Photosynthetic Photon Flux Density Levels Research Articles

Varying Light Intensities Affect Lettuce Growth and Physiology in Controlled Indoor Environments

Agriculture in controlled environments has gained popularity over time. Compared to traditional agriculture, controlled environments emerge as an alternative to mitigate the negative impacts of conventional farming methods. However, controlled environment agriculture, particularly plant factories with artificial lighting, incurs higher electricity costs, primarily for supplemental lighting and dehumidification of the cultivation area. Given these high costs, it is crucial to understand how efficiently plants utilize available light to convert it into biomass. This understanding can be used to design lighting strategies to reduce electricity usage. In this study, we cultivated ‘Rex’ lettuce (Lactuca sativa) plants on a soilless substrate and used an ebb-and-flow system for irrigation and fertilization. Plants grew in varying photosynthetic photon flux density (PPFD) levels ranging from 125 to 375 µmol·m−2·s−1 and were assessed for various physiological responses. Our findings revealed that plants exposed to higher light levels exhibited greater final dry weight, increased photosynthetic activity, higher water use efficiency, and accelerated growth compared to those under lower light conditions. Notably, plants subjected to higher light intensities did not show a significant increase in transpiration, suggesting a potential trade-off between energy expenditure on supplemental lighting and dehumidification. This finding opens the possibility of reducing energy consumption for dehumidification and achieving economic savings by subjecting plants to optimal growing conditions for shorter durations. This depends on whether higher savings on dehumidification are achieved compared to the energy required to maintain high PPFD levels.

Light Flux Density and Photoperiod Affect Growth and Secondary Metabolism in Fully Expanded Basil Plants.

Indoor production of basil (Ocimum basilicum L.) is influenced by light spectrum, photosynthetic photon flux density (PPFD), and the photoperiod. To investigate the effects of different lighting on growth, chlorophyll content, and secondary metabolism, basil plants were grown from seedlings to fully expanded plants in microcosm devices under different light conditions: (a) white light at 250 and 380 μmol·m-2·s-1 under 16/8 h light/dark and (b) white light at 380 μmol·m-2·s-1 under 16/8 and 24/0 h light/dark. A higher yield was recorded under 380 μmol·m-2·s-1 compared to 250 μmol·m-2·s-1 (fresh and dry biomasses 260.6 ± 11.3 g vs. 144.9 ± 14.6 g and 34.1 ± 2.6 g vs. 13.2 ± 1.4 g, respectively), but not under longer photoperiods. No differences in plant height and chlorophyll content index were recorded, regardless of the PPFD level and photoperiod length. Almost the same volatile organic compounds (VOCs) were detected under the different lighting treatments, belonging to terpenes, aldehydes, alcohols, esters, and ketones. Linalool, eucalyptol, and eugenol were the main VOCs regardless of the lighting conditions. The multivariate data analysis showed a sharp separation of non-volatile metabolites in apical and middle leaves, but this was not related to different PPFD levels. Higher levels of sesquiterpenes and monoterpenes were detected in plants grown under 250 μmol·m-2·s-1 and 380 μmol·m-2·s-1, respectively. A low separation of non-volatile metabolites based on the photoperiod length and VOC overexpression under longer photoperiods were also highlighted.

Optimization of Photoautotrophic Growth Regimens of Scenedesmaceae alga: The Influence of Light Conditions and Carbon Dioxide Concentrations

Improving methods for landless production of bioproducts is considered an important stage in the development of the modern bioeconomy. In this context, microalgal biomass is one of the most promising sources of valuable substances due to its rich biochemical composition. Despite the high adaptability of microalgae to various environmental factors, the effectiveness of cultivation systems depends on precisely selected parameters. Both the light conditions and the supply of inorganic carbon sources are key in determining the efficiency of photoautotrophic cultivation. In this work, the effect of a high daily photosynthetic photon flux density (PPFD) ranging from 37.44 to 112.32 mol m−2 day−1 on the growth and productivity of a novel Scenedesmaceae alga, strain EZ-B1, was assessed. The next stage of cultivation consisted of selecting the optimal CO2 concentration. Improved performance of microalga during cultivation in a photobioreactor was achieved at 112.32 mol m−2 day−1 (24 h photoperiod) and by supplying 2% CO2, as evidenced by the high biomass productivity (0.69 g L−1 day−1), total biomass yield (5.23 g L−1), and ammonium nitrogen consumption rate. The data obtained suggest that a higher level of PPFD led to the highest growth rate of the novel strain and the highest biomass productivity, which, in practice, will increase production capacity.

