Phytochrome Photostationary State Research Articles

Estimation of time course in phytochrome photostationary state under artificial light for controlling plant growth.

A model to estimate the time course of a phytochrome photostationary state (PSS) under an arbitrary light environment was developed. It is the solution of differential equations that use conversion rates between active and inactive forms of previously reported phytochromes. The model estimated that 90% of the PSS changes were completed using approximately 3.4 mmol m-2 of integrated end-of-day far-red light irradiation, and 99% of the changes were completed with approximately 6.9 mmol m-2 irradiation. Although these values were affected by the spectral photon flux density of the far-red light. They were consistent with previous results that examined dose requirements of far-red irradiation. The rate at which the PSS changes approached equilibrium was maximized under a red light, followed by far-red, green, and blue light. This estimation method could be used to control phytochrome responses for horticulture via artificial lighting.

Far-red radiation increases flower and fruit abortion in sweet pepper (Capsicum annuum L.)

• FR (far-red) applied whole day or end of day stimulates flower and fruit abortion. • FR decreases dry matter partitioned to fruits, due to a reduced fruit number. • FR stimulates total shoot biomass production. • FR increases the fraction of dry matter partitioned to the stem. Fruit set is a crucial plant developmental process, determining yield in many crops. Pepper ( Capsicum annuum L.) is a crop with poor fruit set as typically about two-thirds of all flowers abort. A higher light integral improves fruit set. However, the role of light spectrum has hardly been investigated. Opportunities for detailed investigation of light spectrum effects on fruit set have strongly increased with the introduction of narrow-band LED lighting. To investigate whether additional far-red light (FR) influences the fruit set of sweet pepper, a climate chamber experiment was conducted. Four light treatments were applied to pepper plants grown under 130 µmol m −2 s −1 of red/white LED light. Treatments consisted of different intensities of FR (0, 50, 100 µmol m −2 s −1 ) applied throughout the day or applied at the end of day (EOD, 30 min, 30 µmol m −2 s −1 ). Treatments resulted in phytochrome photostationary state (PSS) values of 0.88, 0.77. 0.70 and EOD 0.16, respectively. Fruit set was determined 3 weeks after the last anthesis of the studied flowers. Additional FR light reduced fruit set in sweet pepper, regardless of whether FR light was provided during the whole day or only at the end of the day. Meanwhile, FR led to more stem elongation, more upright branches, and more dry mass partitioning to stems. Additional FR during the daytime increased total shoot dry weight but not when FR was applied at the end of day. Possible reasons for stimulated flower and fruit abortion by additional FR are discussed.

The Power of Far-Red Light at Night: Photomorphogenic, Physiological, and Yield Response in Pepper During Dynamic 24 Hour Lighting.

Supplemental light is needed during the winter months in high latitude regions to achieve the desired daily light integral (DLI) (photoperiod × intensity) for greenhouse pepper (Capsicum annuum) production. Peppers tend to have short internodes causing fruit stacking and higher labor time for plant maintenance when grown under supplemental light. Far-red light can increase internode length, and our previous study on tomatoes (Solanum lycopersicum) also discovered monochromatic blue light at night during continuous lighting (CL, 24 h) increased stem elongation. Furthermore, the use of low-intensity, long photoperiod lighting can reduce light fixture costs and overall electricity costs due to lower power prices during the night. Therefore, we investigated the use of blue and/or far-red light during the night period of CL to increase stem elongation. Three pepper cultivars with different internode lengths/growing characteristics (‘Maureno,’ ‘Gina,’ and ‘Eurix’) were used to investigate the effects on plant morphology in a short experiment, and one cultivar ‘Maureno’ was used in a long experiment to assess the impact on fruit yield. The five lighting treatments that were used are as follows: 16 h of white light during the day followed by either 8 h of darkness (16W – control), white light (24W), blue light only (16W + 8B), blue + far-red light (16W + 8BFR), or far-red light only (16W + 8FR). Calculated nighttime phytochrome photostationary state (PSS) was 0.833, 0.566, 0.315, and 0.186 for 24W, 16W + 8B, 16W + 8BFR, and 16W + 8FR respectively. All five treatments had the same DLI in photosynthetically active radiation (PAR) and far-red light. The 16W + 8BFR and 16W + 8FR treatments significantly increased internode length compared to 16W and 24W but neither was more impactful than the other. The 16W + 8B treatment also increased internode length but to a lesser extent than 16W + 8BFR and 16W + 8FR. This indicates that a nighttime PSS of 0.315 is sufficient to maximize stem elongation. Both 16W + 8B and 16W + 8BFR drove photosynthesis during the nighttime supporting a similar yield compared to 16W. Therefore, 16W + 8BFR is the most potential lighting strategy as it can lead to a greater reduction in the light fixture and electrical costs while maintaining yield and enhancing internode length.

