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Utilizing quantum dot (QD) luminescent films as a greenhouse covering material is an innovative method of modifying the greenhouse light spectrum. The QD films convert a portion of high-energy ultraviolet and blue photons to lower-energy photons. Previous research has shown that the application of QD films in greenhouses led to improved crop yields of red lettuce and tomatoes. However, the underlying mechanism of the yield increases has not been fully explored. We quantified the effects of solar spectral shifts attributable to QD films on plant morphology, radiation capture, and, subsequently, crop yield. Green and red leaf lettuces and basil were grown in a greenhouse under four treatments: regular-concentration QD film (reg QD film); high-concentration QD film (high QD film); color-neutral polyethylene (PE) film; and control treatment without any films. Compared to the reg QD film, the high QD film converted a higher fraction of blue photons into longer-wavelength photons, resulting in enhanced leaf expansion, stem elongation, and shoot fresh weight of red lettuce and basil compared with those grown under the PE film without spectral modifications. No significant growth differences were observed between the control and high QD film treatments of red lettuce and basil despite a 23% reduction in the average daily light integral (DLI) under the high QD film treatment. Compared to that grown under the control treatment, green lettuce grown under the high QD film treatment had a similar total leaf area but reduced shoot biomass; this was likely associated with reductions in leaf thickness and chlorophyll content. In contrast, the red lettuce showed more pronounced leaf expansion and reduced leaf anthocyanin content under the high QD film, which likely helped to offset the reduction in DLI. Overall, our results indicated that modifying the solar spectrum with QD films as greenhouse covering material could result in improved crop radiation capture and yield in greenhouse production of lettuce and basil. However, the spectral shifts caused by the QD films may affect crop quality attributes, such as anthocyanin levels and the production of other beneficial secondary metabolites. This effect on crop quality should be carefully considered and requires further study.

UV-A radiation (320–400 nm) is an abiotic stressor that may be used to enhance the production of beneficial secondary metabolites in crops such as leafy vegetables. However, tradeoffs between enhanced phytochemical contents and overall growth/yield reductions have been reported. The responses varied depending on the UV-A intensity, spectral peak, exposure time, species, and varieties. We quantified the changes in growth, morphology, photosynthesis, and phenolic contents of sweet basil grown under a base red/blue/green LED light with four supplemental UV-A intensity treatments (0, 10, 20, and 30 W·m−2) in an indoor environment over 14 days. The objective was to determine whether UV-A radiation could be utilized to improve both yield and quality of high-value sweet basil in a controlled production environment. Biomass harvested at 14 days after treatment (DAT) was highest under mild-intensity UV-A treatment of 10 W·m−2 and lowest under high-intensity UV-A treatment of 30 W·m−2. The total leaf area and the number of leaves were significantly lower under the 30 W·m−2 treatment than under the 10 and 20 W·m−2 treatments at 14 DAT. The maximum quantum efficiency of photosystem II (PSII) for photochemistry (Fv/Fm) showed a gradual decrease under the 20 and 30 W·m−2 treatments from 3 to 14 DAT, whereas Fv/Fm remained relatively constant under the 0 and 10 W·m−2 treatments over the entire 14 days. The leaf net photosynthesis rate showed a significant decrease of 17.4% in the 30 W·m−2 treatment compared to that in the 10 W·m−2 treatment at 14 DAT. Phenolic contents (PAL enzyme activity, total phenolic concentration, and antioxidant capacity) were the highest under the 20 W·m−2 treatment, followed by the 10, 30, and 0 W·m−2 treatments. Overall, our results indicate that the biomass production and accumulation of beneficial phenolic compounds in sweet basil varied depending on the intensity and duration of UV-A application. Mild UV-A radiation (10–20 W·m−2) can be a beneficial stressor to improve sweet basil yield and quality over relatively long-term cultivation.

