A case of the blues

Blue light is radiation between 400 and 500 nanometers (nm). Before the introduction of light-emitting diodes (LEDs), greenhouse growers would look to metal halide (MH) lamps as a supplemental light source to provide additional blue light to their crops. With the improvements in LEDs for horticultural lighting, comes the ability to produce narrow-spectrum light in a variety of wavelengths. Blue light, usually somewhere between ~430 and 450 nm, is now commonly included or available for many LED lights (Fig. 1). One of the primary reasons blue light was originally included along with red light in LED arrays was because they are the two wavelengths the chlorophyll absorb most. However, there are other benefits to using blue light in crop production aside from promoting photosynthesis.

First, one of the main interests in including or adding blue light to crops grown in a greenhouse or indoors is to control growth. The effect of red light and far-red light on plant growth is fairly well-known: When the amount of red light relative to far-red light is low, stem elongation is promoted. Alternatively, when the amount of red light relative to far-red light is high, elongation is suppressed. Blue light also has a similar growth-suppressing effect on plants, as it can suppress cell elongation and stem and leaf expansion. As the amount of blue light increases, the degree of growth inhibition increases. Usually, anywhere from five to 30 percent blue light provides adequate growth control without excessive stunting.

For ornamental and edible crops with purple foliage, color development can be less than ideal during the late fall, winter and early spring under low-light conditions. The purple coloration, anthocyanin, is typically formed in response to high-light intensities; the pigment helps reduce the amount of light energy so plants are not stressed. However, if you are interested in improving the foliage color of red lettuce, purple fountain grass or other red/purple-pigmented foliage, blue light can be used during low-light times of the year to stimulate color development in foliage. Additionally, blue light does not have to be used throughout production; providing blue light for around one or two weeks is sufficient to produce the desired color.

Blue light can also alter the chemical composition of plants, which in turn can add value to crops. For example, blue light can increase the abundance of phytochemicals in culinary herbs, potentially improving flavor and aroma. Additionally, blue light can promote accumulation of antioxidants in other leafy greens, which in turn enhances their nutritional value.

Blue light is a high-energy wavelength, and there are some practical implications this has, most notably energy consumption. The high energy of blue light means more power is required to produce blue light. For example, when three physically identical LED arrays varying only in the proportion of red and blue light provided were mounted the same distance from a greenhouse bench and provided 70 µmol·m^–2·d^–1 at plant height, the amount of energy used to run the lights increased from 1.80 to 2.57 kilowatt hours per day as the proportion of blue changed from 0 percent (100 percent red) to 30 percent (70 percent red). Try to only provide as much blue light as you need to in order to achieve your goals, since providing more blue light than you need can needlessly increase your energy bill.

In addition to energy consumption, you’ll want to consider employee safety and comfort when using blue light. High proportions of blue light can be uncomfortable to work around for extended periods of time, with some getting uncomfortable or feeling nauseous after a while. Aside from trying to minimize the time working around blue light, you can use special glasses that reduce the strain on eyes resulting from too much blue light.

With the recent advances in lighting technology, providing blue light to manipulate plant growth, development and value to improve horticulture crops grown in greenhouses and controlled environments is easier than ever before. This is one case of the blues that is not sad!

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu