Low Photosynthetic Photon Flux Research Articles

Plant responses to UV-A1 radiation are genotype and background irradiance dependent

A large fraction of solar UV radiation that reaches the Earth’s surface is in the UV-A1 waveband (350–400 nm). Despite its prevalence, it is unknown how strongly plant genotypes differ in their response to UV-A1, and how their responses to UV-A1 depend on the photosynthetic photon flux density (PPFD). We grew several horticultural (tomato, cucumber, and two lettuce cultivars) and a model species (Arabidopsis thaliana) under low (LL; 150 μmol m−2 s−1) and high PPFD (HL; 550 μmol m−2 s−1), each of which was paired with UV-A1 (peaking at 365 nm) at irradiances of 0, 20, and 100 µmol m−2 s−1. Arabidopsis was the most strongly affected, as under LL, addition of UV-A1 resulted in early flowering, changes in leaf shape, and strong decreases in shoot dry weight (∼40%). In all species, UV-A1 exposure induced photoinhibition (low Fv/Fm) only under LL, but not under HL. Under HL, exposure to UV-A1 also induced strong decreases in the concentrations of UV-absorbing compounds and anthocyanins in arabidopsis (Columbia-0) and lettuce cv. Klee, but not in other genotypes. Altogether, we found that UV-A1 had only mild effects on the morphology of horticultural species (e.g., petiole angle, leaf shape and curvature), did not significantly alter biomass or leaf biochemical compound concentrations, and these effects were almost entirely unaffected by background PPFD. In contrast, these traits were strongly affected by UV-A1 in arabidopsis, highlighting the importance of not relying solely on model species when exploring environmental effects on plants. We conclude that plant responses to UV-A1 radiation depend on genotype and background irradiance.

Light response of gametophyte in Adiantum flabellulatum: transcriptome analysis and identification of key genes and pathways.

Light serves not only as a signaling cue perceived by plant photoreceptors but also as an essential energy source captured by chloroplasts. However, excessive light can impose stress on plants. Fern gametophytes possess the unique ability to survive independently and play a critical role in the alternation of generations. Due to their predominantly shaded distribution under canopies, light availability becomes a limiting factor for gametophyte survival, making it imperative to investigate their response to light. Previous research on fern gametophytes' light response has been limited to the physiological level. In this study, we examined the light response of Adiantum flabellulatum gametophytes under different photosynthetic photon flux density (PPFD) levels and identified their high sensitivity to low light. We thereby determined optimal and stress-inducing light conditions. By employing transcriptome sequencing, weighted gene co-expression network analysis, and Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses, we identified 10,995 differentially expressed genes (DEGs). Notably, 3 PHYBs and 5 Type 1 CRYs (CRY1s) were significantly down-regulated at low PPFD (0.1 μmol m-2 s-1). Furthermore, we annotated 927 DEGs to pathways related to photosynthesis and 210 to the flavonoid biosynthesis pathway involved in photoprotection. Additionally, we predicted 34 transcription factor families and identified a close correlation between mTERFs and photosynthesis, as well as a strong co-expression relationship between MYBs and bHLHs and genes encoding flavonoid synthesis enzymes. This comprehensive analysis enhances our understanding of the light response of fern gametophytes and provides novel insights into the mechanisms governing their responses to light.

Influence of Day and Night Temperature and Radiation Intensity on Growth, Quality, and Economics of Indoor Green Butterhead and Red Oakleaf Lettuce Production

