The spectral quality of sunlight reaching plants remains a path for optimization in greenhouse cultivation. Quantum dots represent a novel, emission-tunable luminescent material for optimizing the sunlight spectrum in greenhouses with minimal intensity loss, ultimately enabling improved light use efficiency of plant growth without requiring electricity. In this study, greenhouse films containing CuInS2/ZnS quantum dots were utilized to absorb and convert ultraviolet and blue photons from sunlight to a photoluminescent emission centered at 600 nm. To analyze the effects of the quantum dot film spectrum on plant production, a 25-week tomato trial was conducted in Dutch glass greenhouses. Plants under the quantum dot film experienced a 14% reduction in overall daily light integral, resulting from perpendicular photosynthetically active radiation transmission of 85.3%, mainly due to reflection losses. Despite this reduction in intensity, the modified sunlight spectrum and light diffusion provided by the quantum dot film gave rise to 5.7% improved saleable production yield, nearly identical total fruiting biomass production, 23% higher light use efficiency (g/mol), 10% faster vegetative growth rate, and 36% reduced tomato waste compared to the control, which had no additional films. Based on this result, materials incorporating quantum dots show promise in enabling passive, electricity-free spectrum modification for improving crop production in greenhouse cultivation, but extensive controlled crop studies are needed to further validate their effectiveness.

Quantum dots (QDs) represent suitable inorganic luminescent materials for optimizing the spectrum in greenhouses because they strongly absorb UV light, which is not used for photosynthesis, and a portion of blue light, and emit light towards longer wavelengths that is more photosynthetically efficient for plants. Quantum dots have optimal optical properties due to their high photoluminescence (PL) quantum yield (QY), size-tunable optical properties realized in manufacturing, and inherent photostability compared to organic dyes. Owing to their small size (<10 nm), the absorption profile and peak PL emission of QDs can be tuned during manufacturing by simply changing the size of the nanoparticles.

The modified sunlight spectrum and light diffusion provided by the quantum dot film gave rise to 5.7% improved saleable production yield, nearly identical total fruiting biomass production, 23% higher light use efficiency (g/mol), 10% faster vegetative growth rate, and 36% reduced tomato waste compared to the control, which had no additional films.

25-Week Tomato Trial

Saleable Production Yield: Relative Change: +5.7% 

Fruit Biomass Production:  Results: nearly identical

Light Use efficiency (g/mol): Relative Change: +23%

Vegetable Growth Rate: Relative Change: +10% Faster

Tomato Waste: Relative Change: -36%

Results 

The objectives of this study are to evaluate the effects of an altered sunlight environment resulting from the application of a retrofit QD film on the growth and fruit production of greenhouse-grown tomato plants. The hypothesis is that a red-shifted spectrum and increased light diffusion compared to a control greenhouse compartment will overcome a reduction in light intensity to result in higher fruit production and increased vegetative growth metrics.

Throughout the experiment, vegetative growth metrics were monitored by selecting two rows with eight tomato stems in each compartment and the following parameters were measured weekly: vine length, head thickness, number of set trusses, number of flowering trusses, number of fruits set, leaf length, number of leaves, flowering speed, and ripening time. On average, plants under the QD film grew 2.1 cm/week faster than control plants, corresponding to a +9.7% increase in vine growth. The average weekly leaf length showed a 3.8-cm increase under the QD film, a +9.5% increase. Other growth metrics showed negligible differences between the plants grown in the test and control compartments. A t-test on two samples assuming unequal variances was performed on vegetative growth metrics using each measurement week for n = 23, resulting in a p-values (two tails). p-values < 0.05 indicate that the difference in the measured metric was significant over the variance in the dataset. The data show statistical significance for both vine growth and leaf length, but no statistical significance for the other metrics, although production of one additional leaf per plant on average, over the duration of the trial, was relatively close to significant. While internodal spacing was not measured in this study, the observed increase in leaf number, leaf length, and vine length indicates more vigorous growth under the QD film. Flowering speed and fruit set data for the trial can be found in the supplemental information within the trial document.

Despite a 14% reduction in overall DLI under the QD film, this altered spectral and diffuse light environment employed over tomato plants gave rise to a +10% faster vegetative growth rate (vine stem and leaf length) and -36% reduced fruit waste, resulting in an overall +5.7% improved saleable production yield per unit area. With a +4.4% increase in Far-Red light content under the QD film, the increase in vine stem elongation and leaf length are not surprising, especially when accompanied by a reduced DLI.

Plant Trial Study Details

Trial Length: 25-week plant trial

Tomato Cultivator: beefsteak variety ‘Merlice’

Quantum Dot Film: 1.25 m wide, 350 µm thick

Increased Far-Red Light Content: +4.4%

Saleable Production Yield per unit area: +5.7%