Sulfur (S) is an essential plant nutrient that regulates plant growth and metabolism. However, S is often absent from certain one-bag hydroponic fertilizers designed to provide a complete and balanced mixture of nutrients. We quantified the effects of S supplementation on the growth, morphology, and photosynthesis of lettuce grown in a deep-water culture hydroponic system. Two lettuce (Lactuca sativa) cultivars, green butterhead Rex and red oakleaf Rouxai, were grown using a prepackaged fertilizer specially formulated for reverse osmosis (RO) and other low-alkalinity water sources. The base nutrient solution was mixed using Jack's FeED 12-4-16 fertilizer and RO water at a nitrogen concentration of 100 mg.L21 (control). Three S supplementation treatments were implemented over a 4-week production period: 10 mg.L21 supplemental S (provided using MgSO4); 20 mg.L21 supplemental S (MgSO4); and a treatment using H2SO4 (instead of nitric acid) for pH adjustment. In both lettuce cultivars, shoot fresh and dry mass, total leaf area, leaf photosynthetic rate, total chlorophyll content, and leaf S concentration with all three S supplementation treatments increased significantly compared with those of the control. In contrast, the ratio of shoot dry mass to fresh mass, root dry mass, and percentage of root dry mass (i.e., root dry mass/total shoot and root mass) were significantly higher with the control treatment. Notably, 'Rouxai' lettuce grown in the control treatment had intense red coloration with a 216.6% to 288.9% increase in the anthocyanin index. There were no statistical differences in any of the growth and morphological parameters among the three S supplementation treatments. Overall, we observed significantly enhanced lettuce growth and photosynthetic performance with S supplementation, resulting in a 144.0% to 215.9% increase in shoot fresh mass in the two cultivars compared with the control. Thus, we recommend that at least 10 mg.L21 of S should be supplemented when growing lettuce hydroponically to ensure optimal plant growth, especially when S is absent or low in the fertilizer and water source. [ABSTRACT FROM AUTHOR]

Automated irrigation systems based on soil moisture sensor measurements can reduce water and fertilizer use while adequately meeting plant water requirements. In this study, the effects of substrate volumetric water content (θ, v/v) on the flowering of 17-month-old Doritaenopsis Queen Beer 'Mantefon' (from the time of deflasking) were examined. The plants were transplanted in plastic pots (10.5 cm width × 9.5 cm height) filled with sphagnum moss and the θ of sphagnum moss was maintained at 0.2, 0.3, 0.4, or 0.5 m³·m-3 using an automated drip irrigation system. Plants grown at a θ threshold of 0.2 m³·m-3 had thinner leaves and lower SPAD value than those grown at higher θ thresholds. The net CO2 uptake of the uppermost fully expanded leaf increased with increasing θ between 0.2 and 0.4 m³·m-3, but there was no significant difference in the net CO2 uptake between plants grown at 0.4 and 0.5 m³·m-3 thresholds. The number of flower buds at the time of the first open flower was lower in plants grown at θ thresholds of 0.2 and 0.3 m³·m-3 as compared with that in the plants grown at 0.4 and 0.5 m³·m-3 thresholds. Early flower abscission, flower bud dropping, and flower senescence during the 2 weeks after flowering occurred in 55% and 30% of the plants at 0.2 and 0.3 m³·m-3 thresholds, respectively, whereas plants at θ thresholds of 0.4 and 0.5 m³·m-3 had negligible flower abscission. Although vegetative growth parameters were similar among θ thresholds of 0.3 m³·m-3 or higher, plants grown at a θ threshold of 0.3 m³·m-3 produced fewer flowers than those grown at 0.4 and 0.5 m³·m-3 thresholds. During the 83-day experimental period, the system irrigated the plants ≈0.79, 1.93, 2.46, and 2.84 L/pot at θ thresholds of 0.2, 0.3, 0.4, and 0.5 m³·m-3, respectively. Overall, 0.4 m³·m-3 was considered to be an optimal threshold θ level for producing high-quality Doritaenopsis Queen Beer 'Mantefon' during the flowering period with most efficient water use. [ABSTRACT FROM AUTHOR]