Daily Light Integral Research Articles

Interaction of the Far-red Radiation Intensity and CO2 Concentration during Early Indoor Production Stages of Red Leaf Lettuce

Although far-red (FR) radiation (700–750 nm) is beyond the traditional photosynthetically active radiation (PAR) waveband (400–700 nm), recent findings suggest equivalent or similar photosynthetic action when applied along with shorter PAR wavelengths. Potential interactive effects of FR with other environmental factors that affect photosynthesis, such as CO2, have not been widely established. In the present study, the effects of four FR photon flux densities (0, 20, 40, or 60 µmol·m−2·s−1) substituting for red (R; 600–700 nm) in combination with three CO2 concentrations (400, 800, or 1200 µmol·mol−1) were investigated at three distinctive stages of young red lettuce production. We tested whether the photomorphogenic response of young plant leaves to FR would result in a higher light interception area, as well as whether interactive effects of CO2 with FR might modify the effects of FR alone. The total photon flux density (TPFD) was maintained at a low limiting intensity of 200 µmol·m−2·s−1 (daily light integral: 11.5 mol·m−2·d−1), consistent with indoor commercial setpoints for sole-source lighting. Although blue (B; 400–500 nm) was held constant at 20 µmol·m−2·s−1 for all treatments, different combinations of R and FR were adjusted. Red oakleaf lettuce (Lactuca sativa ‘Rouxai’) used as a model test crop was harvested at 14 days (small baby greens), 18 days (standard baby greens), and 22 days (teen greens) after sowing. Crop productivity was highest at 800 µmol·mol−1 CO2 for small and standard baby stages. At the teen green stage, plant productivity was similar for the two elevated CO2 concentrations. The FR substitution for R did not affect dry biomass accumulation at all CO2 concentrations tested, thus supporting equivalent photosynthetic action. A photomorphogenic effect of FR that elongated but did not increase the area of leaves persisted from the earliest to the most mature stage of crop development tested. The FR inclusion in the light recipe resulted in longer, thinner leaves with lower biomass compared to those without FR. At higher FR fluxes, purple pigmentation reduction occurred. However, the interaction of elevated CO2 with FR radiation counteracted purple pigment reduction caused by FR alone. Substitution of FR in the light recipe for indoor production of young ‘Rouxai’ lettuce at low limiting light intensity did not improve productivity, even in combination with elevated CO2.

Trait component analysis of lettuce in response to daily light integrals at two growth stages

AbstractLittle is known about the relative impact of lighting duration and light intensity on lettuce production, as well as whether daily light integrals (DLIs) play a significant role during different plant growth stages. Four DLIs were tested: 8.64, 11.52, 12.96, and 17.28 mol m−2 day−1 as supplemental lighting, which were obtained from a combination of supplemental light intensities at 200 and 300 µmol m−2 day−1 for 12 and 16 h. Sunlight was used as control. A direct correlation was observed between DLIs and yield, total fresh weight, and total dry weight. Under supplemental lighting, relative growth rate (RGR) increased due to an increase in net assimilation rate (NAR) rather than in leaf area ratio (LAR). Plants in the seedling stage were more sensitive to an elevation in DLI than in the head stage. This was confirmed by a greater increase in NAR compared to their corresponding control under the same DLI. Increasing DLI also improved lettuce quality via reducing nitrate and increasing protein content. Higher DLIs (12.96 and 17.28 mol m−2 day−1) led to a decrease in maximum quantum yield of photosystem II and an increase in performance index. In conclusion, manipulating RGR through NAR adjustments proved to be more effective than changes in LAR, and adjusting supplemental DLI at each stage was necessary to achieve a larger NAR and, consequently, a larger RGR.

Daily Light Integral and Far-Red Radiation Influence Morphology and Quality of Liners and Subsequent Flowering and Development of Petunia in Controlled Greenhouses