Lettuce (Lactuca sativa) is among the most consumed vegetables worldwide and is primarily field-grown; however, indoor agriculture enables year-round, precise production. Through precise manipulation of the mean daily temperature (MDT) and photosynthetic photon flux density (PPFD), crop color, morphology, and yield can be altered. Therefore, we quantified how MDT and PPFD interact and developed models predicting yield and economic viability. Eleven days after sowing, green butterhead lettuce ‘Rex’ and red oakleaf lettuce ‘Rouxaï RZ’ were transplanted into six deep-flow hydroponic tanks with day/night and MDTs of 22/15 °C (20 °C), 25/18 °C (23 °C), or 28/21 °C (26 °C), under light-emitting diodes providing a low or high PPFD of 150 or 300 µmol·m−2·s−1 for 17-h·d−1. As PPFD increased, shoot fresh mass (SFM) of ‘Rex’ increased by 29% (33.4 g). SFM of ‘Rouxaï RZ’ and shoot dry mass (SDM) of both cultivars was influenced by the interaction of MDT and PPFD. The greatest ‘Rouxaï RZ’ SFM (158.8 g) and SDM (6.42 g) were recorded at >20 °C MDT under the high PPFD; the lowest SFM (76.0 g) and SDM (3.17 g) occurred at 20 °C under the low PPFD. Similarly, ‘Rex’ SDM was greatest (7.36 g) and lowest (3.78 g) under the aforementioned MDTs and PPFDs. Increasing from the low to high PPFD increased tipburn incidence on ‘Rouxaï RZ’ from 0 to 25% and ‘Rex’ from 47 to 100%. ‘Rouxaï RZ’ had darker yellow-red foliage at lower MDTs under the high PPFD. A high MDT and low PPFD resulted in a lighter green. Finally, for the greatest SFM, while reducing energy costs as interpreted from the economic analysis, we recommend growing ‘Rex’ and ‘Rouxaï RZ’ under a PPFD of 150 and 300 µmol·m−2·s−1, respectively, at an MDT of 23 to 26 °C depending on the cost of temperature control.

Photosynthetic responses of Eulophia dentata, Bletilla formosana, and Saccharum spontaneum under various photosynthetic photon flux density conditions

This study aimed to determine the photosynthetic performance and differences in chlorophyll fluorescence (ChlF) parameters between Eulophia dentata and its companion species Bletilla formosana and Saccharum spontaneum when subjected to different photosynthetic photon flux density (PPFDs). Leaf surfaces were then illuminated with 50, 100 (low PPFDs), 300, 500, 800 (moderate PPFDs); 1,000; 1,500; and 2,000 (high PPFDs) μmol m-2.s-1, and the ChlF parameters were measured during the whole process. Increasing nonphotochemical quenching of ChlF and decreasing potential quantum efficiency of PSII, actual quantum efficiency of PSII, and quantum efficiency ratio of PSII in dark recovery from 0-60 min were observed in all leaves. A significant and negative relationship was detected between energy-dependent quenching (qE) and photoinhibition percent in three species under specific PPFD conditions, whereas a significant and positive relationship was detected between photoinhibitory quenching (qI) and photoinhibition percent. The qE and qI can be easily measured in the field and provide useful ecological indexes for E. dentata species restoration, habitat creation, and monitoring.

Effects of supplementing green light to red and blue light on the growth and yield of lettuce in plant factories

Red and blue light-emitting diodes (LEDs) are widely used as light sources in plant factories. Plants grown under red and blue light only produces purplish-gray light environment where plant leaves look purplish-gray. Under that light condition, it makes difficulty to check health status of plants specially insect and disease infected leaves by human naked eye. But the leaf color turns green when inspected under full spectrum of light environment. In this regard, the use of green light to red and blue create a white light environment which is congenial for the grower. Likewise, CO2 assimilation of green light is usually lower compared to red and blue light because of its lower absorptance under low PPFD. But at higher PPFD (≥500 µmol m−2 s−1), green light shows higher CO2 assimilation rate compared to red and blue light through uniform distribution of full spectrum (red, green and blue) of light into the plant canopy and the lower leaves. In this study, optimal intensity of green light supplementation to the red and blue LEDs was evaluated for better growth, yield, and quality of lettuce. In experiment I and II, green light was added with red and blue light where total photosynthetic photon flux density (PPFD) was increased for the additional green light. In experiment III, 0, 10, 20, 30, 40, 50, 60, 70, 80 µmol m−² s−1 green (G) light was supplemented to 235 µmol m−² s−1 red (R) and 59 µmol m−² s−1 blue (B) light maintaining 294 µmol m−² s−1 total PPFD. Lettuce plants were cultivated hydroponically in three-step vertical grow beds using half-strength of Enshi nutrient solution. The temperature was maintained at 20 ± 2 °C at day and night and the day length was 16 h. In experiment I and II, the fresh mass of lettuce grown under the combination of R, B and G LEDs was found higher than those cultivated under R and B LEDs only. In experiment III, lettuce plants produced high fresh masses when 30 µmol m−² s−1 of G light was supplemented with R and B lights maintaining the ratio 211:30:53 (R:G:B). By supplementing excessive G light (≥50 µmol m−² s−1) with R and B lights, lettuce shoot fresh mass declined. The various combinations of irradiation of R, G and B LED did not significantly affect the contents of ascorbic acid and mineral nutrients. It is recommended to supplement 30 µmol m−2 s−1 of green light to red and blue light for a higher yield of lettuce. Therefore, the suitable LED (R:G:B) combination would be 72% of red, 10% of green and 18% of blue for lettuce cultivation following recycled hydroponics in plant factories.