Effects of Green Light on Elongation Do Not Interact with Far-Red, Unless the Phytochrome Photostationary State (PSS) Changes in Tomato.

Simple SummaryThis paper focuses on the role of phytochromes (phys) in the interaction between green light and far-red light effects on “shade avoidance syndrome”. We grew wild type and phy mutants of tomato under a set of light conditions with different combinations of green, blue, red, and far-red light. Partial (20%) replacement of red/blue by green light in the absence of far-red light hardly affected the tomato plant morphology. However, when the spectrum contained far-red light, partially replacing red/blue by green light resulted in more elongation, which was associated with a lower phytochrome photostationary state (PSS) value. There was no effect of partial substitution of red/blue with green light when the PSS was kept constant. Thus, this study has revealed an interaction between green and far-red light effects on elongation unless PSS was kept constant. Green light was often a bit neglected in photobiology, but now an increasing number of researchers are realizing that green light deserves more attention. This study advances the understanding of light quality and plant growth and finding the optimal spectrum when growing plants under LED lighting in controlled environment agriculture.Green light (G) could trigger a “shade avoidance syndrome” (SAS) similarly to far-red light. We aimed to test the hypothesis that G interacts with far-red light to induce SAS, with this interaction mediated by phytochromes (phys). The tomato (Solanum lycopersicum cv. Moneymaker) wild-type (WT) and phyA, phyB1B2, and phyAB1B2 mutants were grown in a climate room with or without 30 µmol m−2 s−1 G on red/blue and red/blue/far-red backgrounds, maintaining the same photosynthetically active radiation (400–700 nm) of 150 µmol m−2 s−1 and red/blue ratio of 3. G hardly affected the dry mass accumulation or leaf area of WT, phyA, and phyB1B2 with or without far-red light. A lower phytochrome photostationary state (PSS) by adding far-red light significantly increased the total dry mass by enhancing the leaf area in WT plants but not in phy mutants. When the background light did not contain far-red light, partially replacing red/blue with G did not significantly affect stem elongation. However, when the background light contained far-red light, partially replacing red/blue with G enhanced elongation only when associated with a decrease in PSS, indicating that G interacts with far-red light on elongation only when the PSS changes.

Blue and Far-Red Light Affect Area and Number of Individual Leaves to Influence Vegetative Growth and Pigment Synthesis in Lettuce.

Published work indicates that high percentage of blue light can enhance pigment levels but decreases growth, while addition of far-red light to growth light can increase quantum efficiency and photosynthesis in leafy greens. Combining high-energy blue light with low-energy far-red light may increase both vegetative growth and pigment levels. However, the effect of high-energy blue and low-energy far-red light on the vegetative growth and pigments synthesis is unclear. This information can be potentially useful for enhancing the levels of pigments with nutritional value (e.g., beta-carotene and anthocyanins) in the produce grown in vertical farms. We grew romaine lettuce (cv. Amadeus) under similar light intensity (approximately 130 μmol⋅m–2⋅s–1) but different proportions of red: blue: far-red including 90:10: 0 (“High-R”), 50: 50: 0 (“High-B”), and 42: 42: 16 (“High-B+FR”) for 31 days. Results indicated that canopy area and leaf photosynthetic rate of lettuce plants was reduced in the High-B, thereby reducing plant growth. We did not observe photosynthesis enhancement in the High-B+FR. Instead, plants clearly showed photomorphogenic effects. The phytochrome photostationary state (PSS) decreased with far-red addition, resulting in reduced leaf number per plant. This was likely to shift the allocation of resources toward elongation growth for shade avoidance. Further, we observed an increase in the area of individual leaves, canopy area, and shoot dry weight in the High-B+FR. However, these appear to be an indirect consequence of decreased leaf number per plant. Our results also indicate that changes in expansion growth at individual leaf scale largely regulated pigment concentration in plants. As individual leaf area became smaller (e.g., High-B) or larger (e.g., High-B+FR), the levels of pigments including chlorophylls and beta-carotene increased or decreased, respectively. Area of individual leaves also positively influenced canopy area (and likely light interception) and shoots dry weight (or vegetative growth). Our study provides additional insights into the effects of high-energy blue and low-energy far-red light on individual leaf number and leaf growth, which appear to control plant growth and pigment levels in lettuce.

Improving the Predictive Value of Phytochrome Photoequilibrium: Consideration of Spectral Distortion Within a Leaf.

The ratio of active phytochrome (Pfr) to total phytochrome (Pr + Pfr), called phytochrome photo-equilibrium (PPE; also called phytochrome photostationary state, PSS) has been used to explain shade avoidance responses in both natural and controlled environments. PPE is commonly estimated using measurements of the spectral photon distribution (SPD) above the canopy and photoconversion coefficients. This approach has effectively predicted morphological responses when only red and far-red (FR) photon fluxes have varied, but controlled environment research often utilizes unique ratios of wavelengths so a more rigorous evaluation of the predictive ability of PPE on morphology is warranted. Estimations of PPE have rarely incorporated the optical effects of spectral distortion within a leaf caused by pigment absorbance and photon scattering. We studied stem elongation rate in the model plant cucumber under diverse spectral backgrounds over a range of one to 45% FR (total photon flux density, 400–750 nm, of 400 μmol m–2 s–1) and found that PPE was not predictive when blue and green varied. Preferential absorption of red and blue photons by chlorophyll results in an SPD that is relatively enriched in green and FR at the phytochrome molecule within a cell. This can be described by spectral distortion functions for specific layers of a leaf. Multiplying the photoconversion coefficients by these distortion functions yields photoconversion weighting factors that predict phytochrome conversion at the site of photon perception within leaf tissue. Incorporating spectral distortion improved the predictive value of PPE when phytochrome was assumed to be homogeneously distributed within the whole leaf. In a supporting study, the herbicide norflurazon was used to remove chlorophyll in seedlings. Using distortion functions unique to either green or white cotyledons, we came to the same conclusions as with whole plants in the longer-term study. Leaves of most species have similar spectral absorbance so this approach for predicting PPE should be broadly applicable. We provide a table of the photoconversion weighting factors. Our analysis indicates that the simple, intuitive ratio of FR (700–750 nm) to total photon flux (far-red fraction) is also a reliable predictor of morphological responses like stem length.

Modeling morphological adaptations of bell pepper (Capsicum annuum) to light spectra

Plant growth lighting research has typically focused on the effects of specific lamps with spectra resulting from the various materials used in building the lamps (eg HPS, Metal Halide, etc). In recent years a single waveband or the pairing of two wavebands as become common as part of LED lamp technology. In this work, we developed eight models to predict height, stem diameter, shoot dry weight, leaf area, dry weight partitioning between leaves and the rest of the shoot, specific leaf weight, and two compactness scores of pepper seedlings. Bell pepper plants were grown under eight light treatments with various combinations of blue, green, red, and far-red light. Physiological measurements were recorded, and the data were fit using general linear models based on spectral quality. Our models explained 82%–98% of the between-groups variation for predicted variables, demonstrating the ability of empirical models to accurately predict growth outcomes for bell pepper seedlings based on light quality. Phytochrome photostationary state, a measure of active to inactive phytochrome under steady illumination, was the most impactful variable in every model, supporting the importance of phytochrome in regulating a wide variety of physiological responses. Blue light played an important role in reducing plant height and increasing stem diameter, partitioning of dry weight into leaves, and both compactness scores, while green light increased leaf area.