Petunia stands as the top-selling bedding plant in the U.S., and improved lighting control in greenhouses holds the potential to reduce crop production time and optimize crop quality. This study investigated the impact of four distinct daily light integral (DLI) conditions with and without supplemental far-red (FR) radiation on the growth of petunia liners and subsequent development of finish plants. Two experiments were conducted in spring (9 April to 18 June 2021) and winter (28 October 2021 to 6 January 2022). Petunia cuttings were rooted in a common environment and then transferred to four greenhouse sections with different DLI treatments: 6, 9, 12, and 15 mol·m−2·d−1 for four weeks. Within each DLI condition, half of the plants were exposed to 28 μmol·m−2·s−1 supplemental FR radiation for 16 h daily (equivalent to 1.61 mol·m−2·d−1 light integral). The number of flower buds and open flowers were tracked daily. Representative liners were destructively harvested and evaluated after four weeks of lighting treatments. The remaining plants were transplanted and moved to a common DLI condition of 15 mol·m−2·d−1 for an additional three weeks before being destructively harvested and evaluated as finish plants. The primary finding reveals the promoting effect of DLI on flowering, branching, morphology, and biomass accumulation of petunia liners, with many effects persisting into the finish stage. A threshold DLI of 9 mol·m−2·d−1 was identified, as lower DLI (6 mol·m−2·d−1) resulted in extensive stem elongation, rendering the plants unmarketable. Higher DLI levels were found to be optimal in terms of flowering and morphology. Supplemental FR accelerated flowering by up to three days in the summer experiment and up to 12 days in the winter experiment. However, FR had limited impact on the number of flower buds and open flowers, branching, and shoot and root weight of the finish plants. Interactions between DLI and FR were observed on some parameters, whereby FR effects were more pronounced under lower DLI. Overall, both higher DLI and supplemental FR exhibited beneficial effects, but DLI had a more pronounced effect. Thus, DLI during petunia liner production appears more important than adding FR. This study well simulated the commercial propagation and production of petunia plants, providing practical insights for decision-making regarding lighting strategies.

Growth, phytochemical, and phytohormonal responses of basil to different light durations and intensities under constant daily light integral

Horticulture in controlled environments has been increasingly used to tackle limitations on crop production. As a crucial environmental factor, light regulate plant growth and metabolism. In the present study, basil plants were subjected to different light durations and intensities considering constant daily light integral (DLI). The lighting environment included 200, 300, and 400 µmol m− 2 s− 1 intensities for 18, 12, and 9 h, respectively. DLI amounted to 12.96 mol m− 2 d− 1 among all light treatments (LI200 for 18 h, LI300 for 12 h, and LI400 for 9 h). Half of the plants under each light treatment were exposed to 30 µmol m− 2 s− 1 of far-red light. The results indicated the general negative impact of LI400/9 on the growth of basils. Exposure to far-red light hurt the growth of the shoot, while it enhanced stem and petiole elongation. This effect was due to higher gibberellin accumulation, which resulted in shade avoidance responses. Exposure to far-red light also reduced anthocyanin and flavonoid contents, as two important nutritional components. Soluble carbohydrates increased, while storage carbohydrates decreased by increasing lighting duration/decreasing light intensity or by far-red light inclusion. The lowest antioxidant activity was detected in LI400/9. In the LI200/18, the highest level of auxin and the lowest level of cytokinin were detected, while the LI300/12 exhibited the highest level of gibberellin hormone. Low light intensity and long photoperiod enhanced plant biomass and phytochemical production and are recommended for basil production in controlled environments.

Sinusoidal LED light recipes can improve rocket edible biomass and reduce electricity costs in indoor growth environments.

Accumulation of edible biomass by crop plants relies on maintenance of a high photosynthetic rates across the photoperiod, with assimilation rate (A) generally responding to increasing light intensity in a hyperbolic fashion. In natural environments light fluctuates greatly over the course of the day, however in Controlled Environmental Agricultural (CEA) systems, light intensity can be supplemented or precisely controlled using LEDs to create near optimum conditions. In such indoor growth environments light is often delivered as a square wave and recommendations to horticulturalists are given in the form of Daily Light Integrals (DLI). However, this does not take into account the slow photosynthetic induction at the start of the photoperiod and the decline of A towards the end of the photoperiod, which has been demonstrated by several previous studies. Square wave light regimes therefore potentially cause suboptimal photosynthetic utilization of the applied lighting and waste electricity. Here we have adapted light recipes to gradually increase and decrease in intensity to take account of these findings. We demonstrate that, utilising a sinusoidal light regime capped at 250 μmol m-2 s-1, it is possible to increase edible biomass of rocket (by ca. 20%) compared to square wave delivered at 250 at the same DLI. Additionally, this can be achieved using less electricity (0.6%), therefore reducing energy costs and improving profitability. We suggest that capping maximum light intensity at 250 µmol m-2 s-1 improves the operating efficiency of PSII photochemistry (Fq'/Fm') also known as the photosynthetic efficiency by maintaining A later in the photoperiod. We show that a higher electron transfer rate (ETR) is maintained in these treatments over the photoperiod compared to higher light intensity caps, resulting in a greater Daily Photochemical Integral (DPI). We attribute this to less NPQ due to a greater sink capacity for the end products of electron transport, ATP and NADPH, as A is kept high for longer.