Effects of Daily Light Integral on Compact Tomato Plants Grown for Indoor Gardening

Our objective was to characterize the growth, physiological responses, fruit yield, and quality of tomato (Solanum lycopersicum L.) plants grown under different daily light integrals (DLIs) and photoperiods. In experiment I, nine compact tomato cultivars were grown indoors using broadband white light-emitting diode (LED) fixtures. Plants were grown under low (10.4 mol·m−2·d−1) and high (18.4 mol·m−2·d−1) DLIs with 12 and 16 h photoperiods, respectively, and two intermediate DLIs of 13.8 mol·m−2·d−1 with either 12 or 16 h photoperiods. In experiment II, three compact tomato cultivars were grown under the same low DLI with either 8 or 12 h photoperiods, and the same high DLI with either 12 or 16 h photoperiods. Generally, higher DLIs decreased plant growth and increased the fruit yield. Changing the DLI delivery strategy by adjusting the photoperiod and photosynthetic photon flux density (PPFD) did not have major effects on the growth, yield, and fruit quality of the compact tomato plants evaluated in this study, even though net photosynthesis increased under higher PPFDs in experiment II. Although several cultivars were affected by intumescence, only two cultivars showed treatment responses, for which the severity was generally higher in lower PPFDs using the same DLI.

Elevated light intensity compensates for nitrogen deficiency during chrysanthemum growth by improving water and nitrogen use efficiency

Identifying environmental factors that improve plant growth and development under nitrogen (N) constraint is essential for sustainable greenhouse production. In the present study, the role of light intensity and N concentrations on the biomass partitioning and physiology of chrysanthemum was investigated. Four light intensities [75, 150, 300, and 600 µmol m−2 s−1 photosynthetic photon flux density (PPFD)] and three N concentrations (5, 10, and 15 mM N L−1) were used. Vegetative and generative growth traits were improved by increase in PPFD and N concentration. High N supply reduced stomatal size and gs in plants under lowest PPFD. Under low PPFD, the share of biomass allocated to leaves and stem was higher than that of flower and roots while in plants grown under high PPFD, the share of biomass allocated to flower and root outweighed that of allocated to leaves and stem. As well, positive effects of high PPFD on chlorophyll content, photosynthesis, water use efficiency (WUE), Nitrogen use efficiency (NUE) were observed in N-deficient plants. Furthermore, photosynthetic functionality improved by raise in PPFD. In conclusion, high PPFD reduced the adverse effects of N deficiency by improving photosynthesis and stomatal functionality, NUE, WUE, and directing biomass partitioning toward the floral organs.

Photosynthetic Efficiency and Leaf Traits of Shorea leprosula Grown Under the Sun and Shade Conditions in Lentang Forest Reserve

Rehabilitation of degraded forests can be a huge challenge as these areas usually face problems including weed invasion, nutrient-deficient soil, lack of seed source and inability of native tree species to colonise or regenerate naturally. Human intervention is thus needed to accelerate the process of rehabilitation. In this regard, using the right species is as important as providing the right environment to encourage the survival and establishment of seedlings. Therefore, the aim of this study was to compare the photosynthetic characteristics and leaf traits of Shorea leprosula trees grown under the sun and shaded environments in a degraded forest. Shade trees were found to have about 20% higher leaf dry weight and 5% higher leaf area as well as significantly (p < 0.05) more chlorophyll than leaves on sun trees. Leaves in shade trees attained maximum photosynthesis at lower photosynthetic photon flux density (PPFD) compared to sun trees. When comparing both sun and shade trees, fast chlorophyll induction curve showed no significant differences in maximum quantum yield of PSII photochemistry (Fv/Fm) and thus was almost unaffected by the leaf ambient light environment. This indicated that although shade trees received lower light intensity compared to sun trees, the physiological responses indicated that the former were capable of minimising light limitation through foliar morphological and physiological adaptations. Our results thus suggested that S. leprosula, being an economically valuable timber species, can be planted in areas with partial shade or in gaps for the purpose of rehabilitation.

