Horticultural lighting technologies have improved dramatically over the past century, but manipulation of light spectrum is a relatively new concept. Since plants tend to absorb red and blue light most strongly (according to absorption spectra), other wavelengths have been disregarded as unnecessary for plant growth and development. As LED technology has progressed, the ability to provide individual spectra has as well. Pink/purple light fixtures have flooded the horticultural lighting market as a result.

Regardless of this influx of products, research on plant lighting continues to explore the varying interactions that plants have with light and have started to discourage the idea that plants only need two individual spectra for optimal growth. While there is still much disagreement over the primary biological function of light in the 550 nm to 600 nm range as well as in the far-red and ultraviolet (UV) wavebands, research has shown that there are many functions and benefits to incorporating these spectra into a growing space.

Human and plant light perception use many of the same molecules to detect photons; however, our eyes are more easily fooled than plant photoreceptors. Narrow band red, blue, and green light (RGB), when mixed in the right proportion, are perceived as white light to the human eye. However, plants are quite aware that they are receiving three individual spectra and their growth habits will demonstrate that.

Broad spectrum white light can come in many forms depending on the source. To the human eye, we mostly perceive these different spectra as cooler (blue) or warmer (orange/red) depending on whether the luminar is a metal halide or high-pressure sodium lamp, or in the case of LEDs and fluorescent bulbs, what type of phosphor coating is used. To a plant however, each individual wavelength may promote a different growth habit and photomorphogenic response.

Incoming photons are absorbed by pigments, which absorb light as energy, and photoreceptors, which perceive light as a signal. When absorbed by the most well-studied pigment, chlorophyll, photons can drive photosynthesis and growth. However, chlorophyll, with its wide-ranging absorption spectrum, is not enough to efficiently harvest light by itself. The “antenna complex” is a concept in biology that describes how accessory pigments such as carotenoids can assist both in capturing light that chlorophyll does not absorb, or dissipating excess light as heat (non-photochemical quenching) when photosynthetic reaction centers are overloaded with incoming energy.

Accessory pigments are primarily carotenoids such as beta-carotene, lutein, zeaxanthin, antheraxanthin, and violaxanthin. These pigments are yellow to orange in color and absorb light most strongly in the range of 450 nm to 550 nm. Some of these pigments change forms based on lighting conditions through processes called epoxidation and de-epoxidation. If radiant exposure is too high, damage can occur to the photosynthetic apparatus, so it is important for a plant to be able to deal with this incoming energy. Under pure sunlight where radiant exposure may fluctuate throughout the day, the antenna complex adjusts to accept or dissipate light:

1. When radiant exposure is low, violaxanthin will capture photons and transfer this energy to chlorophyll, improving efficiency of light absorption.
2. When radiant exposure is high, violaxanthin is de-epoxidated (converted) into zeaxanthin which then dissipates excess photons as heat.

Beta-carotene functions similarly to violaxanthin and lutein functions like zeaxanthin but without the interconversion process called the “Xanthophyll Cycle.” The flow of energy between pigments occurs spontaneously as they are “excited” by light particles. Interestingly, carotenoids that protect plants from light and improve their ability to capture light can also serve similar functions within the eyes of many animal species. There are several other plant pigments unassociated with the photosynthetic light-harvesting complex including anthocyanin and lycopene. Though these compounds do absorb light, their main function is to protect cells and DNA from damaging UV radiation as well as scavenge “free radicals” such as hydrogen peroxide, preventing further cellular damage.

Editor’s Note: These molecules that protect plants from high light levels are typically dark, resulting in the deep purple or deep reds observed in some plants. Red lettuce is a great example of this.

Photoreceptors, in most cases, are proteins that are paired with a “chromophore” that absorbs certain wavelengths of light and then send a signal to the plant, ultimately influencing photomorphogenesis. There are several different types of photoreceptors and their light absorption ranges overlap:

1. Cryptochromes use light in the range of 300 nm to 500 nm though it most strongly absorbs at 350 nm (UV-A) and 450 nm (blue). This receptor, when it is excited by light, prevents elongation of hypocotyls (main stem of seedlings) and even mediates flowering and photoperiod in some species.
2. Phototropins are blue/UV-A absorbing photoreceptors but with a much stronger absorption peak at 450 nm and are thought to regulate phototropism (process in which plants move in response to light), stomatal aperture (opening and closing of stomatal pores), movement of chloroplasts (photosynthetic organelles containing chlorophyll) within leaf cells, and the inhibition of leaf expansion.
3. Phytochromes are some of the more famous photoreceptors because they can strongly influence flowering. It is a little known fact that phytochromes actually absorb light in the range of 300 nm to 800 nm. Most of the known functions however, are a result of the absorption peaks at 660 nm in the P_r form and 730 nm in the P_fr form.