Design, development and experimental evaluation of a concentrator agrivoltaic system with integrated spectrally splitting Fresnel lens

The main aim of this research is to develop and evaluate a cost-effective, innovative agrivoltaic system that optimizes land use by simultaneously producing agricultural products and generating electricity on the same land. In this regard, a highly transparent concentrator agrivoltaic system that utilizes the spectral splitting property is developed. The system consists of a Fresnel lens coated with a dichroic thin film and a receiver made of silicon solar cells. In this system, the sunlight is first focused by a Fresnel lens and then split into two spectral ranges by the dichroic film: (i) photosynthetically active radiation (PAR), which passes through the film and can be utilized by the plants, and (ii) the remaining spectrum, which is reflected to the receiver to generate electricity. Cherry tomatoes and mint were planted under the system. The results showed that the receiver reduces the intensity of the focused radiation by almost 3.28 times. The optical efficiency of the receiver is 83.1% and the electricity generated is up to 18.1 watts. The photosynthetic photon flux density (PPFD) under the agrivoltaic system ranges from 229 to 778 μmol/m2. s, while the daily light integral (DLI) varies between 13.5 and 27 mol/m2.d. Cherry tomatoes grown in the shade of the agrivoltaic system are larger, heavier and have a deeper red hue, while the mint plants have larger, greener, and longer leaves. In addition, the agrivoltaic system provides cooler soil temperatures, ranging from 3.66°C to 9.53°C, and retains 5.7% to 8.4% more moisture, resulting in less water consumption. The total initial cost of the system was USD 587 and included the agricultural and photovoltaic components and maintenance. Over a period of ten years, the cumulative costs are likely to amount to USD 1,165. However, the system has strong economic potential, starting at an annual income of USD 27 and expected to exceed USD 15,000. This analysis indicates a favorable return on investment that should pay for itself within six years.

Modifying the Ambient Light Spectrum Using LED Lamps Alters the Phenolic Profile of Hydroponically Grown Greenhouse Lettuce Plants without Affecting Their Agronomic Characteristics.

The growth and development of green lettuce plants can be modulated by the prevailing light conditions around them. The aim of this study was to evaluate the effect of ambient light enrichment with different LED light spectra on agronomic characteristics, polyphenol concentration and relative gene expression of enzymes associated with polyphenol formation in 'Levistro' lettuce grown hydroponically in a Nutrient Film Technique (NFT) system for 28 days in a greenhouse. The spectra (blue:green:red:far-red) and red:blue (R:B) ratios obtained by enriching ambient light with Blue (B), White (W), Blue-Red (BR) and Red (R) LED light were B: 47:22:21:10, 0.5:1; W: 30:38:23:9, 0.8:1; BR: 33:15:44:8, 1.3:1 and R: 16:16:60:8, 3.8:1, respectively, and photosynthetically active radiation (PAR) under the different treatments, measured at midday, ranged from 328 to 336 µmoles m-2 s-1. The resulting daily light integral (DLI) was between 9.1 and 9.6 mol m-2 day-1. The photoperiod for all enrichment treatments was 12 h of light. The control was ambient greenhouse light (25:30:30:15; R:B = 1.2:1; PAR = 702 µmoles m-2 s-1; DLI = 16.9 mol m-2 day-1; photoperiod = 14.2 h of light). Fresh weight (FW) and dried weight percentage (DWP) were similar among the enrichment treatments and the control. The leaf number increased significantly under BR and R compared to B lights. The relative index of chlorophyll concentration (RIC) increased as plants grew and was similar among the enrichment treatments and the control. On the other hand, the concentration of chlorogenic acid and chicoric acid increased under BR and B lights, which was consistent with the higher relative expression of the coumarate 3-hydroxylase enzyme gene. In view of the results, it is inferred that half of the PAR or DLI is sufficient to achieve normal growth and development of 'Levistro' lettuce plants, suggesting a more efficient use of light energy under the light enrichment treatments. On the other hand, the blue and combined blue-red lights promoted the accumulation of phenolic compounds in the leaves of 'Levistro' lettuce plants.