A novel external reflecting raceway pond design for improved biomass productivity

Outdoor raceway ponds for microalgae cultivation suffer from low biomass productivity due in part to the low photosynthetic photon flux received by the microalgae culture in the mornings and evenings and during the winter months at middle and high latitudes. This study explores the use of external mirrors to reflect additional sunlight onto the culture and increase the incident solar flux at critical times of the day and year. Four designs cultivating Chlorella vulgaris were considered: a raceway pond without mirrors (Configuration A), a pond oriented along the north/south axis with dual east/west mirrors (Configuration B), a pond oriented along the east/west axis with a single north mirror (Configuration C), and a solar tracking rotating pond with a single mirror (Configuration D). The biomass productivity was predicted by coupling the simulated radiative field within the culture to a microalgae growth kinetics model accounting for photolimitation and biomass loss due to respiration. Two different locations were considered, namely Los Angeles, CA, USA and Saint-Nazaire, France. The use of mirrors was predicted to increase the daily culture-area-based and volumetric biomass productivity at both locations and all months of the year. Configuration D yielded the highest biomass productivity year-round. Configuration B had a higher biomass productivity than Configuration C in the summer months and vice versa in the winter months. Overall, Configuration C was considered to be the simplest and most cost-effective method to increase raceway productivity. Indeed, this configuration improved the raceway pond volumetric and culture-area-based biomass productivity by as much as 73% in the winter months compared to Configuration A. Additionally, the impact of operational parameters (initial biomass concentration and culture depth) and design parameters (pond length-to-width ratio and mirror height) were assessed to provide practical recommendations for maximizing biomass productivity.

Influence of Monochromatic Light Irradiation as Pre-rooting Treatment on Rooting of Cuttings in Spray-type Chrysanthemums

The influences of monochromatic light irradiation as pre-rooting treatment on the rooting of cuttings in spray-type chrysanthemum cultivars ‘Sei Prince’ and ‘Remidas’ examined using light-emitting diodes. The unrooted cuttings were irradiated with red (660 nm), green (520 nm), and blue (450 nm) light and fluorescent light at around 60 mmol m-2 s-1 of photosynthetic photon flux density (PPFD) for 24 h continuously at 20℃ for 7 days. In ‘Remidas,’ the rooting percentages of cuttings were high above 85% irrespective of light wavelength. In ‘Sei Prince,’ red light irradiation promoted the rooting of cuttings compared to the other light irradiation. When the cuttings were irradiated with red light at 59.4, 37.4, and 12.4 mmol m-2 s-1 of PPFD, a decrease in rooting percentage was not observed even at a low PPFD in ‘Remidas.’ On the other hand, in ‘Sei Prince,’ the rooting percentage of cuttings decreased at the lowest PPFD. Thus, it was shown that monochromatic light irradiation for 7 d before rooting could promote the rooting of spray-type chrysanthemum cuttings, however, not only light wavelength but also light intensity that could promote rooting differed between two cultivars.

Limited light intensity and low temperature: Can plants survive freezing in light conditions that more accurately replicate the cold season in temperate regions?

Plants in temperate regions have evolved mechanisms that enable them to survive sudden temperature drops. Experiments with plants grown in long-day conditions, in which they are most sensitive to freezing stress, indicate that the cold acclimation mechanism is light-dependent and does not fully operate under low light intensity. However, winter annuals like Arabidopsis thaliana Col-0 germinate in the fall, overwinter as rosettes, and thus must acclimate under short photoperiods and low irradiance. Thus, we have analysed effects of variations in light intensity in plants grown under short-day photoperiod at the 1.14 growth stage (14 rosette leaves). Plants were acclimated at 4 °C for seven days under control and limited-light conditions: 100 and 20 μmol m-2s-1 photosynthetic photon flux density (PPFD), respectively. All cold-acclimated plants accumulated molecular markers reportedly associated with acquired freezing tolerance, including proline, sucrose, cold-responsive gene transcripts, dehydrins and low temperature-induced proteins. Observed changes (and similarity of freezing stress survival rates of plants in both light conditions) indicate that low PPFD did not inhibit the cold acclimation process. The molecular analysis identified distinct PPFD-specific adaptation mechanisms manifested in contrasting contents of anthocyanins, cytokinin conjugates, photosystem proteins, and enzymes involved in protein, energy, and reactive oxygen species metabolism. Finally, the results identify putative proteins and metabolite markers correlating with susceptibility to freezing stress of non-acclimated plants grown under low PPFD. Our data show that Arabidopsis plants grown under short-day photoperiods can be fully cold-acclimated under limited light conditions, employing standard and PPFD-specific pathways.

Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms.

Red and blue light are traditionally believed to have a higher quantum yield of CO2 assimilation (QY, moles of CO2 assimilated per mole of photons) than green light, because green light is absorbed less efficiently. However, because of its lower absorptance, green light can penetrate deeper and excite chlorophyll deeper in leaves. We hypothesized that, at high photosynthetic photon flux density (PPFD), green light may achieve higher QY and net CO2 assimilation rate (An) than red or blue light, because of its more uniform absorption throughtout leaves. To test the interactive effects of PPFD and light spectrum on photosynthesis, we measured leaf An of “Green Tower” lettuce (Lactuca sativa) under red, blue, and green light, and combinations of those at PPFDs from 30 to 1,300 μmol⋅m–2⋅s–1. The electron transport rates (J) and the maximum Rubisco carboxylation rate (Vc,max) at low (200 μmol⋅m–2⋅s–1) and high PPFD (1,000 μmol⋅m–2⋅s–1) were estimated from photosynthetic CO2 response curves. Both QYm,inc (maximum QY on incident PPFD basis) and J at low PPFD were higher under red light than under blue and green light. Factoring in light absorption, QYm,abs (the maximum QY on absorbed PPFD basis) under green and red light were both higher than under blue light, indicating that the low QYm,inc under green light was due to lower absorptance, while absorbed blue photons were used inherently least efficiently. At high PPFD, the QYinc [gross CO2 assimilation (Ag)/incident PPFD] and J under red and green light were similar, and higher than under blue light, confirming our hypothesis. Vc,max may not limit photosynthesis at a PPFD of 200 μmol m–2 s–1 and was largely unaffected by light spectrum at 1,000 μmol⋅m–2⋅s–1. Ag and J under different spectra were positively correlated, suggesting that the interactive effect between light spectrum and PPFD on photosynthesis was due to effects on J. No interaction between the three colors of light was detected. In summary, at low PPFD, green light had the lowest photosynthetic efficiency because of its low absorptance. Contrary, at high PPFD, QYinc under green light was among the highest, likely resulting from more uniform distribution of green light in leaves.

The Impact of Carbon Dioxide Concentrations and Low to Adequate Photosynthetic Photon Flux Density on Growth, Physiology and Nutrient Use Efficiency of Juvenile Cacao Genotypes

Cacao (Theobroma cacao L.) was grown as an understory tree in agroforestry systems where it received inadequate to adequate levels of photosynthetic photon flux density (PPFD). As atmospheric carbon dioxide steadily increased, it was unclear what impact this would have on cacao growth and development at low PPFD. This research evaluated the effects of ambient and elevated levels carbon dioxide under inadequate to adequate levels of PPFD on growth, physiological and nutrient use efficiency traits of seven genetically contrasting juvenile cacao genotypes. Growth parameters (total and root dry weight, root length, stem height, leaf area, relative growth rate and net assimilation rates increased, and specific leaf area decreased significantly in response to increasing carbon dioxide and PPFD. Increasing carbon dioxide and PPFD levels significantly increased net photosynthesis and water-use efficiency traits but significantly reduced stomatal conductance and transpiration. With few exceptions, increasing carbon dioxide and PPFD reduced macro–micro nutrient concentrations but increased uptake, influx, transport and nutrient use efficiency in all cacao genotypes. Irrespective of levels of carbon dioxide and PPFD, intraspecific differences were observed for growth, physiology and nutrient use efficiency of cacao genotypes.

The Physiological Response of Lettuce to Red and Blue Light Dynamics Over Different Photoperiods.

