Photomorphogenesis: How Light Colors Affect Plants Growth
by /u/SuperAngryGuy of /r/HandsOnComplexity
Photomorphogenesis has to do with light sensitive proteins, and unlike photosynthesis, can be very wavelength dependent in a plant's response. The phytochromes are predominately red and far red with Pr peaking around 660nm. The blue sensitive proteins are the crytochromes and phototropins have what's known as the 'three finger blue action response' with peaks at roughly 430nm, 450nm, and 470nm depending on the specific protein.
In most papers, blue is counted as 400-500nm, green is 500-600nm, red is 600-700nm, and far red is 700-750nm (or so). This means in many papers that cyan and yellow/amber will count as green light although their photosynthesis rates are different. You'll see in the McCree curve that yellow/amber light is very efficient.
Phototropin protein absorbs light best at blue wavelengths (original source)
BLUE
Blue light decreases acid growth (which is different than growth through photosynthesis). Excess acid growth, or 'stretching', in seedlings/veg is all about greater cell expansion in the stem that we get from lower lighting levels or not enough blue light. We typically only want as much blue in a light source to help prevent any excess stem elongation. Blue photons have much more energy needed for photosynthesis, and this extra energy is wasted as heat that the plant has to dissipate. The associated blue light sensitive proteins are the phototropins and cryptochromes.
Several studies have shown that some blue light is necessary for normal growth and development, but the effects of blue light appear to be species-dependent and may interact with other wavelengths of light as well as photosynthetic photon flux (PPF). Overall, the low blue light from warm white LEDs increased stem elongation and leaf expansion, whereas the high blue light from cool white LEDs resulted in more compact plants. The optimal amount of blue light likely changes with plant age because plant communities balance the need for rapid leaf expansion, which is necessary to maximize radiation capture, with prevention of excessive stem elongation. Cope and Bugbee. Spectral Effects of Three Types of White Light-emitting Diodes on Plant Growth and Development: Absolute versus Relative Amounts of Blue Light
View related studies and papers (7)
- Park and Runkle. Spectral effects of LED on plant growth, visual color quality, and photosynthetic photon efficacy
- Waralee Phansurin et al. Comparison of Growth, Development, and Photosynthesis Under White or Red-blue LED lights
- Hideo Yoshida et al. Effects of varying light quality from single-peak blue and red LED
- Haijie Dou, Genhua Niu, and Mengmeng Gu. Basil Plants Influenced by Substituting Green Light for Partial Red and/or Blue Light
- Jie He et al. Plant Growth and Photosynthetic Characteristics under Different Blue and Red LEDs
- Li-Jun Liu et al. COP1-Mediated Ubiquitination of CONSTANS Is Implicated in Cryptochrome Regulation of Flowering
- Cope and Bugbee. Spectral Effects of Three Types of White Light-emitting Diodes on Plant Growth and Development: Absolute versus Relative Amounts of Blue Light
GREEN
In healthy cannabis, 80-90% of green light is being absorbed and available for photosynthesis. Green is the opposite of blue in photomorphogenesis responses in that green causes stretching also called the shade avoidance responses. Pretty much anything blue does, green does the opposite.
Maximum external quantum efficiency (EQE) of different commercial nitride and phosphide LEDs (spheres), illustrating the green gap problem. (original source)
Green can help make leaves larger due to the reversibility of blue light sensitive proteins and this in turn increases the LAI (leaf area index) for greater light capture. Green light can also cause the stomata (gaseous exchange pores) of plants to close a bit more than normal, which is the opposite of blue light.
Green LEDs are relatively electrically inefficient which is why they are not commonly used in grow lights. In physics/engineering this is known as the green gap (graph). We do, however, perceive green light much higher than red or blue light, so for display purposes this inefficiency matters less.