A Systematic Literature Review on Parameters Optimization for Smart Hydroponic Systems

Hydroponics is a soilless farming technique that has emerged as a sustainable alternative. However, new technologies such as Industry 4.0, the internet of things (IoT), and artificial intelligence are needed to keep up with issues related to economics, automation, and social challenges in hydroponics farming. One significant issue is optimizing growth parameters to identify the best conditions for growing fruits and vegetables. These parameters include pH, total dissolved solids (TDS), electrical conductivity (EC), light intensity, daily light integral (DLI), and nutrient solution/ambient temperature and humidity. To address these challenges, a systematic literature review was conducted aiming to answer research questions regarding the optimal growth parameters for leafy green vegetables and herbs and spices grown in hydroponic systems. The review selected a total of 131 papers related to indoor farming, hydroponics, and aquaponics. The review selected a total of 123 papers related to indoor farming, hydroponics, and aquaponics. The majority of the articles focused on technology description (38.5%), artificial illumination (26.2%), and nutrient solution composition/parameters (13.8%). Additionally, remaining 10.7% articles focused on the application of sensors, slope, environment and economy. This comprehensive review provides valuable information on optimized growth parameters for smart hydroponic systems and explores future prospects and the application of digital technologies in this field.

Intra-canopy LED lighting outperformed top LED lighting in improving tomato yield and expression of the genes responsible for lycopene, phytoene and vitamin C synthesis

Greenhouses located at high latitudes and in cloudy areas often experience a low quality and quantity of light, especially during autumn and winter. This low daily light integral (DLI) reduces production rate, quality, and nutritional value of many crops. This study was conducted on Sakhiya RZ F1 tomato plants to evaluate the impact of LED lights on the growth and nutritional value of tomatoes in a greenhouse with low daily light due to cloudy weather. The treatments included LED growth lights in three modes: top lighting, intra-canopy lighting, and combined top and intra-canopy lighting. The results showed that although the combined top and intra-canopy lighting reached the maximum increase in tomato yield, exposure to intra-canopy LED lighting alone outperformed in tomato fruit yield increase (28.46%) than exposure to top LED lighting alone (12.12%) when compared to no supplemental lighting during the entire production year. Intra-canopy exposure demonstrated the highest increase in tomato lycopene (31.3%), while top and intra-canopy lighting exhibited the highest increase in vitamin C content (123.4%) compared to the control. The LED light treatment also had a very positive effect on the expression of genes responsible for metabolic cycles, including Psy1, LCY-β, and VTC2 genes, which had collinearity with the increase in tomato fruit production.

Morphological and Physiological Changes of Hydroponic Lettuce Grown in Varying Potassium Concentrations and an Adaptive Lighting Control System

We investigated the growth dynamics of hydroponic lettuce (Lactuca sativa) driven by the influence that potassium (K+) has on crop growth. This study aimed to determine whether increased K+ concentrations under different daily light integrals (DLIs) in a hydroponic system will boost growth of greenhouse lettuce. This study was conducted within a controlled glass greenhouse environment with varying DLIs achieved by integrating an adaptive lighting control system over a 16-hour photoperiod. We used three K+ treatments of 200, 400, or 600 mg⋅L−1 K+ and six DLI lighting treatments of 11.1, 12.9, 14.6, 15.9, 16.9, and 17 mol⋅m−2⋅d−1. We found that increasing K+ did not increase shoot dry weight, leaf area, or specific leaf area with increasing DLIs. Although K+ and DLI had an interacting effect on the root dry weight fraction, leaf chlorophyll content, and quantum yield of photosystem II, the K+ treatments did not increase or decrease with increasing DLIs. The influencing factor was DLI, which led to increases in shoot dry weight and leaf area, whereas a decrease in specific leaf area was observed with increasing DLIs. Ultimately, adding supplemental concentrations of K+ did not enhance lettuce growth, nor did these effects show any increase with increasing DLIs.