This study aimed to evaluate the effect of dynamic red and blue light parameters on the physiological responses and key metabolites in lettuce and also the subsequent impact of varying light spectra on nutritive value. We explored the metabolic changes in carotenes, xanthophylls, soluble sugars, organic acids, and antioxidants; the response of photosynthetic indices [photosynthetic (Pr) and transpiration (Tr) rates]; and the intracellular to ambient CO2 concentration ratios (Ci/Ca) in lettuce (Lactuca sativa L. “Lobjoits Green Cos”). They were cultivated under constant (con) or parabolic (dyn) blue (B, 452 nm) and/or red (R, 662 nm) light-emitting diode (LED) photosynthetic photon flux densities (PPFDs) at 12, 16, and 20 h photoperiods, maintaining consistent daily light integrals (DLIs) for each light component in all treatments, at 2.3 and 9.2 mol m–2 per day for blue and red light, respectively. The obtained results and principal component analysis (PCA) confirmed a significant impact of the light spectrum, photoperiod, and parabolic profiles of PPFD on the physiological response of lettuce. The 16 h photoperiod resulted in significantly higher content of xanthophylls (neoxanthin, violaxanthin, lutein, and zeaxanthin) in lettuce leaves under both constant and parabolic blue light treatments (BconRdyn 16 h and BdynRdyn 16 h, respectively). Lower PPFD levels under a 20 h photoperiod (BdynRdyn 20 h) as well as higher PPFD levels under a 12 h photoperiod (BdynRdyn 12 h) had a pronounced impact on leaf gas exchange indices (Pr, Tr, Ci/Ca), xanthophylls, soluble sugar contents, and antioxidant properties of lettuce leaves. The parabolic PPFD lighting profile over a 16 h photoperiod (BdynRdyn 16 h) led to a significant decrease in Ci/Ca, which resulted in decreased Pr and Tr, compared with constant blue or red light treatments with the same photoperiod (BconRdyn and BdynRcon 16 h). Additionally, constant blue lighting produced higher α + β-carotene and anthocyanin (ARI) content and increased carotenoid to chlorophyll ratio (CRI) but decreased biomass accumulation and antioxidant activity.

Only Extreme Fluctuations in Light Levels Reduce Lettuce Growth Under Sole Source Lighting.

The cost of providing lighting in greenhouses and plant factories can be high. In the case of variable electricity prices, providing most of the light when electricity prices are low can reduce costs. However, it is not clear how plants respond to the resulting fluctuating light levels. We hypothesized that plants that receive a constant photosynthetic photon flux density (PPFD) will produce more biomass than those grown under fluctuating light levels. To understand potential growth reductions caused by fluctuating light levels, we quantified the effects of fluctuating PPFD on the photosynthetic physiology, morphology, and growth of ‘Little Gem’ and ‘Green Salad Bowl’ lettuce. Plants were grown in a growth chamber with dimmable white LED bars, alternating between high and low PPFDs every 15 min. The PPFDs were ∼400/0, 360/40, 320/80, 280/120, 240/160, and 200/200 μmol⋅m−2⋅s–1, with a photoperiod of 16 h and a DLI of ∼11.5 mol⋅m−2⋅day–1 in all treatments. CO2 was ∼800 μmol⋅mol–1. Plants in the 400/0 μmol⋅m−2⋅s–1 treatment had ∼69% lower An,30 (net assimilation averaged over 15 min at high and 15 min at low PPFD) than plants grown at a PPFD of 320/80 μmol⋅m−2⋅s–1 (or treatments with smaller PPFD fluctuations). The low An,30 in the 400/0, and to a lesser extent the 360/40 μmol⋅m−2⋅s–1 treatment was caused by low net assimilation at 360 and 400 μmol⋅m−2⋅s–1. Plants grown at 400/0 μmol⋅m−2⋅s–1 also had fewer leaves and lower chlorophyll content compared to those in other treatments. The four treatments with the smallest PPFD fluctuations produced plants with similar numbers of leaves, chlorophyll content, specific leaf area (SLA), dry mass, and leaf area. Chlorophyll content, An,30, and dry mass were positively correlated with each other. Our results show that lettuce tolerates a wide range of fluctuating PPFD without negative effects on growth and development. However, when fluctuations in PPFD are extreme (400/0 or 360/40 μmol⋅m−2⋅s–1), chlorophyll levels and An,30 are low, which can explain the low poor growth in these treatments. The ability of lettuce to tolerate a wide range of fluctuating light levels suggests that PPFD can be adjusted in response to variable electricity pricing.