Absorption of green light is used to stimulate photosynthesis deep within the leaf and canopy profile, contributing to carbon gain and likely crop yield. In addition, green light also contributes to the array of signalling information available to leaves, resulting in developmental adaptation and immediate physiological responses. Within shaded canopies this enables optimization of resource-use efficiency and acclimation of photosynthesis to available irradiance. Smith et al. Don’t ignore the green light: exploring diverse roles in plant processes
View related studies and papers (14)
- Terashima et al. Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light
- Sun et al. Green Light Drives CO2 Fixation Deep within Leaves
- Moriwaki et al. Nitrogen-improved photosynthesis quantum yield is driven by increased thylakoid density, enhancing green light absorption
- Nishio. Why are higher plants green? Evolution of the higher plant photosynthetic pigment complement
- Johkan et al. Effect of green light wavelength and intensity on photomorphogenesis and photosynthesis in Lactuca sativa
- Marosvölgyi and van Gorkom. Cost and Color of Photosynthesis
- Snowden. Effects of Blue and Green Light on Plant Growth and Development at Low and High Photosynthetic Photon Flux
- Snowden, Cope, Bugbee. Sensitivity of Seven Diverse Species to Blue and Green Light: Interactions With Photon Flux
- Wang and Folta. Contributions of green light to plant growth and development
- Wang, Zhang and Folta. Green light augments far-red-light-induced shade response
- Zhang and Folta. Green light signaling and adaptive response
- Griffin-Nolan et al. Green light drives photosynthesis in mosses
- Battle and Jones. Cryptochromes integrate green light signals into the circadian system
RED & FAR RED
For photomorphogenesis responses, red and far red are opposites like blue and green are opposites. Red can help keep a plant more compact but not nearly to the degree of blue. The associated red/far red light sensitive proteins are the phytochromes. Red photons have a lower energy with a higher theoretical PPE of about 5.51 uMol/joule (660nm) compared to blue of 3.76 uMol/joule (450nm). The higher efficacy is one reason why red LEDs are being added to white LEDs: what's held them back is their electrical efficiency (red and blue LEDs use different semiconductor material).
Far red chlorophyll fluorescence imaging of a leaf showing damage not normally visible
Far red causes greater stretching like green light and contributes to the shade avoidance responses. It 'may' help put short day plants 'to sleep' faster. Far red may in some plants may be able to drive photosynthesis efficiently though the Emerson enhancement effect. About 50% of far red light is reflected off plant leaves, and also transmits easily though leaves.
Far-red light (700–800 nm) is integral to initiating shade responses which can increase plant growth. The far-red gradient study revealed that increasing supplemental far-red light increased leaf length and width, which was associated with increased projected canopy size (PCS). The higher PCS was associated with increased cumulative incident light received by plants, which increased dry matter accumulation. Far-red light provided by LEDs increases the canopy size to capture more light to drive photosynthesis. Legendre and van Iersel. Supplemental Far-Red Light Stimulates Lettuce Growth
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- Zhen and Bugbee. Far-red photons have equivalent efficiency to traditional photosynthetic photons
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- Zhen, Haidekker and van Iersel. Far-red light enhances photochemical efficiency in a wavelength-dependent manner
- Elkins and van Iersel. Supplemental Far-red LED Increases Growth of Foxglove Seedlings Under Sole-source Lighting
- Pettai et al. Photosynthetic activity of far-red light in green plants
- Pettai et al. The long-wavelength limit of plant photosynthesis
- Dorokhov et al. The effect of far-red light on the productivity and photosynthetic activity of tomato
- Yongran Ji et al. Far‐red radiation stimulates dry mass partitioning to fruits by increasing fruit sink strength in tomato
- Lee, Park and Oh. Growth and Cell Division of Lettuce Plants under Various Ratios of Red to Far-red LED
- Yang et al. Effects of the red:far-red light ratio on photosynthetic characteristics
- Yang et al. Effect of interactions between light intensity and red:far-red ratio on photosynthesis
- Zhang et al. Disentangling the effects of PAR and red to far-red ratio on plant photosynthesis under canopy shading
- Zhang et al. Overhead supplemental far-red light stimulates tomato growth under intra-canopy lighting with LEDs