Growth and Physiological Responses of Norway Spruce (Picea abies (L.) H. Karst) Supplemented with Monochromatic Red, Blue and Far-Red Light

Monochromatic red light (R) supplementation is more efficient than blue light (B) in promoting Norway spruce (Picea abies (L.) H. Karst) growth. Transcriptome analysis has revealed that R and B may regulate stem growth by regulating phytohormones and secondary metabolites; however, the effects of light qualities on physiological responses and related gene expression in Norway spruce require further study. In the present study, three-year-old Norway spruce seedlings received sunlight during the daytime were exposed to monochromatic B (460 mm), monochromatic R (660 nm), monochromatic far-red light (FR, 730 nm), and a combination of three monochromatic lights (control, R:FR:B = 7:1:1) using light-emitting diode (LED) lamps for 12 h after sunset for 90 day. Growth traits, physiological responses, and related gene expression were determined. The results showed that light quality significantly affected Norway spruce growth. The stem height, root collar diameter, and current-year shoot length of seedlings treated with R were 2%, 10% and 12% higher, respectively, than those of the control, whereas seedlings treated with B and FR showed significantly lower values of these parameters compared with that of the control. The net photosynthetic rate (Pn) of seedlings under R treatment was 10% higher than that of the control, whereas the Pn values of seedlings treated with FR and B were 22% and 33%, respectively, lower than that of the control. The ratio of phosphoenolpyruvate carboxylase to ribulose-1,5-bisphosphate carboxylase/oxygenase (PEPC/Rubisco) of seedlings after the R treatment (0.581) was the highest and 3.98 times higher than that of the seedlings treated with B. Light quality significantly affected the gibberellic acid (GAs) levels, which was 13% higher in seedlings treated with R (6.4 g/100 ng) than that of the control, whereas, the GAs level of seedlings treated with B and FR was 17% and 19% lower, respectively, than that of the control. In addition, seedlings treated with R achieved the lowest ratio of leaf chlorophyll content to fresh weight (8.7). Compared to the R and control treatments, seedlings received FR treatment had consistently lower values of the quantum yield of electron transport beyond QA− (primary quinone, ϕEo) and efficiency, with which a trapped exciton moves an electron into the electron transport chain beyond QA− (ψo), while higher values of the relatively variable fluorescence at the J step and normalized relatively variable fluorescence at the K step (Wk). The values of ϕEo, ψO, VJ and Wk in seedlings treated with B were similar to those in the control group. The expression of genes associated with light signal transduction, such as PHYTOCHROME C (PHYC), ELONGATED HYPOCOTYL5 (HY5), CONSTITUTIVE PHOTOMORPHOGENIC 1-2 (COP1-2), and PHYTOCHROMEINTERACTING FACTOR 3 (PIF3), was significantly higher in seedlings under B treatment than those under other light treatments. Nevertheless, significant differences were not observed in the expression of COP1-2, HY5, and PIF3 between the R treatment and the control. The expression value of COP1-2 was significantly lower in R than FR light treatments. In conclusion, compared with the control, R promotes, whereas B and FR inhibit Norway spruce growth, which was accompanied by physiological changes and genes expression regulation that may be relate to a changing phytochrome photostationary state (PSS) with the supplemental R in seedlings.