Variations in photoperiods and their impact on yield, photosynthesis and secondary metabolite production in basil microgreens

BackgroundThe effects of different photoperiods on plant phytochemical synthesis can be improved by adjusting the daily light integral. Photoperiod is one of the most important environmental factors that control growth, plant’s internal rhythm and the synthesis of secondary metabolites. Information about the appropriate standard in terms of photoperiod for growing basil microgreens as one of the most important medicinal plants is limited. In this study, the effects of five different photoperiods, 6 (6 h × 3 cycles), 8 (8 h × 2 cycles), 16, 18, and 24 h day− 1 on the yield, photosynthesis and synthesis of secondary metabolites of three cultivars and one genotype of basil microgreens in floating system were evaluated. The purpose of this research was to determine the feasibility of using permanent light in growing basil microgreens and to create the best balance between beneficial secondary metabolites and performance.ResultsThe results showed that the effects of photoperiod and cultivar on all investigated traits and their interaction on photosynthetic pigments, antioxidant capacity, total phenolic compounds, proline content and net photosynthesis rate were significantly different at the 1% level. The highest levels of vitamin C, flavonoids, anthocyanins, yield and antioxidant potential composite index (APCI) were obtained under the 24-h photoperiod. The highest antioxidant capacity was obtained for the Kapoor cultivar, and the highest total phenolic compound and proline contents were measured for the Ablagh genotype under a 24-h photoperiod. The highest yield (4.36 kg m− 2) and APCI (70.44) were obtained for the Ablagh genotype. The highest nitrate content was obtained with a photoperiod of 18 h for the Kapoor cultivar. The highest net photosynthesis rate was related to the Violeto cultivar under a 24-hour photoperiod (7.89 μmol CO2 m− 2 s− 1). Antioxidant capacity and flavonoids had a positive correlation with phenolic compounds and vitamin C. Yield had a positive correlation with antioxidant capacity, flavonoids, vitamin C, APCI, and proline.ConclusionsUnder continuous light conditions, basil microgreens resistance to light stress by increasing the synthesis of secondary metabolites and the increase of these biochemical compounds made basil microgreens increase their performance along with the increase of these health-promoting compounds. The best balance between antioxidant compounds and performance was achieved in continuous red + blue light. Based on these results, the use of continuous artificial LED lighting, due to the increase in plant biochemical with antioxidant properties and yield, can be a suitable strategy for growing basil microgreens in floating systems.

Sole-source Lighting of Lettuce that Increased Yield Had No Negative Effect on Postharvest Longevity

The effects of sole-source lighting on the growth and yield of hydroponically grown lettuce have been extensively studied, but research of postharvest performance is limited. We grew frill-leaf lettuce (Lactuca sativa) ‘Green Incised’ and ‘Hydroponic Green Sweet Crisp’ hydroponically in an indoor vertical research farm under daily light integrals (DLIs) of 12 or 18 mol⋅m−2⋅d−1 and the following three ratios of blue (B; 400–499 nm) and red (R; 600–699 nm) light from light-emitting diode fixtures: B5:R95, B20:R80, and B35:R65. We postulated that biomass accumulation would increase with the DLI and decrease with the B light fraction, and that postharvest longevity would increase with the DLI and the B light fraction. As expected, shoot fresh weight, leaf length and width, leaf number, and relative chlorophyll content (SPAD; ‘Green Incised’ only) decreased as the proportion of B light increased from 5% to 35%. Decreasing the DLI from 18 to 12 mol⋅m−2⋅d−1 reduced the shoot fresh weight and leaf number of both cultivars. Leaves of ‘Green Incised’ were up to 27% wider under B5:R95 and 60% longer under B5:R95 at 12 mol⋅m−2⋅d−1 than those under treatments with a higher DLI or more B light. The shoot fresh weight of ‘Hydroponic Green Sweet Crisp’ was greatest when grown under B5:R95 at 18 mol⋅m−2⋅d−1 and decreased as B light increased or DLI decreased. At the time of harvest, leaves of each cultivar and treatment were placed in clamshells and stored at 7 °C in darkness and evaluated for decay. ‘Green Incised’ that grew under B35:R65 and a DLI of 18 mol⋅m−2⋅d−1 had the shortest storage life, with 9.5 d and 11.4 d for replications 1 and 2, respectively, which were ∼2.5 to 4.0 d and 1.4 to 3.6 d earlier, respectively, than the storage life of lettuce grown under other treatments. In contrast, ‘Hydroponic Green Sweet Crisp’ was not influenced by light quality or DLI and had a storage life of 12.6 to 13.3 d and 13.5 to 14.3 d for replications 1 and 2, respectively. Therefore, a B light fraction between 5% and 20% and a DLI of 18 mol⋅m−2⋅d−1 produced high-yielding frill-leaf lettuce with a relatively long storage life.