Phytochromes are constantly changing form and reach a “photoequilibrium” that is regulated by spectral ratio and PPFD present in the growing environment. Depending on the photoequilibrium of phytochrome, different signals may be sent down different metabolic pathways within the plant. These signals can regulate many processes including germination, seedling establishment, stem elongation, leaf expansion, and of course flowering and photoperiod. Different ratios of R:FR (red to far red) light received by a plant will dictate how the plant develops in terms of compactness, flower size, flower number, etc. There are several other newly discovered and under-researched photoreceptors (UV-B receptor correlated with anthocyanin accumulation) but we will not discuss those in this article.

Since there is significant overlap in the absorption spectra of these photoreceptors, most photomorphogenic responses are co-regulated. Some responses may be turned on and off by one receptor, but the expression of that response can be amplified by another receptor. The enigmatic “circadian clock” (or circadian rhythm) that regulates so many functions within plants is the culmination of activity from multiple photoreceptors based on photoperiod, light spectrum, and PPFD. This rhythm of growth patterns within a plant strongly influences photomorphogenic outcomes; however, just like photosynthesis, there is an action spectrum for all photomorphogenic responses dictated by a mixture of signals from these photoreceptors and does not necessarily mirror the absorption spectrum.

When considering whether or not to use narrow band lighting, there are two primary considerations to ponder:

1. Whether or not your plants are already exposed to broad spectrum light (solar for greenhouses, or a broad-spectrum fixture for sole-source lighting applications)
2. What crops you are growing.

When plants are already exposed to broad spectrum lighting from a sole-source fixture, it makes sense to supplement with narrow band lighting only if there is a desired photomorphogenic effect that your crop can’t achieve without being exposed to a specific waveband. However, if you are growing under solar radiation and a high DLI, your crop may not be as responsive to changes in light spectrum, as solar radiation is quite broad already and may drown out the photomorphogenic benefits of narrow band lighting.

Another aspect to consider when growing under solar radiation is whether or not you need to increase your DLI. If you supplement with a narrow-band fixture as a method of increasing DLI, you may see inconsistencies in product quality as solar radiation increases and decreases throughout the year, exposing your crops to differing amounts of sunlight and narrow band light. If your DLI is constant and you only wish to induce a photomorphogenic response such as coloring, compactness, or rooting, it may make sense for you to supplement more blue light. However, DLI often has more of an impact on desired traits than spectrum. If you are growing a flowering/fruiting crop (like cannabis) and wish to encourage more flower/fruit growth (with a sufficient DLI and photoperiod) it may be beneficial to supplement more red light, as 660 nm light encourages phytochrome responses in many species, which sends signals throughout the plant to encourage reproductive growth.

Narrow band lighting can provide acceptable growth for many species, so long as there are no significant fluctuations in DLI independent of the portion supplied by the light fixture. However, plants use a variety of different photoreceptors and pigments that cooperatively regulate growth and development. Plants developed these photomorphogenic responses under broad spectrum light and it is very rare for a certain species to express a response to narrow band lighting that cannot also be achieved by broad spectrum lighting given sufficient DLI. For consistent product quality, broad spectrum fixtures are a safer choice. Different species can have varying responses to changes in light spectrum. Research is constantly ongoing that is attempting understand how individual crops respond to different light spectra, and in some cases there is clear evidence about what type of lighting is best for a crop. If you are uncertain about the response your crop may have, supplementing with broad spectrum light has a proven track record of improving crop quality, consistency, and yield.

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1. Website: https://fluence.science/
2. Email: info@fluencebioengineering.com
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Plants have co-evolved with soil microbes over hundreds of millions of years. When bacteria colonized the Earth and transformed the atmosphere over three billion years ago, they created conditions that made it possible for soil fungi to evolve (approximately 900 millions of years ago). Together, bacteria and fungi have shaped Earth’s soil structure and created habitable conditions for the evolution of plants around 700 millions of years ago.

Soil microbes are ubiquitous, meaning they are abundant in almost all terrestrial environments. For example, more microbes can be found in one gram of soil than there are people on the Earth! This is important because these tiny soil microbes play a huge role in supporting plant growth.