Improvement of Growth and Morphology of Vegetable Seedlings with Supplemental Far-Red Enriched LED Lights in a Plant Factory

Although light-emitting diode (LED) lamps have been broadly applied in horticultural production to improve plant yield and quality, compared to natural light there is a disadvantage in the lack of far-red light in the LED spectrum. Far-red light has been studied widely to control plant growth and development. Therefore, this study aimed to find the effect of supplemental far-red-enriched LED lights to control the growth of tomato, red pepper, cucumber, gourd, watermelon and bottle gourd seedlings. The treatments were cool white LED:far-red LED at ratios of 5:0, 5:1, 5:2 and 5:3. The growth of tomato and red pepper seedlings, including hypocotyl length, was correlated to far-red light and light intensity. The phytochrome photostationary state (PSS) value of maximum hypocotyl length by supplemental far-red-enriched light ranged from 0.69 to 0.77 in tomato and red pepper seedlings. Although hypocotyl lengths of cucumber and watermelon were greatly affected by PSS, the PSS value for maximum hypocotyl length was lower than for tomato and red pepper. These results show that manipulating supplemental far-red enrichment can be used to control vegetable seedling growth with some variation among plant species.

Maximum elongation growth promoted as a shade-avoidance response by blue light is related to deactivated phytochrome: a comparison with red light in four microgreen species

To clarify detailed patterns of responses to blue light associated with decreasing phytochrome activity, the growth and morphology traits of arugula, cabbage, mustard, and kale microgreens were compared under the treatments: (1) R, pure red light; (2) B, pure blue light; (3) BRF0, (4) BRF2, (5) BRF4, and (6) BRF6: unpure blue lights created by mixing B with low-level (6%) R, and further adding 0, 2, 4, and 6 μmol m−2 s−1 of far-red light, respectively. The calculated phytochrome photostationary state (PPS) value, indicating phytochrome activity, gradually decreased in the order of R (0.89), BRF0 (0.69), BRF2 (0.65), BRF4 (0.63), BRF6 (0.60), and B (0.50). Generally, the elongation growth (including stem extension rate, hypocotyl length, or petiole length) under blue lights increased with the decreasing PPS values, showing the highest and lowest sensitivity for arugula and mustard, respectively. However, the elongation promoted by blue lights gradually became saturated once the PPS values decreased below 0.60, a level which deactivates phytochrome. Other plant traits, such as biomass allocation and plant color, varied with increasing shade-avoidance responses to blue lights with decreasing PPS values relative to R, and these traits reached saturation at a similar PPS value as elongation. The response sensitivity was highest in elongation growth for arugula and cabbage, and highest in plant color for kale and mustard. This suggests that deactivated phytochrome contributes to the maximum elongation promotion as a shade-avoidance response induced by blue light, although the response sensitivity varies with plant traits and species.

Adventitious rooting of Chrysanthemum is stimulated by a low red:far-red ratio

Adventitious rooting, a critical process in the vegetative propagation of many ornamentals, can be affected by both light intensity and light quality. We investigated the use of spectral light quality to improve adventitious rooting of Chrysanthemum morifolium cuttings by applying different combinations of blue, red and far-red light. Additionally, unrooted cuttings were treated before planting with two auxin transport inhibitors (TIBA and NPA) to study the effect of light quality on auxin biosynthesis and/or transport. Results showed that lowering the R:FR ratio (decreasing the phytochrome photostationary state, PSS) improved rooting significantly and decreased the inhibiting effect of the auxin transport inhibitor NPA. An extra decrease of PSS by adding blue light to a red + far-red spectrum further enhanced rooting. In contrast, adding blue light to solely red light decreased rooting, an effect which was more pronounced in combination with the auxin transport inhibitors TIBA and NPA. Our results show that phytochrome plays a role in adventitious root formation through the action of auxin, but that also blue light receptors interact in this process.

Growth and phenolic compounds of Crepidiastrum denticulatum under various blue light intensities with a fixed phytochrome photostationary state using far-red light

