Plant responses to their environment are complex and have evolved over hundreds of millions of years. Our timeframe is shorter than that so I hope you will forgive me if I simplify a bit.
The starting point to understand plant responses, is to think like a plant.
Plants have two essential prerogatives. To photosynthesize and to reproduce. Photosynthesis is the conversion of light energy into chemical energy, which fuels all downstream plant processes. Reproduction is an imperative that most of us can relate to.
The incredible variability we see in the plant kingdom are the result of evolutionary responses in service to the basic requirements of photosynthesis and reproduction.
Plants are sessile organisms. They are rooted in place, and don’t have the ability to change their environment, or to take a vacation if they don’t like the weather. For this reason, plants have evolved the ability to radically change their physical form based on external stimuli. This ability is known as plasticity.
For plants to intelligently change their physical form, they need to collect detailed information about their light environment. This collection of light information is referred to as photo perception. Photo perception enables plants to continuously analyze the light environment in order to formulate physical responses to optimize photosynthetic potential. This ability to change form, or morphology based on light information is known as photomorphogenesis.
Information about the light environment is perceived by plant photoreceptors. These photoreceptors are biochemicals which change state when exposed to specific wavelengths and intensities of light. There are several different classes of photoreceptors and the cumulative information they collect informs the plants morphology through interactions with other signaling systems, including phytohormones, as mediated by the plants genome.
Photoreceptors collect multiple levels of information on the quality, quantity, duration, and direction of light energy.
Let’s start with light quality.
When we talk about light quality, we are referring to the spectral composition of the light environment. We perceive this spectral composition as color. Plants perceive light quality as different wavelengths of light energy.
The portion of the light spectrum that has direct photosynthetic value is known as Photosynthetically Active Radiation, or PAR.
PAR is defined as the portion of the electromagnetic spectrum between 400 and 700 nanometers in length. We perceive PAR as white light.
Light wavelengths that fall just outside of PAR can also have effects on plant health, and morphology.
Light wavelengths of 280-400 nanometers in length are known as ultraviolet, or UV wavelengths. Although these light qualities have limited photosynthetic potential, they have important effects on plants, not all of them good.
UV light can damage cells, and plants have evolved systems to limit, or repair cell damage caused by UV radiation. These systems are referred to as being photoprotective.
UVR8 is the primary photoreceptor responsible for UV photoprotection signaling. This photoreceptor functions on a signaling pathway to stimulate biosynthesis of photoprotective pigments like anthocyanine, which is the pigment which gives plants their purple coloration.
At the other end of PAR, we have far-red light. Far-red light is classified as light spectra between 700 and 780 nanometers in length.
Far-red light has limited photosynthetic potential but has potent effects on plant morphology. In far-red enriched environments, which occur below dense canopies of leaves, plants will often exhibit Shade Avoidance Response. This response involves rapid elongation to outgrow the surrounding canopy. This response is not based on light intensity, but on light quality, specifically the ratio of red to far-red light.
Phytochromes are the class of photoreceptors that absorb light energy primarily in the red to far-red portion of the light spectrum with some absorption in the blue spectrum, particularly at high intensities.
Phytochromes exist in two interconvertible states depending on the quality of light they absorb. Phytochrome red is the red-light absorbing state. When this pigment is saturated, it converts to phytochrome far-red.
These states of phytochrome are constantly interconverting to mirror the light environment. This process is known as photoequilibrium. During the dark period, or skotoperiod, the phytochrome far-red slowly converts back to phytochrome red. This daily pattern of the interconversion of phytochrome red and phytochrome far-red creates a rhythm which enables photoperiod perception. This rhythm is known as the circadian clock and informs many biological processes.
Light duration, or photoperiod is the most reliable indicator of seasonal changes in the environment. Through photoperiod perception, plants can determine not only the current lighting conditions, but also anticipate future environmental conditions and formulate an appropriate reproductive response.