Growth responses of tomato plants to different wavelength ratios of amber, red, and blue light

The differential effects of light quality and quantity are under scrutiny so that energy-efficient plant production may be optimized. This study aimed to determine the combined effects of amber (595 nm), red (635 nm), and blue (445 nm) wavelengths at different light intensities on tomato plant growth. Plants were treated for 12 days with combined light-emitting diode (LED) wavelengths, at a photosynthetic photon flux density (PPFD) ranging from 232 to 1660 μmol m−2s−1 with a 16 h d−1 photoperiod. Mean fresh mass of tomato plants treated with amber-red light (AR) was 22% greater than plants treated with blue-amber (BA) light and 67% greater than blue-red (BR) light. Plants treated with BA exhibited 36% greater fresh mass than plants treated with BR light. Mean dry mass of plants treated with AR light was 30% greater compared to BR light, and the dry mass of plants treated with BA and BR was statistically similar. Plants grown under BR had 28% and 35% smaller leaf area compared with those grown under BA and AR, respectively. Mean height of the plants treated with AR increased by 24 and 52% compared to plants grown under BA and BR lights, respectively. Maximum stem diameter was observed at 1000 μmol m−2s−1 for AR and BA, and 1200 μmol m−2s−1 for BR. Specific leaf area (SLA) decreased with the increase of PPFD levels up to 1000 μmol m−2s−1 for all the light treatments. A PPFD above 1000 μmol m−2s−1 showed an increase in SLA for plants cultivated with AR and AB. SLA for plants treated with RB was stable above 1000 μmol m−2s−1 PPFD levels. Plants treated with BR light had 25 and 38% higher chlorophyll content compared to plants treated with BA and AR light, respectively. Maximum yield and chlorophyll content data indicate that using Chl content to correlate full plant photosynthetic and growth potential is inappropriate. Findings suggest that a high percentage of amber light (>75%) in a full-spectrum LED regimen may augment controlled environment crop production.

The Influence of End-of-Day Blue Light on the Growth, Photosynthetic, and Metabolic Parameters of Lettuce at Different Development Stages.

This study evaluates the effect of end-of-day blue (EOD B) light on the physiological response of lettuce (Lactuca sativa, Lobjoits Green Cos) at different phenological development stages. Plants were grown in a controlled environment growth chamber (day/night temperature 21 ± 2 °C; relative air humidity 60 ± 5%) under the light of light-emitting diodes (LEDs) consisting of 5% blue (B; 450 nm), 85% red (R; 660 nm), and 10% green (G; 530 nm) photosynthetic photon flux density (PPFD) at 200 µmol m−2 s−1 for 16 h d−1 (BRG, control) for 8, 15, and 25 days (BBCH 12, BBCH 14, and BBCH 18, respectively). For the EOD B treatments, lettuce plants were additionally illuminated with 100% of B light at 30 and 60 µmol m−2 s−1 PPFD for 4 h d−1 (B30 and B60, respectively). The results show that EOD B light caused the elevated shoot elongation of lettuce plants regardless of their growth stages. However, leaf width increased only in more developed lettuce plants (BBCH 18). EOD B light negatively affected the development of new leaves and fresh weight, except for seedlings (BBCH 12). Most photosynthetic and spectral leaf indices also decreased when lettuce was treated with EOD B light, especially under the PPFD level of 60 µmol m−2 s−1. Moreover, the changes in metabolic parameters such as DPPH free radical activity, free proline content, and H+-ATPase activity in lettuce showed a plant response to unfavorable conditions to EOD B light.