Editor’s Note: Very few places on Earth’s surface lack soil microbes. Typically these locations would be volcanic.

Bacterial and fungal species work together in clusters (i.e. consortia) to support plant growth in the rhizosphere (AKA the soil root zone) primarily by delivering nutrients and preventing disease. Soil bacteria and fungi continually increase soil nutrient availability by transforming unavailable nutrients into bioavailable forms for plant uptake (Editor’s Note: A good example would be lichens, which can take nutrients locked inside of rocks and gradually break them into a usable form). Microbes also act as a biofertilizer by releasing critical nutrients when they die. Without microbes, plants wouldn’t have the constant supply of nutrients they need to grow.

Beyond nutrient cycling, microbes also produce hormones and other chemicals that stimulate plant growth. Microbes can also prevent pathogen infection by inducing systemic acquired resistance in plants and by coating root surfaces to physically shield the plant from getting infected by pathogens. In the remainder of this article, we will discuss the key characteristics of soil bacteria and fungi, and highlight how they function to support plant growth.

Soil bacteria represent some of the smallest single-celled organisms on Earth. Scientists have classified well over ten million unique bacterial species and, like most life on Earth, bacterial species can occur in a number of different shapes and sizes (referred to as morphologies) including spheres, rods and spirals (Fig 1). There are approximately 5×10³⁰ bacterial cells covering the surface of the Earth (AKA 5,000,000,000,000,000,000,000,000,000,000 cells!). This means that the cumulative soil bacterial biomass across the world is greater than all plant and animal biomass combined.

Free-living soil bacteria can function in a wide range of environmental conditions and live in symbiotic (mutualistic or parasitic) relationships with plants. These bacteria are well known to improve soil nutrient conditions in the rhizosphere (i.e. root zone), filter out soil contaminants and heavy metals, and compete with disease-causing organisms.

In modern cannabis cultivation practices, beneficial bacteria are primarily used as inoculants to promote plant growth by increasing plant nutrient uptake. One means by which bacteria accomplish this is through the release of specialized proteins called extracellular enzymes into soil and soilless media. Enzymes are not living things, but are chemical catalysts that function to break down nutrient-rich molecules into bioavailable forms. Soil bacteria can produce many different functional groups of enzymes to cycle important macronutrients such as nitrogen and phosphorus for plant uptake.

Soil bacteria also make nutrients available by facilitating ion exchange. They accomplish this by secreting organic acid compounds into soil and soilless media, which act to release chemically-bound nutrients for plant uptake. For example, phosphorus (P) is an essential plant macronutrient that is not readily bioavailable in soils due to its high sorption capacity. Even when soluble phosphate fertilizers are added to soils and other growth media, up to 70% can become immediately unavailable due to chemical binding and transformations. Beneficial bacteria can naturally unlock bound Phosphorous, transforming it back into available forms to maximize its availability for plant uptake.

Plant growth is also often limited by iron because of the very low solubility of ferric iron (Fe³⁺). Iron is essential for plants because it plays a crucial role in metabolic processes such as DNA synthesis, respiration, and photosynthesis. Beneficial bacteria can mobilize iron for plant uptake by producing compounds called siderophores (i.e. high-affinity, iron-chelating compounds), which acquire and make Fe³⁺ ions readily available for plant uptake.

Some beneficial bacteria produce a class of hormones called auxins, which are essential for plant root formation and shoot development (Editor’s Note: IBA is one of the first auxins discovered). Beneficial bacteria can produce other plant hormones as well, such as gibberellins and cytokines, both of which have been shown to stimulate shoot development.

Lastly, beneficial soil bacteria induce disease resistance by producing compounds such as lipopolysaccharides, salicylic acid, and siderophores, which active plant defense mechanisms such as phytoalexins (a chemical that inhibits parasite growth) and other pathogen-related proteins. Beneficial soil bacteria can also physically coat plant roots – acting as a biological shield against pathogenic bacteria, fungi and viruses.

Scientists have classified seven primary fungal groups (AKA phyla) and over 70,000 unique fungal species. Soil fungi have filamentous (hairlike) structures called hyphae that grow throughout the soil and along plant root surfaces (Fig 2a); also referred to as mycelium when growing in concentrated tufts (Fig 2b). Fungi produce microscopic spores, providing a protective sheath similar to those of plant seeds, allowing fungi to protect their DNA from environmental damage while spreading to other regions (Fig 2c).