Another important class of photoreceptors are the Cryptochromes. Cryptochromes are primarily UV, blue, and green light receptors. They are called cryptochromes due to their cryptic nature. Multiple photoreceptors absorb light within overlapping spectra, making it very difficult for early researchers to determine which photoreceptors are responsible for various plant responses.
Very few plant responses are the result of information from only one class of photoreceptors. Rather plants measure the light environment through cross talk between the various signaling systems.
Cryptochromes have a role in many aspects of plant development. In general, they oppose stem elongation, and favour leaf expansion. In many plants, cryptochromes have an accessory role in photoperiod perception, and entraining the circadian clock.
Another class of photoreceptors that guide plant development are the Phototropin receptors. Phototropins are blue and UV light receptors that enable plants to determine light direction, and to grow toward the light. This phenomenon is known as Phototropism.
Perception of light direction enables plants to continuously optimize light capture through movement of leaves. Photoreceptors within plant leaf cells will also translocate to optimize their position for light energy capture.
Light intensity is also an important metric in plant photo perception. We measure light intensity by calculating the total number of photons within the range of PAR that a plant receives per square meter per second. This measure is referred to as Photosynthetic Photon Flux Density, or PPFD, and is calculated in umol.
A plants photosynthetic potential is determined by the light intensity, or PPFD multiplied by the photoperiod duration. This calculation gives us the Daily Light Integral, or DLI which represents the total amount of light the plant receives during the photoperiod. The DLI is measured in mol, with 1 mol equaling some ridiculous number of photons.
So, we have examined Phytochromes, Cryptochromes, Phototropins, and UVR8. These are the primary signaling systems that plants utilize to determine light quality, quantity, duration, and direction.
The cumulative information the plant acquires through these signaling systems enables the plant to optimize its morphology, or physical form for photosynthesis in various light conditions.
These chemical signals interact with other proteins, plant hormones and with the plants genome to direct growth and determine a plants morphology.
Now that we have examined how plants perceive light, and how they change form based on the information provided by photo receptors, let’s turn to the main course, which is of course photosynthesis.
The primary classes of photoreceptors used in photosynthesis are Chlorophylls, and Carotenoids.
Chlorophylls primarily absorb light energy in the blue and red portions of the light spectrum and convert this light energy into chemical energy, primarily in the form of saccharides, or simple carbohydrates.This process takes place within the chloroplast; an organelle present in high numbers in leaf cells.
Carotenoids are photoreceptors containing many types of pigments classed as Carotenes, and Xanthophylls. These pigments serve both photoprotective, and photosynthetic roles within the chloroplast.
Carotenoids absorb light in wavelengths that are not as efficiently absorbed by chlorophylls. These wavelengths are primarily in the blue to green portion of the light spectrum. Carotenoids transfer the light energy that is harvested to the chlorophylls for further synthesis.
Carotenoids also have a photoprotective role. When chlorophylls are over saturated with light energy, carotenoids have the ability to dissipate the excess light energy as heat.
Chlorophylls, and carotenoids will organize their location within the chloroplast to optimize light capture. Under high fluence light conditions the carotenoids primarily serve as photoprotective, and photosynthetic support systems. Under lower light conditions, particularly deep in the canopy, where the light quality is green shifted due to absorption of blue and red light by the upper leaves, the carotenoids perform a critical role in photosynthesis.
An understanding of how plants interact with sunlight gives us important insights in creating an ideal spectrum for horticultural lights. Historic LED manufacturers have focused on providing red and blue light only, as these represent the peak absorption spectra of chlorophyll. This can result in plants inability to optimize their morphology for photosynthetic efficiency, as the light information they require is not available in the environment.
Provision of light in narrow bandwidth red and blue spectra can also lead to photobleaching under very high light intensities, as the supporting, and photoprotective roles of carotenoids are diminished by the limited spectra available.
Plants can adapt and perform under many different light conditions. When we seek to optimize an artificial light environment for plant performance, we are learning that broad spectrum, high fluence lighting technologies provide the most complete plant response.
– By Cannabis Crop Consultant, Stewart Maxwell