Growth and metabolism of basil grown in a new-concept microcosm under different lighting conditions

To simulate a typical market-oriented cultivation in laboratory, plants of basil (Ocimum basilicum L.) were grown from seedling to flowering stages in a new-concept microcosm device that enables roots and aerial parts to grow as under real crop conditions. To test the device efficacy, two microcosms were used with the same lighting architecture, temperature and photoperiodic conditions and with two different light spectra, white (W) or blue-red (BR), displaying a similar spectral power in the blue region. Plant growth, biomass yield, photosynthetic efficiency and nutrient uptake were determined. An innovative analytical approach for secondary metabolic profile was also developed to determine basil quality. The plants grew vigorous and healthy for the whole cultivation period and under both the lighting regimes, giving a biomass yield similar to those of basil grown under conventional greenhouse and field conditions. The two lighting regimes differently affected plant growth and yield, with the BR light, that was characterized by a higher photosynthetic photon flux density (PPFD), associated to higher plants, earlier flowering and greater yield. In the average, fresh and dry aerial biomasses per plant were about 250 g and 41 g under BR and about 114 g and 9 g under W light. Higher concentrations of major nutrients were detected in plants under W light, thus indicating that yield levels and major nutrient concentrations are not necessarily related to each other. Similar Fv/Fm (0.76–0.78) and ETR values were observed under the two light regimes, possibly indicating that plants under long lasting cultivation can adapt to different light regimes to reach similar photosynthetic efficiency levels. Different secondary metabolic profiles were detected in tissues sampled at the end of cultivation period and previously unreported profiles for basil were also recorded, possibly indicating that the extent of plant growth affects secondary metabolism in basil, in addition to light spectrum and PPFD level. This is the first report of basil grown under microcosm conditions from seedlings to adult plants. Our results indicate that the microcosm based-technology is effective in simulating a typical market-oriented cultivation and that long lasting cultivation emphasizes the effects of different environmental conditions on plant growth and metabolism.

Estimating Light Acclimation Parameters of Cucumber Leaves Using Time-Weighted Averages of Daily Photosynthetic Photon Flux Density.

Leaves acclimate to day-to-day fluctuating levels of photosynthetic photon flux density (PPFD) by adjusting their morphological and physiological parameters. Accurate estimation of these parameters under day-to-day fluctuating PPFD conditions benefits crop growth modeling and light environment management in greenhouses, although it remains challenging. We quantified the relationships between day-to-day PPFD changes over 6 days and light acclimation parameters for cucumber seedling leaves, including leaf mass per area (LMA), chlorophyll (Chl) a/b ratio, maximum net photosynthetic rate (Pnmax), maximum rate of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Vcmax), and maximum rate of electron transport (Jmax). The last two parameters reflect the capacity of the photosynthetic partial reactions. We built linear regression models of these parameters based on average or time-weighted averages of daily PPFDs. For time-weighted averages of daily PPFDs, the influence of daily PPFD was given a specific weight. We employed three types of functions to calculate this weight, including linear, quadratic, and sigmoid derivative types. We then determined the trend of weights that estimated each parameter most accurately. Moreover, we introduced saturating functions to calibrate the average or time-weighted averages of daily PPFDs, considering that light acclimation parameters are usually saturated under high PPFDs. We found that time-weighted average PPFDs, in which recent PPFD levels had larger weights than earlier levels, better estimated LMA than average PPFDs. This suggests that recent PPFDs contribute more to LMA than earlier PPFDs. Except for the Chl a/b ratio, the average PPFDs estimated Pnmax, Vcmax, and Jmax with acceptable accuracy. In contrast, time-weighted averages of daily PPFDs did not improve the estimation accuracy of these four parameters, possibly due to their low response rates and plasticity. Calibrating functions generally improved estimation of Chl a/b ratio, Vcmax, and Jmax because of their saturating tendencies under high PPFDs. Our findings provide a reasonable approach to quantifying the extent to which the leaves acclimate to day-to-day fluctuating PPFDs, especially the extent of LMA.

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.

Carbon Dioxide Concentrations and Light Levels on Growth and Mineral Nutrition of Juvenile Cacao Genotypes

