In Plants, Chlorophyll Absorbs Light

In order to choose the best horticultural light, it’s helpful to understand which light is best for photosynthesis. Photosynthesis converts light energy into sugars for plant growth. The first step of photosynthesis happens when the leaves soak up light using chlorophyll pigments. Chlorophyll is located in plant cells. More specifically, it’s located in the chloroplasts within plant cells. These pigments are set within the walls of the chloroplast. The chlorophylls group together in a bundle called the light harvesting complex (LHC)

The LHC has a reaction center and an antenna. The reaction center is a single chlorophyll A molecule (Figure 1). The antenna has more chlorophyll and also a mix of other pigments like xanthophylls, and carotenes (Figure 1). It’s shaped like a funnel and catches incoming light. The antenna bounces the light energy from chrlorophyll to chlorophyll until it ends up at the reaction centre. Once at the reaction centre, light energy is converted into electrons that are used for photosynthesis. Most pigments in the LHC are chlorophylls (~ 65 % chlorophyll)! There are also xanthophylls (~ 29 %) and carotenes (~ 6 %) [1].

Which Light is Best for Chlorophyll?

About 10 different kinds of chlorophylls exist in plants and each absorbs a unique mix of light colours [1]. Chlorophyll A and B (chl A and B) appear green to our eyes. However, they can appear anywhere from yellow-green to blue-green.  Usually, there is more chl A than chl B because chl A is the only pigment in the reaction center. The ratio of chl A to chl B (chl A:B) ranges from about 1 to 6, depending on the species and environment. For example, tomato has a chl A:B of 2.14, lettuce is about 3.30, and chrysanthemum is about 1.20 [2, 3]. Under shade or stress conditions, the amount of chl B increases to catch more light in the LHC.

Figure 2: Colours of light absorbed by the main plant pigments like chlorophyll A and B and beta-carotene. Plants absorb a lot of blue and red light, but can use other colours of light as well.

Chlorophyll is needed for photosynthesis, so it is important to provide the colours of light that both chlorophyll A and B can soak up. So which wavelengths of light do chl A and chl B use? Chlorophyll A likes light at 430 nm and 660 nm best. Chlorophyll B prefers light at 450 nm and 640 nm (Figure 2). These wavelengths correspond to the blue and red parts of the spectrum. When plants are given a combination of both blue and red light, they grow better than if they were given just red or blue light [4]. The red-blue light combination increases chlorophyll content, photosynthetic rate, leaf area and number, and plant biomass for many different species [3. 4, 5, 6, 7].

In addition to red and blue light, chl A and chl B use small amounts of ultraviolet, green, and yellow light too (Figure 2). The addition of small amounts of green and yellow light to red-blue light enhances plant growth compared to just red-blue light [7, 8].  To optimize plant yields, grow your plants using a light that satisfies as many of the chl A and chl B wavelength requirements. This means that you should choose a light that has high amounts of both blue and red light.