When Reds & Blues Collide: Understanding Light-Related Plant Physiology

Light is all around us every day. We can see it and manipulate it to a certain extent, but what exactly is it? Scientific literature says light behaves as both a wave and a particle, and wavelength information can tell us about the energy contained within light.

The human eye perceives different wavelengths of light as different colors, and multiple waves of light can be produced by the same light source, leading to separate waves being perceived as one combined color.

The particle nature of light emerges when light differentiates into discrete packets of energy called photons. Beams of light are described by referring to individual wavelengths, a.k.a. colors or energy, and intensity, a.k.a. counts of individual photons per unit time.

How Do Plants Sense Light?

To understand how different types of lights affect plants, a basic understanding of photosynthesis is required. Photosynthesis is, in broad terms, the process used by plants or other photosynthetic organisms to convert light energy into chemical energy. First, light reaches the plant surface.

Some wavelengths of light are reflected (bounced) off the plant, some are transmitted (shine all the way through the plant) and some are absorbed. The apparent color of plants is determined by the light that is reflected away.

The reflected light has little to do with photosynthesis, since most of the energy is directed away from the plant. Some wavelengths of light are captured by chlorophyll, which are green pigments within special structures called chloroplasts inside of plant cells. Ultimately, this light energy is used to convert carbon dioxide and water into carbohydrates to fuel growth processes and release oxygen into the environment.

Chlorophyll pigments are better at catching some wavelengths of light than others, so not all light is efficiently absorbed. While chlorophyll is the most abundant light-absorbing compound found within plants, it is not the only material sensing and absorbing incoming energy.

Over the past several decades, researchers have been able to isolate other light-sensing compounds within many plants, and identify what kind of light they are most responsive to. Phytochromes are one such family of compounds, which are sensitive to red and far-red ratios.

Another group of compounds called cryptochromes are compounds that sense light at the other end of the visible spectrum, in the blue and ultraviolet ranges. Plants exposed to each of these ranges of light in the electromagnetic spectrum can stimulate a variety of physiological changes as they grow and develop.

Red Light

The majority of the light plants are exposed to is red light. In fact, chlorophyll is most efficient at absorbing light with wavelengths around 660 nm, which is solidly in the red range (650-730 nm) of the spectrum. The light that chlorophyll absorbs primarily drives the synthesis of compounds used for energy and building materials within plant cells. However, overexposure to red light can have detrimental effects on plants.

When red light intensities are too high, chlorophyll synthesis may become suppressed. Leaves lacking chlorophyll will have a bleached-white appearance, since chlorophyll is responsible for the green color of leaves. Beyond overexposure, the exclusive use of red light can also lead to interesting and often unwanted physiological changes. Leaves and stems in plants grown under red light alone become elongated, and flower initiation in some species can be extremely delayed.

Far-red light between 730 and 770 nm also plays an important role in plant development. Increased amounts of far-red light typically indicate a shaded environment. Phytochromes give the plant a way to sense relative ratios of red and far-red light. When this shading effect is detected, physiological changes are triggered that lead to the stretching of plant leaves and stems. This behavior evolved to enable plants to reach above their neighbors competing with them for light, offering the tallest plants the advantage of unshaded lighting.

Blue Light

A dramatically smaller fraction of the type of light plants use is blue light. Though just as much of the visible spectrum is considered blue (400-500 nm in wavelength) as red, plants need only a small percentage of the total light received to be blue for effective plant growth. As little as 1% blue light increases photosynthesis rates and shoot dry matter compared with plants grown under red light alone.

Some blue light is absorbed by chlorophyll, but some is also absorbed by chryptochromes. Cryptochrome response to blue light has been shown to be involved in stomatal control and stem elongation of plants. Specifically, with exposure to higher percentages of blue light, plants tend to become more compact and flower earlier in some species.

Combining Red and Blue Light in a Growroom

While each color of light has its advantages and disadvantages when it comes to plant growth, the overwhelming consensus is that a combination of blue and red light produces the best results. Plants grown under a blue-red light source will have a more typical leaf shape, have more biomass and experience higher rates of photosynthesis than plants grown under either color alone.

High-quality LED grow lights will include both red (640-670 nm) and blue (430-450 nm) LEDs for maximum plant productivity. Some lights may be designed to deliver a fixed ratio of red and blue light, while others allow for control of each color individually. Manufacturers have even started to include far-red (730-740 nm) LEDs into their lights.

Understanding how to tilt the balance towards either red or blue at key points in the growth cycle can lead to important changes in plant physiology. For example, controlling flower production can have dramatically different effects in the cultivation of different species.

Suppressing flower initiation through blue-red light ratios is beneficial when growing lettuce, leading to a longer growth period with larger biomass when other factors such as temperature or photoperiod might otherwise trigger bolting. However, earlier flowering in strawberries may lead to more fruit produced over a longer period of time.

An effective LED grow light should include LEDs in the blue and red range. Including green LEDs can make plants appear more natural to the human eye, but does not always offer significant advantages for growth. Including far-red light has a number of applications that many growers are currently investigating. Including white light can make visual identification of disease symptoms easier, and make it more comfortable to look at plants with the naked eye.

Photobiology is the study of how light interacts with living organisms and is a growing field with new research being published every year. More information on how individual crops respond to specific light treatments can be found in horticultural science publications, many of which are made available to the public.