Modelling of Soybean (Glycine max (L.) Merr.) Response to Blue Light Intensity in Controlled Environments.

Low photosynthetic photon flux density (PPFD) under shade is associated with low blue photon flux density (BPFD), which independent from PPFD can induce shade responses, e.g., elongation growth. In this study, the response of soybean to six levels of BPFD under constant PPFD from LED lighting was investigated with regard to morphology, biomass and photosynthesis to increase the knowledge for optimizing the intensity of BPFD for a speed breeding system. The results showed that low BPFD increased plant height, leaf area and biomass and decreased leaf mass ratio. Photosynthetic rate and internode diameter were not influenced. A functional structural plant model of soybean was calibrated with the experimental data. A response function for internode length to the perceived BPFD by the internodes was derived from simulations and integrated into the model. With the aim to optimize lighting for a speed breeding system, simulations with alternative lighting scenarios indicated that decreasing BPFD during the growth period and using different chamber material with a higher reflectance could reduce energy consumption by 7% compared to the experimental setup, while inducing short soybean plants.

The Combined Conditions of Photoperiod, Light Intensity, and Air Temperature Control the Growth and Development of Tomato and Red Pepper Seedlings in a Closed Transplant Production System

Understanding environmental factors is essential to maximizing the biomass production of plants. There have been many studies on the effects of the photosynthetic photon flux (PPF), photoperiod and air temperature as separate factors affecting plants, including under a closed transplant production system (CTPS). However, few studies have investigated the combined effects of these factors on plant growth. Germinated tomato and red pepper seedlings were transferred to three different photoperiods with five different photosynthetic photon fluxes (PPFs) at an air temperature of 25/20 °C to investigate plant growth under a different daily light integral (DLI). Three different air temperatures, 23/20, 25/20, and 27/20 °C (photo/dark periods), with five different PPFs were used to examine plant growth under different DIFs (difference between the day and night temperature). Increasing the DLI from 4.32 to 21.60 mol·m−2·d−1, either by increasing the photoperiod or PPF, improved the growth of seedlings in both cultivars. However, when comparing treatments that provided the same DLI, tomato seedlings had s significantly higher growth when grown under longer photoperiods and s lower PPF. Even in higher DLI conditions, reduced growth due to higher PPF indicated that excessive light energy was a limiting factor. At 23 and 25 °C, tomato seedlings showed similar correlation curves between growth and PPF. However, at the higher temperature of 27 °C, while the slope of the curve at low PPFs was similar to that of the curves at lower temperatures, the slope at high PPFs was flatter. On the other hand, red pepper seedlings displayed the same correlation curve between growth and PPF at all tested temperatures, and red pepper plants accumulated more dry weight even at higher temperatures. These results suggested that the combination effect was more useful to observe these overall tendencies, especially in reacting to a second factor. This will provide us with more information and a deeper understanding of plant characteristics and how they will behave under changing environments.

Increasing Growth of Lettuce and Mizuna under Sole-Source LED Lighting Using Longer Photoperiods with the Same Daily Light Integral

Light recommendations for horticultural crops often focus on the optimal daily light integral (DLI) without regard to how that light is delivered throughout each day. Because photosynthesis is more efficient at lower photosynthetic photon flux density (PPFD), we hypothesized that longer photoperiods with lower PPFD results in faster growth than shorter photoperiods with higher PPFD and the same DLI. We quantified the effect of different photoperiods, all providing the same DLI, on photosynthesis and growth of two leafy greens. Mizuna (Brassica rapa var. japonica) and lettuce (Lactuca sativa) “Little Gem” were grown from seed in a controlled environment chamber (20 °C and 819 µmol·mol−1 CO2) under six photoperiods (10, 12, 14, 16, 18, and 20 h). LED fixtures provided white light and PPFD was adjusted so each treatment received a DLI of 16 mol·m−2·d−1. Mizuna and lettuce were harvested 30 and 41 days after planting, respectively. Longer photoperiods with lower PPFD increased light interception, chlorophyll content index, quantum yield of photosystem II, and aboveground biomass, but decreased instantaneous CO2 assimilation of lettuce and mizuna. Aboveground biomass increased 16.0% in lettuce and 18.7% in mizuna in response to increasing the photoperiod from 10 to 20 h. In summary, extending the photoperiod and lowering PPFD increases growth of lettuce and mizuna by increasing light interception and the quantum yield of photosystem II.