This study aimed to evaluate the effect of the red (R)-to-blue (B) ratio of light on the growth and phenolic compounds of Crepidiastrum denticulatum under a phytochrome photostationary state (PSS) for growth in a plant factory with artificial light (PFAL) using light-emitting diodes (LEDs). Three-week-old C. denticulatum seedlings were transplanted into a PFAL where the air temperature, relative humidity, CO2 concentration, and light period were set at 20 °C, 60%, 1000 μmol·mol−1, and 16 h, respectively. Three controls were used with different ratios of R to B light without supplemental FR light: 8:2, 7:3, and 6:4 (based on chip number; 130 µmol·m−2·s−1 photosynthetic photon flux density). For the treatments, the same R to B light ratios as in the controls were used but supplemented with FR light, plus a treatment with only R light supplemented with FR, set to a PSS of 0.71. Growth characteristics and total phenolic and individual phenolic contents were measured after a 6-week treatment. When the R light ratio increased, shoot dry weight, leaf length, leaf width, and leaf area increased regardless of supplemental FR light, and the R8B2 with FR light treatment was the most effective, with significantly higher values (1.6–2 times) than the control. FR irradiation did not have any negative effect on total phenolic content, chlorogenic acid, caffeic acid, and chicoric acid per unit dry weight (g); thus, the R8B2 with FR treatment had significantly higher total phenolic and individual phenolic contents per shoot (43% and 52–62%, respectively). Thus, supplemental lighting with FR LEDs was found to be effective to enhance the growth and bioactive compounds of C. denticulatum in a PFAL installed with a R and B lighting system, and the effectiveness could be changed by the RB ratio, with 8:2 considered the proper ratio.

Proposed Product Label for Electric Lamps Used in the Plant Sciences

Electric lamps are widely used to supplement sunlight (supplemental lighting) and daylength extension (photoperiodic lighting) for the production of horticultural crops in greenhouses and controlled environments. Recent advances in light-emitting diode (LED) technology now provide the horticultural industry with multiple lighting options. However, growers are unable to compare technologies and LED options because of insufficient data on lamp performance metrics. Here, we propose a standardized product label that facilitates the comparison of lamps across manufacturers. This label includes the photosynthetically active radiation (PAR) efficacy, PAR conversion efficiency, photon flux density output in key wave bands, as well as the phytochrome photostationary state (PSS), red/far red ratio, and graphs of the normalized photon flux density across the 300–900 nm wave band and a horizontal distribution of the light output.

Enhancing Flower Stalk Emergence in <i>Phalaenopsis</i> by Red Light Supplementation

A key step in regulating the flowering of Phalaenopsis is to control the emergence of the flower stalk or spiking. Light quality is an important factor to regulate plant reproduction. We used either red or blue light emitting diodes (LEDs) to substitute for 10% photosynthetic photon flux emitted by white fluorescent tubes as the control (WC) at approximately 70 μmole&middotm-2&middots-1 to examine the effects of enhanced red (WR) or blue (WB) light on spiking and the dawn-to-dusk fluctuations in the photosynthetic pigments and carbohydrates of Phalaenopsis aphrodite, which were grown in 7.5 cm diameter pots for six weeks. WR treatment significantly elevated the ratio of red to far-red light and the level of phytochrome photostationary state in concurrence with an increase in the spiking ratio and length, but had little effect on the concentrations of chlorophyll a and chlorophyll b in the leaf, levels of soluble sugars such as glucose, fructose, sucrose, and insoluble starch in the leaf and shortened stem, the locus of spiking, when compared to the other two treatments. Evidently, the spiking of Phalaenopsis is improved by dependent on the relative amount of active phytochrome expressed in the photostationary state.

Can a high red: Far red ratio replace temperature-induced inflorescence development in Phalaenopsis?

