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 400-500 nm, green is 500-600 nm, red is 600-700 nm, and far red is 700-750 nm (or so). This means in many papers that cyan and yellow/amber will count as green light although their photosynthesis rates are different. Cyan has lower photosynthesis rates due to the higher absorption of cyan light by carotenoids, which is only 30-70% efficient at transferring energy to chlorophyll. 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 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

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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

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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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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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