CEA optimizes environmental controls for year-round production, exceeding field agriculture yields tenfold. Photosynthetically active radiation (PAR), quantified by photosynthetic photon flux density (PPFD) and daily light integral (DLI), is crucial for plant growth. However, spectral quality, impacting photosynthetic quantum yield (QY), is also vital. Studies show that both light quantity and quality affect plant morphology, with optimal spectra varying among species. CEA is particularly advantageous in extreme environments like space, where NASA and other agencies are developing systems ranging from LED-lit growth chambers to deployable greenhouses. Currently, controlling light quality is limited, with photoselective films reducing light intensity. Quantum dots (QDs), offering high photon conversion efficiency and tunable emission, present an alternative for modifying the solar spectrum without electricity. This study uses CuInS₂/ZnS QDs in films to down-convert UV and blue photons to orange and red photons, aiming to enhance lettuce growth in a controlled environment.

The literature highlights the importance of both light quantity and quality in plant growth. Several studies demonstrate that optimal light spectra vary depending on the plant species and light intensity. In high-light intensity environments, modifying the spectral composition can significantly impact plant growth, as demonstrated by experiments showing reduced dry mass in tomatoes with increased blue light. The use of CEA in space exploration is also discussed, highlighting the need for efficient and lightweight light modification technologies. Existing technologies like photoselective films have limitations, motivating the exploration of alternative materials such as quantum dots.

Two CuInS₂/ZnS QD films were created, one emitting at 600 nm (orange, O-QD) and the other at 660 nm (red, R-QD), chosen based on lettuce's photosynthetic action spectrum. Red romaine lettuce was grown in a custom-built plant growth test chamber (PGTC) with three zones: one under each QD film and a control (C) zone under a polyethylene film. The PGTC maintained uniform environmental conditions (temperature, CO2, nutrient solution) across zones, isolating spectral quality as the variable. Light intensity and spectral distribution were measured using spectroradiometers and quantum sensors before and between experiments to ensure consistency. After 28 days, edible fresh mass, edible dry mass, and total leaf area were measured. Statistical analysis (two-tailed Student's t-test) compared the growth parameters under different light treatments. A model was then developed to estimate the potential benefits of QD films in space applications by convolving the QD film absorption spectra with solar spectra for AM 1.5 (Earth) and AM 1.0 (space), accounting for QD quantum yield, isotropic emission, and out-of-PAR emissions.

The O-QD (600 nm) and R-QD (660 nm) films significantly enhanced lettuce growth. Compared to the control, edible dry mass increased by 13% (O-QD) and 9% (R-QD), edible fresh mass by 11% under both QD films, and total leaf area by 8% (O-QD) and 13% (R-QD). Even with a 12% reduction in DLI under the R-QD film, statistically significant increases were observed in biomass, indicating improved light use efficiency (LUE). The LUE was significantly higher under both QD films. The model for space applications predicted potential increases in PPFD under AM 1.0 conditions (space), with a relative increase of +2.6% for the O-QD film and a decrease of -2.4% for the R-QD film. Considering additional losses in a real system, the net improvement of PPFD for the O-QD film under AM 1.0 was +2.6%, while under the AM 1.5 conditions it would be -1.6%. For the R-QD film, the net PPFD would be -2.4% and -5% under AM 1.0 and AM 1.5 conditions, respectively. This improvement in PPFD would result in an additional benefit of 3.4% and 2.1% respectively for the O-QD and R-QD films, according to the linear relationship between DLI and yield for lettuce. However, for terrestrial applications under AM 1.5, a net increase in PPFD was not observed.

The study successfully demonstrated that modifying the solar spectrum using CuInS₂/ZnS QD films enhances lettuce growth in a controlled environment. The increased biomass under both QD films, despite a DLI reduction in the R-QD treatment, indicates improved photosynthetic efficiency, attributable to the spectral shift towards photosynthetically more efficient wavelengths. The model suggests the potential for further yield improvements in space due to the higher UV photon availability for conversion, although the gains are less than those observed in the laboratory due to system losses. These findings highlight the potential of QD films to boost crop production in both terrestrial and extraterrestrial CEA systems.

This research introduces a novel approach using CuInS₂/ZnS QD films to enhance plant growth by optimizing spectral quality. The significant improvements in lettuce biomass under different spectral conditions validate the efficacy of this method. The developed model provides insights into the potential for further yield enhancement in space applications. Future research could explore optimizing QD film composition and design for various plant species and light conditions, and implementing this technology in larger-scale CEA systems.

The study focused on red romaine lettuce in a specific controlled environment. The generalizability of these results to other plant species and environments needs further investigation. The model for space applications relies on several assumptions (e.g., 100% quantum yield, uniform emission), which may not entirely reflect real-world conditions. Larger-scale field trials are needed to confirm the findings in more realistic settings.