Soil-borne fungi are generally considered the primary decomposers on Earth. However, the beneficial soil fungi that promote plant health can be more specifically categorized as either decomposers or mutualists. Of these, the three functional categories that we will discuss in this article include saprophytic fungi, ectomycorrhizal fungi and arbuscular mycorrhizal fungi.

Saprophytic fungi are the primary decomposers that live in all terrestrial ecosystems. They live freely in soils and are quite capable of degrading difficult substrates such as lignin and soil pollutants. As decomposers, these fungi improve soil quality by decomposing complex carbon compounds to increase soil organic matter. This is a critical step for nutrient cycling because organic matter helps soils retain nutrients and moisture.

Ectomycorrhizal (EcM) fungi form symbiotic relationships with some plant species by forming a dense hyphal sheath known as the mantle, which surrounds the root surface (Fig 3a). Unlike other mycorrhizae, EcM fungi do not infect their plant host’s cortical cell walls, but instead form a hyphal “net” between the plant’s epidermal and cortical root cells (Fig 3a), commonly known as a Hartig net. EcM fungi primarily benefit plants by transporting nitrogen from the surrounding soil to improve plant nitrogen uptake, functionally acting as extended roots for the plants. In exchange, plants supply the fungi with labile carbon via roots exudates. EcM fungi demonstrate low host specificity, meaning that many different fungal species form symbiotic relationships with many different plant species.

Editor’s Note: EcM fungi commonly feed nitrogen to multiple plants and get fed carbon compounds such as sugar in return.

Glomus is the largest genus of arbuscular mycorrhizal (AM) fungi. Approximately 80% of the plant species on Earth are affiliated with AM fungi. Scientists have identified approximately 85 different AM fungal species.

The symbiotic interactions between plants and AM fungi originated at least 460 million years ago. These ancient relationships evolved from both free-living saprophytic fungi that over time became endosymbiotic with plants, and from parasitic fungal interactions that developed into mutually beneficial relationships. Likewise, AM fungi are taxonomically defined as non-monophyletic (meaning that they not descended from a common evolutionary ancestral group, but several different groups).

AM fungi are differentiated from EcM fungi in several ways. For example, AM fungi infect their hosts by penetrating the root cortical layer (Fig 3b). Instead of transporting nitrogen to the plant, AM fungi promote plant growth by transporting soluble phosphorous to the plant from the surrounding rhizosphere. Likewise, the plant hosts supply AM fungi with labile carbon (via roots exudates) internally at arbuscular exchange sites.

AM fungi generally lack the ability to produce extracellular enzymes that break down nutrients, and strictly serve as a transport vessel by growing with plant roots to extend the capacity of the plant to take up nutrients from the surrounding environment. Nonetheless, many studies have cited that plants that have established symbiotic relationships with AM fungi demonstrate increased drought tolerance.

Although there is no clear evidence to suggest that AM fungi exhibit specific plant hosts for colonization, several studies have shown that AM fungi exhibit plant-specific chemotaxis (the movement of an organism in response to specific chemical stimuli). This suggests that some AM fungi prefer specific plant hosts. Furthermore, environmental conditions such as moisture, soil pH, and cation exchange capacity can have a strong influence on AM fungal colonization with plants. For example, many studies have shown that AM fungal hyphal growth significantly declines in fertilized soils that have higher levels of nutrients. Likewise, research has shown that practices such as reduced tillage and lower fertilizer usage will optimize the establishment of AM fungal colonization with host plants.

Editor’s Note: This is likely because the mutualistic exchange between the host plant and AM fungi has a significant cost to it, and, in rich soils, the cost is too high for the potential gain. In poor soils, the cost for the host plant is small compared to the potential gain.

Many scientific studies have shown that tiny soil microbes play a large role in growing healthy crops and increasing plant yield. While most growth media contains microbes capable of cycling nutrients, there are now commercially-available organic microbial formulations with superior functionality. Modern microbial biostimulants can improve plant growth and maximize yields by increasing nutrient availability, preventing pathogens and stimulating plant growth.

Functionally-targeted microbial inoculants represent the next-generation of green technologies which will support agriculture management across many crops to help farmers meet global production demands. The future success of cultivation practices will depend on new biological solutions to drive sustainable improvements in plant quality and productivity for decades to come

Resources:

Want to get in touch with Mammoth Microbes? They can be reached via the following methods:
1. Website: https://mammothmicrobes.com/
2. Email: info@growcentia.com
3. Phone: (970) 818-3321