In many countries cacao (Theobroma cacao L.) is invariably grown as an understory crop in agroforestry types of cropping systems and subjected to low levels photosynthetic photon flux density (PPFD) due to presence of large number of upper story shade trees with poorly managed canopy structure. In recent years carbon dioxide concentration in the atmosphere is steadily increasing and it is unclear what impact this will have on performance of cacao grown under shade of upper story shade trees. A climatically controlled greenhouse experiment was undertaken to evaluate the effects of ambient and elevated carbon dioxide (400 and 700 μmol·mol-1) and three levels of PPFD (100, 200, and 400 μmol·m-2·s-1) on growth, and macro- and micronutrient use efficiency of three genetically contrasting cacao genotypes (CCN 51, VB 1117 and NO 81). Intraspecific variations were observed in cacao genotypes for growth parameters at ambient to elevated carbon dioxide and low to adequate levels of PPFD. With the exceptions of total root length and leaf area, irrespective of carbon dioxide and PPFD levels, all three genotypes showed significant differences in all the growth parameters. For all the cacao genotypes, increasing PPFD from 100 to 400 μmol·m-2·s-1 and carbon dioxide from 400 to 700 μmol·mol-1 increased overall growth parameters such as leaf, shoot and root biomass accumulation, stem height, leaf area, relative growth rate and net assimilation rate. Irrespective of carbon dioxide and PPFD, invariably genotypes differed significantly in macro-micronutrient uptake parameters such as concentration, uptake, influx, transport and use efficiency. With few exceptions, raising PPFD from 100 to 400 μmol·m-2·s-1 and carbon dioxide from 400 to 700 μmol·mol-1 increased nutrient use efficiency for all the cacao genotypes. Elevated carbon dioxide and adequate PPFD are beneficial in improving cacao growth and mineral nutrient uptake and use efficiency.

Comparison of the Photoautotrophic Growth Regimens of Chlorella sorokiniana in a Photobioreactor for Enhanced Biomass Productivity

Microalgae have a wide industrial potential because of their high metabolic diversity and plasticity. Selection of optimal cultivation methods is important to optimize multi-purpose microalgal biotechnologies. In this research, Chlorella sorokiniana AM-02 that was isolated from a freshwater lake was cultured under various high photosynthetic photon flux density (PPFD) conditions and CO2 gas levels in standard Bold’s basal medium (BBM). Furthermore, a wide range of nitrate levels (180–1440 mg L−1) was tested on the growth of C. sorokiniana. Microalgae growth, pigment concentration, medium pH, exit gas composition, as well as nitrate, phosphate, and sulfate levels were measured during an experimental period. The preferred high PPFD and optimal CO2 levels were found to be 1000–1400 μmol photons m−2 s−1 and 0.5–2.0% (v/v), respectively. The addition of nitrate ions (up to 1440 mg L−1) to the standard growth medium increased final optical density (OD750), cell count, pigment concentration, and total biomass yield but decreased the initial growth rate at high nitrate levels. Our findings can serve as the basis for a robust photoautotrophic cultivation system to maximize the productivity of large-scale microalgal cultures.

Can light‐saturated photosynthesis in lowland tropical forests be estimated by one light level?

AbstractLeaf‐level net photosynthesis (An) estimates and associated photosynthetic parameters are crucial for accurately parameterizing photosynthesis models. For tropical forests, such data are poorly available and collected at variable light conditions. To avoid over‐ or underestimation of modeled photosynthesis, it is critical to know at which photosynthetic photon flux density (PPFD) photosynthesis becomes light‐saturated. We studied the dependence of An on PPFD in two tropical forests in French Guiana. We estimated the light saturation range, including the lowest PPFD level at which Asat (An at light saturation) is reached, as well as the PPFD range at which Asat remained unaltered. The light saturation range was derived from photosynthetic light‐response curves, and within‐canopy and interspecific differences were studied. We observed wide light saturation ranges of An. Light saturation ranges differed among canopy heights, but a PPFD level of 1,000 µmol m−2 s−1 was common across all heights, except for pioneer trees species that did not reach light saturation below 2,000 µmol m−2 s−1. A light intensity of 1,000 µmol m−2 s−1 sufficed for measuring Asat of climax species at our study sites, independent of the species or the canopy height. Because of the wide light saturation ranges, results from studies measuring Asat at higher PPFD levels (for upper canopy leaves up to 1,600 µmol m−2 s−1) are comparable with studies measuring at 1,000 µmol m−2 s−1.

Elongation growth mediated by blue light varies with light intensities and plant species: A comparison with red light in arugula and mustard seedlings