Daily Light Integral, but Not the Photoperiod, Influences the Time to Flower and Finished Quality of Dianthus Specialty Cut Flowers

Because of the burgeoning year-round demand, greenhouse growers across the United States are increasingly becoming interested in producing specialty cut flowers for local and regional markets. However, outdoor or high tunnel production is not possible year-round in northern latitudes because of low temperatures and radiation intensities experienced during the winter and early spring. Additionally, natural short days in these seasons can limit which photoperiodic crops can be grown. Thus, our objectives were to quantify the influence of the photoperiod and daily light integral (DLI) on greenhouse-grown dianthus ‘Amazon Neon Cherry’ and ‘Amazon Rose Magic’ (Dianthus barbatus interspecific) cut flowers during the young plant and finishing stages. Seeds of both cultivars were sown under 9-, 10-, 11-, 12-, 13-, 15-, or 16-hour photoperiods and a DLI of either ≈5 or 10 mol⋅m−2⋅d−1. After 4 weeks, seedlings from several young-plant photoperiods were distributed across 11-, 12-, 13-, 14-, 15-, or 16-hour photoperiods or a 4-hour night interruption (NI) under a DLI of either ≈5 (low) or 14 (moderate) mol⋅m−2⋅d−1 for finishing. The young plant photoperiod generally had a statistical, but not commercial, influence on development and finished cut flower quality, whereas a 16-h finishing photoperiod marginally hastened development compared with an 11-hour finishing photoperiod. Additionally, stems were 11 to 13 cm longer when finished under the 16-hour photoperiod compared with those finished under the 11-hour photoperiod. Day length minimally influenced the time to flower and harvest, indicating a day-neutral flowering response. However, plants finished under a moderate DLI reached visible flower bud and were harvestable 9 to 10 days earlier than those finished under a low DLI. Additionally, ≈99% of cut flowers finished under a moderate DLI were harvestable, whereas only up to 32% and 57% of dianthus ‘Amazon Rose Magic’ and ‘Amazon Neon Cherry’, respectively, finished under a low DLI were harvestable. Although finished stem lengths were comparable between DLI treatments, cut flower stems were up to 29.6% thicker under a moderate DLI. These findings indicate that high-quality greenhouse-grown dianthus ‘Amazon Neon Cherry’ and ‘Amazon Rose Magic’ cut flowers can be produced when grown under any photoperiod between 9 and 16 hours for 4 weeks (during the young plant stage) and finished under any photoperiod between 11 and 16 hours or a 4-hour NI during finishing. If longer stems are desired, then plants can be finished under a 16-hour photoperiod. Young plants should be grown under a moderate DLI ≥10 mol⋅m−2⋅d−1 to promote biomass accumulation and reduce the young plant crop time. Additionally, plants should be finished under a moderate DLI ≥14 mol⋅m−2⋅d−1 to reduce crop time and increase stem thickness and yield.

Temporal Separation of Red and Blue Photons Does Not Increase Photon Capture or Yield of Lettuce

Temporal separation of red (R) and blue (B) (alternating R/B) photons has been reported to increase leaf area, photon capture, and yield of lettuce compared with delivering both colors together (concurrent R+B). We grew three diverse lettuce cultivars (Grand Rapids, Rex, and Red Sails) under concurrent R+B photons (9/1 ratio) and alternating R/B photons (9/1 ratio) under an equal daily light integral (DLI) of either 8.6 or 23 mol⋅m−2⋅d−1. Contrary to five previous studies, we found no increase in either leaf area or fresh mass and dry mass in any of the alternating R/B photon treatments compared with concurrent R+B photons. In fact, at a DLI at 8.6 mol⋅m−2⋅d−1, alternating R/B photons decreased the dry mass of ‘Grand Rapids’ and ‘Rex’ lettuce by 38% and 17%, respectively. Two previous studies reported that photosynthetic rates increased with alternating R/B photons; however, we found that the net assimilation rate was generally decreased by alternating R/B photons. An analysis of images obtained from automated digital photography revealed that the relative expansion rate of leaves was 61% higher during intervals of pure B rather than intervals of pure R photons at the same photosynthetic photon flux density; however, this did not result in a higher leaf area compared with concurrent R+B photons. Overall, our studies do not indicate that alternating R/B photons increase lettuce leaf area or yield compared with concurrent R+B photons.

Luminescent Quantum Dot Films Increase the Radiation Capture and Yield of Lettuce and Sweet Basil Compared to a Traditional/Neutral-density Greenhouse Glazing

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.