Longer Photoperiods with the Same Daily Light Integral Improve Growth of Rudbeckia Seedlings in a Greenhouse

Supplemental light can increase growth and accelerate production of greenhouse crops, but it can be expensive if not provided in a way that promotes efficient use of the light. Dimmable light-emitting diode (LED) fixtures have the potential to reduce lighting costs because the output can be precisely controlled to meet crop needs. Because light is used more efficiently to drive photosynthesis at lower photosynthetic photon flux densities (PPFDs), we hypothesized that providing Rudbeckia fulgida var. sullivantii ‘Goldsturm’ seedlings with the same daily light integral (DLI), spread out over a longer photoperiod and at lower PPFDs, should improve growth. A DLI of 12 mol·m−2·d−1 was provided in a greenhouse over 12, 15, 18, or 21-hour photoperiods from a combination of sunlight and supplemental light from LEDs, using adaptive lighting control. Plants grown without supplemental light had an ≈12-hour photoperiod and received an average DLI of 5 mol·m−2·d−1, ≈58% less light than the four lighting treatments. Lengthening the photoperiod from 12 to 21 hours increased shoot dry mass (30%), root dry mass (24%), plant height (14%), leaf area (16%), and chlorophyll content index (48%), and decreased specific leaf area (26%). There was no significant effect of photoperiod on root mass fraction or compactness. Growth parameters of plants without supplemental light were 26% to 90% smaller compared with those in the 12-hour photoperiod treatment. Treatment effects on canopy size, seen as early as 2 weeks into the study, were correlated with final shoot dry mass. Longer photoperiods did not induce a shade-avoidance response, based on specific leaf area and compactness data. The 24% increase in root dry mass for the plants in the 21-hour photoperiod suggests that cropping cycles can be shortened by 1 to 2 weeks compared with the 12-hour photoperiod. This could result in more crop turns per year and increased profits. In addition, fewer lights would be needed for adequate growth, reducing the capital cost of the lighting system.

Longer Photoperiods with the Same Daily Light Integral Increase Daily Electron Transport through Photosystem II in Lettuce.

Controlled environment crop production recommendations often use the daily light integral (DLI) to quantify the light requirements of specific crops. Sole-source electric lighting, used in plant factories, and supplemental electric lighting, used in greenhouses, may be required to attain a specific DLI. Electric lighting is wasteful if not provided in a way that promotes efficient photochemistry. The quantum yield of photosystem II (ΦPSII), the fraction of absorbed light used for photochemistry, decreases with increasing photosynthetic photon flux density (PPFD). Thus, we hypothesized that the daily photochemical integral (DPI), the total electron transport through photosystem II (PSII) integrated over 24 h, would increase if the same DLI was provided at a lower PPFD over a longer photoperiod. To test this, ΦPSII and the electron transport rate (ETR) of lettuce (Lactuca sativa ‘Green Towers’) were measured in a growth chamber at DLIs of 15 and 20 mol m−2 d−1 over photoperiods ranging from 7 to 22 h. This resulted in PPFDs of 189 to 794 μmol m−2 s−1. The ΦPSII decreased from 0.67 to 0.28 and ETR increased from 55 to 99 μmol m−2 s−1 as PPFD increased from 189 to 794 μmol m−2 s−1. The DPI increased linearly as the photoperiod increased, but the magnitude of this response depended on DLI. With a 7-h photoperiod, the DPI was ≈2.7 mol m−2 d−1, regardless of DLI. However, with a 22-h photoperiod, the DPI was 4.54 mol m−2 d−1 with a DLI of 15 mol m−2 d−1 and 5.78 mol m−2 d−1 with a DLI of 20 mol m−2 d−1. Our hypothesis that DPI can be increased by providing the same DLI over longer photoperiods was confirmed.