Previous studies have indicated that at a photosynthetic photon flux density (PPFD) of 50 or 100 μmol m−2·s−1, pure blue light, compared to red light, can promote plant elongation of de-etiolated seedlings for arugula, but not for mustard. The objective of this study was to investigate whether the promotion effect of blue light can also occur under wider range of PPFD levels. The growth and morphology traits of arugula (Brassica eruca, ‘Rocket’) and mustard (Brassica juncea, ‘Ruby Streaks’) seedlings were compared during the stage from seeding to cotyledon unfolding under two light quality × six light intensity treatments: (1) R, “pure” red light (665 nm); and (2) B, “pure” blue light (440 nm) using light-emitting diode lighting at PPFD levels of 20, 70, 120, 250, 450 and 650 μmol m−2·s−1. Continuous (24-h light/0-h dark) lighting and an air temperature of approximately 22 °C was used for the above treatments. After 7 to 8 days of lighting treatment, regardless of species, B promoted hypocotyl and petiole elongation compared to R at certain PPFD levels, associated with some shade-avoidance responses such as reduced biomass partitioning to cotyledons, cotyledon size, and plant coloring, which varied between species. The blue light promotion effect on hypocotyl elongation was detected at all the tested PPFD levels for arugula, but at a narrower range of PPFD levels (i.e., 250–650 μmol m−2·s−1) for mustard. Also, for arugula, the hypocotyl promotion by blue light decreased with increased PPFD levels. Generally, the promotion effects of blue vs. red light were more obvious for arugula than mustard. This suggests that blue vs. red light can promote elongation growth of de-etiolated seedlings for arugula and mustard, but the promotion effects vary with light intensities and plant species.

Evaluation of growth and quality of hydroponic lettuce at harvest as affected by the light intensity, photoperiod and light quality at seedling stage

In order to obtain high-quality lettuce seedlings, effects of photosynthetic photon flux density (PPFD), photoperiod and light quality at seedling stage on growth and quality of hydroponic lettuce were investigated at different growth stages. Hydroponic lettuce (Lactuca sativa L. cv. Frill ice) seedlings were grown under 12 combinations of two levels of PPFD (200 and 250 μmol m−2 s-1) and two photoperiods (14 and 16 h d-1) under three light quality: light-emitting diodes (LEDs) with red to blue ratio (R:B ratio) of 1.2 and 2.2, and fluorescent lamps with R:B ratio of 1.8 as control. After the 20-day treatment during seedling stage, lettuce seedlings were transplanted to a homogeneous lighting environment for another 20 days before harvest. Results showed that leaf length and ratio of leaf length to leaf width at 20 days after sowing decreased in a logarithmic way with increasing daily light integral and fluorescent lamps resulted in larger seedlings with bigger leaves and higher leaf and root dry weights compared to LEDs. Leaf and root fresh weights of harvested lettuces were higher in the seedlings grown under PPFD at 200 μmol m−2 s-1 with a 16 h d-1 photoperiod, regardless of light quality at seedling stage. Generally, vitamin C content of mature lettuce was lower in seedlings grown under LEDs with R:B ratio of 1.2 and no significant differences were observed in nitrate content among the three lighting sources. In consideration of mature yield and quality, lighting efficiency and electrical consumption of lighting sources, PPFD of 200 μmol m−2 s-1 with a photoperiod of 16 h d-1 provided by LEDs with R:B ratio of 2.2 was suitable for hydroponic lettuce (cv. Frill ice) seedling production.

Computational analysis of the effects of light gradients and neighbouring species on foliar nitrogen

Foliar nitrogen is one of the key traits determining the photosynthetic capacity of trees. It is influenced by many environmental factors that are often confounded with the photosynthetic photon flux density (PPFD), which alone strongly modifies the nitrogen content and other foliar traits. We combined field measurements and computational estimates of light transmittance in 3D stands with different combinations of Scots pine (Pinus sylvestris) and silver birch (Betula pendula) to decouple the effect of PPFD from other potential effects exerted by the species of neighbouring trees on the leaf nitrogen content per unit leaf area (Narea) and leaf mass per area (LMA). Independent of the level of PPFD, silver birch had a significantly lower Narea and LMA when Scots pine was abundant in its neighbourhood compared with the presence of conspecific neighbours. In Scots pine, Narea and LMA were only dependent on PPFD and the branching order of shoots. In both species, the relationships between PPFD and Narea or LMA were nonlinear, especially at intermediate levels of PPFD. The levels of PPFD did not show any dependence on the species of the neighbouring trees. The responses of silver birch suggest that the species composition of the surrounding stand can influence foliar nitrogen, independent of the level of PPFD within the canopy.