- Zhang et al. Morphological and physiological properties of indoor cultivated lettuce in response to additional far-red light
- Ji et al. Dissecting the Genotypic Variation of Growth Responses to Far-Red Radiation in Tomato
- Shibuya et al. The photosynthetic parameters as affected by irradiances with different red:far-red ratios
- Chen et al. Growth and nutritional properties of lettuce affected by mixed irradiation of white and supplemental light
- Owen, Meng and Lopez. Promotion of Flowering from Far-red Radiation Depends on the Photosynthetic DLI
- Lee, Son and Oh. Increase in Biomass and Bioactive Compounds in Lettuce under Various Ratios of Red to Far-red LED Light
- Kalaitzoglou et al. Effects of Far-Red Light on Tomato Plant Growth, Morphology, Light Absorption, and Fruit Production
- Jiang et al. Effect of supplemental far-red light with blue and red LED on leaf photosynthesis, stomatal regulation and development
- Kono et al. Far-Red Light Accelerates Photosynthesis in the Low-Light Phases of Fluctuating Light
- Bojka Kump. The role of far-red light (FR) in photomorphogenesis and its use in greenhouse plant production
- Laisk et al. Action spectra of photosystems II and I and quantum yield of photosynthesis in leaves in State 1
- Laisk et al. Fast cyclic electron transport around photosystem I in leaves under far-red light
- Lysenko et al. Far-Red Spectrum of Second Emerson Effect: A Study Using Dual-Wavelength Pulse Amplitude Modulation Fluorometry
- Thapper et al. Defining the Far-Red Limit of Photosystem II in Spinach
- Park and Runkle. Far-red radiation promotes growth of seedlings by increasing leaf expansion and whole-plant net assimilation
- Park and Runkle. Blue radiation attenuates the effects of the red to far-red ratio on extension growth but not on flowering
- Craig and Runkle. A Moderate to High Red to Far-red Light Ratio from LED Controls Flowering of Short-day Plants
- Meng, Kelly and Runkle. Substituting green or far-red radiation for blue radiation induces shade avoidance and promotes growth
- Sharrock. The phytochrome red/far-red photoreceptor superfamily
- Affandi et al. Far-red light during cultivation induces post harvest cold tolerance in tomato fruit
- Kim et al. Supplemental intracanopy far-red radiation to red LED light improves fruit quality attributes
- Demotes-Mainard et al. Plant responses to red and far-red lights, applications in horticulture
- Lee, Xu, Wang and Rajashekar. The effect of far-red light on the productivity and photosynthetic activity of tomato
- Lee, Xu, Wang and Rajashekar. The Effect of Supplemental Blue, Red and Far-Red Light on Growth and Nutritional Quality
- Murakami et al. A Mathematical Model of Photosynthetic Electron Transport in Response to the Light Spectrum
- Kohler and Lopez. Duration of LED supplemental lighting providing far-red radiation during seedling production influences time to flower
- Hwang et al. Improvement of Growth and Morphology of Vegetable Seedlings with Supplemental Far-Red Enriched LED
- Aguirre-Becerra et al. Effect of Extended Photoperiod with a Fixed Mixture of Light Wavelengths on Tomato Seedlings
UV
Space Buckets can be a great framework for photomorphogenesis experimentation
Ultraviolet is a wild card and I can make no rhyme or reason of it working with a variety of plants. It tends to cause dwarfing when used as an only light source (UVA). For cannabis, the idea is to try to increase trichome and THC levels by adding UV, but some researchers including Bruce Bugbee are saying this does not happen. The only identified UV protein is the UVR8 protein, which is only UVB sensitive, not UVA sensitive (285nm peak sensitivity).
UV light can influence the physiological responses of plants. Interaction between plants and UV light is regulated by photoreceptors such as UV Resistance Locus 8 (UVR8) that enables acclimation to UV-B stress. Although UV in high doses is known to damage quality and production parameters, some studies show that UV in low doses may stimulate biomass accumulation and the synthesis of healthy compounds that mainly absorb UV. Abiotic stress induced by UV exposure increases resistance to insects and pathogens, and reduce postharvest quality depletion. Loconsole and Santamaria. UV Lighting in Horticulture: A Sustainable Tool for Improving Production Quality and Food Safety
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- Chen et al. UVA Radiation Is Beneficial for Yield and Quality of Indoor Cultivated Lettuce
- Loconsole and Santamaria. UV Lighting in Horticulture: A Sustainable Tool for Improving Production Quality and Food Safety
- Romen Naorem. The Secret Beauty of Ultraviolet Radiation in Plants
- Jansen et al. Higher plants and UV-B radiation: balancing damage, repair and acclimation