Phalaenopsis is one of the most economically important ornamental crops in the Netherlands, but due to its growth requirements during its various developmental stages, it is a costly crop to grow in terms of energy. Warming the greenhouse in the winter during the vegetative phase, and cooling the greenhouse to 19°C in the summer to induce flowering costs a great deal of energy. Because flower induction is primarily controlled by altering the balance of plant hormones, it might well be possible to steer flower induction by changing the light spectrum instead of changing the temperature. The objective of the present study was to investigate the role of the light spectrum on bud break and inflorescence elongation in Phalaenopsis. To this end an experiment was designed with two Phalaenopsis cultivars, Quincy and Red Stones in a matrix of temperature and spectral treatments. Light treatments were used with a relatively high red to far-red ratio (R:FR) and therefore a high phytochrome photostationary state (PSS) value of 0.85, and with a relatively lower R:FR ratio resulting in a PSS value of 0.71, similar to that of natural daylight. Our hypothesis was that low temperature treatments can, at least partially, be substituted by a light spectrum with a high PSS value in order to suppress apical dominance of the primary flowering inflorescence and thus realize multiple inflorescences. A cooling period (19°C) of 8 weeks always resulted in a high percentage of multiple inflorescences, both from Red Stones as well as Quincy. This implies that a flower induction period of 8 weeks with cooling is effective, regardless of the light spectrum. An induction period of 8 weeks with “red light” always resulted in a high percentage of multiple inflorescences, regardless of the temperature (19 or 22°C). A cooling period (19°C) of 4 weeks followed by 4 weeks 22°C only resulted in a higher percentage of multiple inflorescences under the light spectrum with a high PSS value during the second 4 week-period. This suggests that the plant hormones responsible for bud break and inflorescence elongation, can be stimulated by light with a high PSS value, as well as by a cool phase. In order to stimulate multiple flowering inflorescences, the temperature and/or light spectrum during the second 4-week-period appears to be more important than during the first 4-week-period of the induction phase. Presumably this is due to suppression of the apical dominance of the first emerging inflorescence by low temperature and/or relatively high R:FR ratio during this phase. This also implies that the energy costly cool-induction period during the summer can be replaced, at least in part, by red light or a relatively high R:FR.

Phytochrome-controlled level of growth substances in etiolated oat seedlings

Irradiation with red light of coleoptiles and leaves of etiolated oat seedlings, causing photoconversion of phytochrome mainly into P<sub>fr</sub>, leads to the release of free auxins and free gibberellins from conjugated forms. The effect of red light is reversible by far-red light irradiation. A correlation between the photostationary state of phytochrome and endogenous abscisic acid content was not found.

Artificial light from light emitting diodes (LEDs) with a high portion of blue light results in shorter poinsettias compared to high pressure sodium (HPS) lamps

Strict control of morphogenesis is essential in production of potted poinsettia. Commonly, this is obtained by the use of plant growth retardants (PGRs), often in combination with early morning temperature drops. Due to negative effects on human health and the environment, the use of PGRs is becoming restricted. Also, energy-saving growth regimes and periods of high temperatures limit effective use of temperature drops. In the present study the use of a high proportion of blue (B) light provided by light emitting diodes [LEDs, 20% blue (B), 80% red (R)] was compared with traditional high pressure sodium (HPS) lamps (5% B) providing similar phytochrome photostationary state to produce compact poinsettia plants. Both in the greenhouse and growth chamber, all cultivars were 20–34% shorter for LED compared to HPS grown plants. Also, leaf and bract area as well as chlorophyll content and total dry matter accumulation were lower under LED. The LED did not delay bract color formation, visible cyathia and flowering compared to HPS, and no difference in post production performance (cyathia/bract abscission or necrosis) between the two light treatments was found. The effect of end of day-red (EOD-R) lighting combination with LED and HPS supplemental lamps during the photoperiod in the greenhouse was also investigated. Reduced stem extension (13%) was observed under HPS only and for one of the two cultivars tested, whereas under the LED regime, there was no effect of EOD-R lighting.