Shining light on diurnal variation of non‐photochemical quenching: Impact of gradual light intensity patterns on short‐term NPQ over a day

AbstractMaximal sunlight intensity varies diurnally due to the earth's rotation. Whether this slow diurnal pattern influences the photoprotective capacity of plants throughout the day is unknown. We investigated diurnal variation in NPQ, along with NPQ capacity, induction, and relaxation kinetics after transitions to high light, in tomato plants grown under diurnal parabolic (DP) or constant (DC) light intensity regimes. DP light intensity peaked at midday (470 μmol m‐2 s‐1) while DC stayed constant at 300 μmol m‐2 s‐1 at a similar 12‐hour photoperiod and daily light integral. NPQs were higher in the morning and afternoon at lower light intensities in DP compared to DC, except shortly after dawn. NPQ capacity increased from midday to the end of the day, with higher values in DP than in DC. At high light ΦPSII did not vary throughout the day, while ΦNPQ varied consistently with NPQ capacity. Reduced ΦNO suggested less susceptibility to photodamage at the end of the day. NPQ induction was faster at midday than at the start of the day and in DC than in DP, with overshoot occurring in the morning and midday but not at the end of the day. NPQ relaxation was faster in DP than in DC. The xanthophyll de‐epoxidation state and reduced demand for photochemistry could not explain the observed diurnal variations in photoprotective capacity. In conclusion, this study showed diurnal variation in regulated photoprotective capacity at moderate growth light intensity, which was not explained by instantaneous light intensity or increasing photoinhibition over the day and was influenced by acclimation to constant light intensity.

Synergetic light and cytokinin treatments mitigate the recombinant protein yield depression induced by high-density cultivation of hydroponically-grown Nicotiana benthamiana.

Plant molecular farming is currently operating a transition from soil-based cultures towardhydroponic systems. In this study, we designed a whole-plant NFT (nutrient film technique) platform for the transient expression of influenza virus-like particles harboring hemagglutinin H1 proteins in Nicotiana benthamiana. In particular, we examined the effects of plant density during the post-infiltration expression phase on plant growth and H1 yield in relation to the daily light integral (DLI) received by the crop and the exogenous application of 6-BAP cytokinin (CK). We expected from previous work that high DLI and CK treatments would stimulate the development of highly productive leaves on axillary (secondary) stems and thereby improve the H1 yield at the whole-plant scale. Increasing plant density from 35.7 to 61 plants m-2 during the post-infiltration phase significantly decreased the proportion of axillary leaf biomass by 30% and H1 yield per plant by 39%, resulting in no additional yield gain on a whole-crop area basis. Adding CK to the recirculated nutrient solution decreased the harvested leaf biomass by 31% and did not enhance the relative proportion of S leaves of the plants as previously reported with foliar CK application. There was a 36% increase in H1 yield when doubling the DLI from 14 to 28 mol m-2 s-1, and up to 71% yield gain when combining such an increase in DLI with the hydroponic CK treatment. Contrary to our expectations, leaves located on the main stem, particularly those from the upper half of the plant (i.e.,eighth leaf and above), contributed about 80% of total H1 yield. Our study highlights the significantly different phenotype (~30% less secondary leaf biomass) and divergent responses to light and CK treatments of NFT-grown N. benthamiana plants compared to previous studies conducted on potted plants.

Supplemental greenhouse lighting increased the water use efficiency, crop growth, and cutting production in Cannabis sativa.

The expanding cannabis production sector faces economic challenges, intensified by freshwater scarcity in the main US production areas. Greenhouse cultivation harnesses sunlight to reduce production costs, yet the impact of greenhouse light levels on crucial production components, such as plant growth, branching, and water use efficiency (WUE), remains poorly understood. This study aimed to assess the effects of combined sunlight and supplemental lighting on the crop's main production components and leaf gas exchange of Cannabis sativa 'Suver Haze' in the vegetative stage. Within a greenhouse, LED lighting provided at intensities of ~150, 300, 500, and 700 µmol m-2 s-1 (18-hour photoperiod), combined with solar radiation, resulted in average daily light integrals of 17.9, 29.8, 39.5, and 51.8mol m-2 d-1. Increasing light levels linearly increased biomass, leaf area, and the number of branches per plant and square meter, with respective rates of 0.26g, 32.5 cm2, and 0.41 branches per mole of additional light. As anticipated, crop evapotranspiration increased by 1.8-fold with the increase in light intensity yet crop WUE improved by 1.6-fold when comparing the lowest and highest light treatments. Moreover, water requirements per unit of plant biomass decreased from 0.37 to 0.24 liters per gram when lighting increased from ~18 to 52mol m-2 d-1, marking a 35% reduction in evapotranspiration. These results were supported by increments in leaf photosynthesis and WUE with light enhancement. Furthermore, our findings indicate that even 52mol m-2 d-1 of supplemental lighting did not saturate any of the crop responses to light and can be economically viable for cannabis nurseries. In conclusion, light supplementation strongly enhanced photosynthesis and plant growth while increasing WUE. Additionally, a comprehensive discussion highlights the shared physiological mechanisms governing WUE in diverse plant species and their potential for water conservation under enhanced lighting conditions.