Effects of Carbon Dioxide and Photosynthetic Photon Flux on Mineral Content in Chrysanthemum Allowing for Growth as a Covariate

The effect of CO2 concentration (330 and 675 μL·L−1) and photosynthetic photon flux (PPF) (mean daily peaks of 550–1400 μmol·m−2·s−1) on total mineral contents in shoots was studied in chrysanthemum [Dendranthema ×grandiflorum (Ramat) Kitam ‘Fiesta’] during three times of the year. Growth (as measured by shoot dry weight) and shoot mineral contents (weight of nutrient per shoot) of hydroponically grown plants were analyzed after 5 weeks. There was a positive synergistic interaction of CO2 concentration and PPF on growth with the greatest growth at high PPF (1400 μmol·m−2·s−1) with high CO2 (675 μL·L−1). When growth was not used as a covariate in the statistical model, both CO2 concentration and PPF significantly affected the content of all eight nutrients. However, after growth was included as a covariate in the model, nutrients were classified into three categories based on whether CO2 concentration and PPF level were needed in addition to growth to predict shoot nutrient content. Neither CO2 concentration nor PPF level was needed for Mg, Fe, and Mn contents, whereas PPF level was needed for N, P, K, and Ca contents, and both CO2 concentration and PPF level were required for B content.

Photosynthetic photon flux density levels affect morphology and bromatology in Cichorium endivia L. var. latifolia grown in a hydroponic system

Abstract Photosynthetic photon flux density (PPFD) in suboptimal or super-optimal levels can modify the biomass accumulation, bromatological composition and appearance of crops. This study was conducted in a hydroponic system to determine whether PPFD could change physiological and bromatological aspects of escarole (Cichorium endivia L. var. latifolia). Three PPFD levels (100%, 70% and 50% of PPFD) were investigated to determine crop’s responses to this factor. Plants of escarole grown under 50% of incident PPFD had converted 0.88 g of dry matter per mol of incident photosynthetically active radiation (42% more efficient compared to unshaded environment), had 26% more lipid content, higher levels of chlorophyll a, b and total (12%, 14% and 13%, respectively), 42% less acid detergent fiber, 34% less cellulose, 47% less lignin, lesser leaf thickness and better visual features. The attenuation of PPFD in sites or seasons where there is high light intensity is an effective method to improve morphological, bromatological and visual characteristics of escarole.

Growth and Accumulation of Secondary Metabolites in Perilla as Affected by Photosynthetic Photon Flux Density and Electrical Conductivity of the Nutrient Solution.

The global demand for medicinal plants is increasing. The quality of plants grown outdoors, however, is difficult to control. Myriad environmental factors influence plant growth and directly impact biosynthetic pathways, thus affecting the secondary metabolism of bioactive compounds. Plant factories use artificial lighting to increase the quality of medicinal plants and stabilize production. Photosynthetic photon flux density (PPFD) and electrical conductivity (EC) of nutrient solutions are two important factors that substantially influence perilla (Perilla frutescens, Labiatae) plant growth and quality. To identify suitable levels of PPFD and EC for perilla plants grown in a plant factory, the growth, photosynthesis, and accumulation of secondary metabolites in red and green perilla plants were measured at PPFD values of 100, 200, and 300 μmol m-2 s-1 in nutrient solutions with EC values of 1.0, 2.0, and 3.0 dS m-1. The results showed significant interactive effects between PPFD and EC for both the fresh and dry weights of green perilla, but not for red perilla. The fresh and dry weights of shoots and leafy areas were affected more by EC than by PPFD in green perilla, whereas they were affected more by PPFD than by EC in red perilla. Leaf net photosynthetic rates were increased as PPFD increased in both perilla varieties, regardless of EC. The perillaldehyde concentration (mg g-1) in red perilla was unaffected by the treatments, but accumulation in plants (mg per plant) was significantly enhanced as the weight of dry leaves increased. Perillaldehyde concentrations in green perilla showed significant differences between combinations of the highest PPFD with the highest EC and the lowest PPFD with the lowest EC. Rosmarinic acid concentration (mg g-1) was increased in a combination of low EC and high PPFD conditions. Optimal cultivation conditions of red and green perilla in plant factory will be discussed in terms of plant growth and contents of medicinal ingredients.

An Adaptive Control Approach for Light-emitting Diode Lights Can Reduce the Energy Costs of Supplemental Lighting in Greenhouses

Supplemental lighting in greenhouses is often needed for year-round production of high-quality crops. However, the electricity needed for supplemental lighting can account for a substantial part of overall production costs. Our objective was to develop more efficient control methods for supplemental lighting, taking advantage of the dimmability of light-emitting diode (LED) grow lights. We compared 14 hours per day of full power supplemental LED lighting to two other treatments: 1) turning the LEDs on, at full power, only when the ambient photosynthetic photon flux (PPF) dropped below a specific threshold, and 2) adjusting the duty cycle of the LEDs so that the LED lights provided only enough supplemental PPF to reach a preset threshold PPF. This threshold PPF was adjusted daily from 50 to 250 μmol·m−2·s−1. Turning the LED lights on at full power and off based on a PPF threshold was not practical since this at times resulted in the lights going on and off frequently. Adjusting the duty cycle of the LED lights based on PPF measurements underneath the light bar provided excellent control of PPF, with 5-minute averages typically being within 0.2 μmol·m−2·s−1 of the threshold PPF. Continuously adjusting the duty cycle of the LED lights reduced electricity use by 20% to 92%, depending on the PPF threshold and daily light integral (DLI) from sunlight. Simulations based on net photosynthesis (An) − PPF response curves indicated that there are large differences among species in how efficiently supplemental PPF stimulates An. When there is little or no sunlight, An of Heuchera americana is expected to increase more than that of Campanula portenschlagiana when a low level of supplemental light is provided. Conversely, when ambient PPF >200 μmol·m−2·s−1, supplemental lighting will have little impact on An of H. americana, but can still results in significant increases in An of C. portenschlagiana (1.7 to 6.1 μmol·m−2·s−1 as supplemental PPF increases from 50 to 250 μmol·m−2·s−1). Adjusting the duty cycle of the LEDs based on PPF levels assures that supplemental light is provided when plants can use that supplemental light most efficiently. Implementing automated duty cycle control of LED grow lights is simple and low cost. This approach can increase the cost effectiveness of supplemental lighting, because of the associated energy savings.

Optimizing LED lighting for space plant growth unit: Joint effects of photon flux density, red to white ratios and intermittent light pulses

The aim of this work were to choose a quantitative optimality criterion for estimating the quality of plant LED lighting regimes inside space greenhouses and to construct regression models of crop productivity and the optimality criterion depending on the level of photosynthetic photon flux density (PPFD), the proportion of the red component in the light spectrum and the duration of the duty cycle (Chinese cabbage Brassica сhinensis L. as an example). The properties of the obtained models were described in the context of predicting crop dry weight and the optimality criterion behavior when varying plant lighting parameters. Results of the fractional 3-factor experiment demonstrated the share of the PPFD level participation in the crop dry weight accumulation was 84.4% at almost any combination of other lighting parameters, but when PPFD value increased up to 500µmol m−2s−1 the pulse light and supplemental light from red LEDs could additionally increase crop productivity. Analysis of the optimality criterion response to variation of lighting parameters showed that the maximum coordinates were the following: PPFD = 500µmol m−2s−1, about 70%-proportion of the red component of the light spectrum (PPFDLEDred/PPFDLEDwhite = 1.5) and the duty cycle with a period of 501µs. Thus, LED crop lighting with these parameters was optimal for achieving high crop productivity and for efficient use of energy in the given range of lighting parameter values.

Flag Leaf Photosynthesis and Stomatal Function of Grain Sorghum as Influenced by Changing Photosynthetic Photon Flux Densities

Photosynthesis (A) and stomatal function research in grain sorghum (Sorghum bicolor (L.) Moench) is limited compared to other crops. Flag leaves from three plants of two hybrids, grown with added N-fertilizer of 0.0, 112, and 224 kg ha−1 near Elizabeth, MS, were measured for A and stomatal functions at growth stages GS6 and GS7. A Li-Cor LI-6400XT set at 355 µmol [CO2], a flow rate of 500 µmol s−1, and a 6400-02 LED light source were used to collect data. Light levels were initially set at 2200 µmol m−2 s−1 indicated photosynthetic photon flux density (PPFD), A was allowed to stabilize, data was recorded, indicated PPFD level was reduced by 200 µmol m−2 s−1, and the process was repeated to a level of 200 µmol m−2 s−1. At GS6 all data were unaffected by N-fertility, hybrids, or years. Data on Ci at GS6 indicated A declines faster with decreasing PPFD than gs. Intrinsic water use efficiency (IWUE) data supports prior research showing stomata function more to regulate water loss and only marginally limit A. Nitrogen fertility was null on A and stomatal functions and minimal on yield; thus no attempt was made to correlate yield with these data.