FINDING THE OPTIMAL GROWTH-LIGHT SPECTRUM FOR GREENHOUSE CROPS

Abstract: Especially in an open crop (e.g., young plants) morphological responses to light quality can affect light interception, crop photosynthesis and growth. Earlier work showed a substantial morphology related biomass increase for young cucumber plants grown under 100% artificial sunlight (ASL) compared with 100% high pressure sodium light (HPS). Here, ASL is used to investigate the effect of HPS and LEDs compared with ASL, when applied supplemental to an ASL background. Tomato plants were grown in a climate room under 17 h ASL (50% of in total 200 µmol PAR m-2 s-1) supplemented with 50% HPS, light emitting diodes LEDs (red/blue), or ASL. The 100% ASL-grown plants produced 32-45% more dry weight, due to a more efficient light interception. As ASL lamps are not energy-efficient enough for commercial production we tried to simplify the solar spectrum while retaining enhanced crop productivity in greenhouses. Red/blue/far-red LEDs, at a ratio inducing the same phytochrome photostationary state (PSS) as natural sunlight, and sulphur-plasma lamps, emitting a continuous spectrum in the PAR-region, were tested and compared with supplemental red/blue LEDs, HPS and ASL in a greenhouse experiment. Red/blue/far-red LEDs resulted in a visual appearance similar to the ASL-plants, while red/blue LEDs produced the most compact morphology. Red/blue/far-red LEDs enhanced dry weight for cucumber (+21%) and tomato (+15%) compared with HPS. Dry weight and compactness were intermediate for sulphur-plasma. The differences were attributable to effects of leaf orientation and positioning on light interception, and not to photosynthesis per unit leaf area. The PSS appears to be a key-factor to control crop morphology, providing a tool to induce ‘sunlight’ crop characteristics to enhance productivity.

Blue light inhibits stomatal development in soybean isolines containing kaempferol‐3‐O‐2G‐glycosyl‐gentiobioside (K9), a unique flavonoid glycoside

ABSTRACTStomata have a fundamental role in controlling plant photosynthesis and transpiration, but very little is known about factors controlling stomatal differentiation and development. Lines of soybean that contain a specific flavonol glycoside, kaempferol‐3‐O‐2‐glycosyl‐gentiobioside (K9), as well as greatly reduced stomatal density, especially on the adaxial epidermis, have been identified. The specific effects of blue light photoreceptors on stomatal development in K9 lines and their isoline pairs containing no K9 were studied. Low irradiances of blue light (7% of total photosynthetically active radiation) added to high irradiances from low‐pressure sodium lamps strongly inhibited stomatal development on the adaxial epidermis of K9 lines, but not in isoline pairs differing putatively in only one gene and lacking K9. Overall, blue light slightly increased stomatal density on the abaxial epidermis in all isolines, demonstrating differential regulation of stomatal development in the upper and lower epidermis. Blue light also caused an increase in leaf area in all isolines, indicating that changes in stomatal density were not the non‐specific result of alterations in leaf area. Morphological studies revealed that the blue light‐induced reduction in stomatal density in K9 lines was due to reduced stomatal initiation as well as aborted or abnormal stomatal development. As the phytochrome photostationary state was kept constant, the results indicate that one or more blue light receptors are involved in the control of stomatal development. This system should be useful for the study of mechanisms controlling stomatal development, even if the photo‐inhibitory response is unique to K9 lines.

Growth and Photomorphogenesis of Pepper Plants under Red Light-emitting Diodes with Supplemental Blue or Far-red Lighting

Light-emitting diodes (LEDs) are a potential irradiation source for intensive plant culture systems and photobiological research. They have small size, low mass, a long functional life, and narrow spectral output. In this study, we measured the growth and dry matter partitioning of 'Hungarian Wax' pepper (Capsicum annuum L.) plants grown under red LEDs compared with similar plants grown under red LEDs with supplemental blue or far-red radiation or under broad spectrum metal halide (MH) lamps. Additionally, we describe the thermal and spectral characteristics of these sources. The LEDs used in this study had a narrow bandwidth at half peak height (25 nm) and a focused maximum spectral output at 660 nm for the red and 735 nm for the far-red. Near infrared radiation (800 to 3000 nm) was below detection and thermal infrared radiation (3000 to 50,000 nm) was lower in the LEDs compared to the MH source. Although the red to far-red ratio varied considerably, the calculated phytochrome photostationary state (phi) was only slightly different between the radiation sources. Plant biomass was reduced when peppers were grown under red LEDs in the absence of blue wavelengths compared to plants grown under supplemental blue fluorescent lamps or MH lamps. The addition of far-red radiation resulted in taller plants with greater stem mass than red LEDs alone. There were fewer leaves under red or red plus far-red radiation than with lamps producing blue wavelengths. These results indicate that red LEDs may be suitable, in proper combination with other wavelengths of light, for the culture of plants in tightly controlled environments such as space-based plant culture systems.