Optimizing Sowing Density for Parsley, Cilantro, and Sage in Controlled Environment Production: Balancing Productivity and Plant Quality

Sowing density is a key management practice influencing productivity and quality of leafy greens and culinary herbs grown in controlled environments. However, research-based information on optimal density is limited for many culinary herbs. This greenhouse study aimed to quantify sowing density impacts on biomass output, individual plant growth, and morphological traits in hydroponically produced ‘Giant of Italy’ parsley (Petroselinum crispum), ‘Santo’ cilantro (Coriandrum sativum), and sage (Salvia officinalis). Seedlings were grown in phenolic foam cubes with 1, 5, 10, 15, or 20 seeds per cell, transplanted into an ebb-and-flow hydroponic system in a glass-glazed greenhouse with 23 °C target average daily temperature, 16-hour photoperiod, a target daily light integral of 13 mol·m−2·d−1, and harvested at 16 to 28 d after transplanting depending on species. ‘Giant of Italy’ parsley and ‘Santo’ cilantro fresh weight per cell increased quadratically by 274% (57.3 g) and 305% (19 g), respectively, as sowing density increased from 1 to 15 seeds per cell, then plateaued as density further increased. Sage fresh weight plateaued at 10 seeds per cell with an increase of 225% (29.2 g) compared with 1 seed per cell. Cilantro and sage dry weight per cell plateaued at 14 and 8 seeds per cell, respectively, and parsley dry weight quadratically increased as sowing density rose up to 20 seeds per cell. Although fresh and dry weight increased, individual plant height, stem diameter, and individual plant dry weight exhibited linear or quadratic declines as sowing density increased, indicating higher sowing densities restricted individual plant growth. In summary, as sowing density increased, fresh and dry weight per cell generally increased but individual plant quality decreased. For the greatest fresh and dry weight, 20, 18, and 10 seeds per cell should be sown for parsley, cilantro, and sage, respectively. However, to balance fresh weight and crop quality, our results suggest sowing density (seeds per cell) targets of 16 seeds for parsley, 18 seeds for cilantro, and 10 seeds for sage.

Estimation of maximum photovoltaic cover ratios in greenhouses based on global irradiance data

In this study, a method for estimating the maximum PV (photovoltaic) cover ratio for plastic greenhouses based on various years of global horizontal irradiance (GHI) data is presented and illustrated with an exemplary site in southeastern Spain. CAMS (Copernicus Atmosphere Monitoring Service) GHI data from 2005 to 2023 were analyzed to estimate the DLI (daily light integral) inside the greenhouses for various PV coverage ratios with East-West or North-South orientation. The conversion from GHI to photosynthetically active radiation is performed with the usage of a regression model from literature based on satellite and measurement data. The shading effect of the PV cover is estimated with a regression model from literature based on radiation distribution simulations in different greenhouse types. The maximum PV cover ratio was derived for different minimal DLI thresholds, corresponding to different crops. The proposed methodology has been tested for the Almería region in southeastern Spain which is characterized by high solar irradiance and can be applied also to other regions with similar climatic conditions. With a required DLI of at least 12 mol/m2/day, a theoretical maximum PV coverage of about 44% is acceptable even in December at the studied site for East-West orientation, while it reaches up to 100% (June) during the year. Further, the maximum PV cover ratios for a DLI threshold range have been calculated and compared with experimental results for plastic greenhouses from literature. In 87.2% of the case studies analyzed from literature, the proposed method showed an agreement in the estimation of the effect of PV shading ratios on marketable crop yields. The study indicates that significant PV cover ratios are theoretically possible even for light demanding crops considering DLI thresholds only and can help to select a useful PV cover ratio in PV greenhouses.