TL;DR: Blue light is a key wavelength for photosynthesis and growth. So buy lights that have blue light, whether for growing cannabis or any other kinds of plants.
Whether you’re buying a light for a houseplant, an indoor cannabis grow, or large-scale plant production, it can be challenging to pick the right light. There are multiple factors to consider and conflicting information online. While light intensity is the most important factor, spectrum can be important too. This article explains how blue wavelengths impact plant growth, based on peer-reviewed research.
Blue Light and Photosynthesis
Blue light is radiation with wavelengths between 450 – 495 nm. It strongly impacts photosynthesis and vegetative growth . This is because the wavelengths of the blue correspond to the wavelengths that photosynthetic pigments absorb the most. Chlorophylls and carotenoids, two key groups of photosynthetic pigments, absorb blue light and convert into chemical energy (sugar) that can be used for plant growth .
Even though blue wavelengths are highly absorbed by chlorophylls and carotenoids, they are slightly less effective than red light for driving photosynthesis . This can be explained by the absorption and action spectrums of photosynthesis (see Figure 1). The absorption spectrum (dotted line) shows which wavelengths are absorbed by chlorophyll and other pigments. On the other hand, the action spectrum (solid line) shows the photosynthesis rate for each wavelength.
Figure 1: The absorption spectrum of chlorophylls and beta carotene correlates with photosynthetic output. Red light is slightly more effective than blue light for driving photosynthesis. (Modified from HyperPhysics Biology)
The key is to compare the difference between the dotted and solid lines. The gap between the two lines in the 450 – 495 nm range means that plants absorb large amounts of blue light, but not all of it is converted into sugar energy. This leaves us to wonder — where does the rest of the blue light go? Some of it gets absorbed by lower-efficiency pigments (like anthocyanin) that don’t contribute to photosynthesis.
This doesn’t mean that plants don’t need blue light — they do. But even better things happen when blue light is paired with red. Photosynthesis happens faster, plants product more protein, more pigments, more leaves, and they generally grow bigger [5, 6, 7, 8, 9].
Blue Light and Stomatal Conductance
Aside from photosynthesis, blue light has other effects on plants. Blue wavelengths increase stomatal conductance. Stomatal conductance is the process of gasses (like CO2) entering and exiting leaves through the pores in the leaf’s surface. Blue light impacts stomatal conductance by increasing the number, density, and size of stomata .
If the effects on photosynthesis didn’t convince you that blue wavelengths are important, this next part might. Gas exchange through the pores is critical for both photosynthesis and leaf cooling. Thus, providing plants with enough blue light is necessary for them to be able to control their temperature.
Relationship to Stem Stretching
Tall, leggy plants are undesirable because the plant can easily fall over. Plants often look nicer when they have a short stem and compact leaves. Stretching happens when a plant doesn’t get enough light and so it grows taller to capture more.
Blue wavelengths are a key way that a plant senses how much light it’s getting. Providing a plant with more blue light helps ensure that the stems stay short. Along similar lines, blue light can also decrease petiole length . Petioles are the small stems connecting the leaves to the stem. For this reason, plants that are given more blue light tend to have more compact leaves [6, 14].
Blue Light and Seedling Growth
Blue light also has an effect on seedling growth: it increases seedling size and the antioxidant concentration [15, 16]. Antioxidants protect plants from UV rays and harmful reactive compounds that can cause major problems for photosynthesis and flowering. Thus, increasing antioxidants in plants may be one way to offset the stresses that come along with intense photosynthesis rates. In addition, blue wavelengths increases the sprouting rate, fresh weight, and protein content compared to other colours of light .
Effect on Flowering
Blue light impacts flowering in two main ways: timing of flowering and flower weight. Through the action of chryptochrome (a light receptor in plants), blue wavelengths can sometimes regulate flowering time. How blue light impacts flowering depends on light intensity and whether the plant is a “long-day” or “short-day” plant. Generally, blue wavelengths cause flowering to happen earlier in long-day plants and later in short-day plants. For example, mustard is a long-day plant and exposing it to blue light causes flowering to happen 20 days earlier than it normally would . In other species, blue wavelengths have no effect on flowering . For some flowers, pea plants, and violets, blue light doesn’t change flowering time at all !
Impact on Cannabinoids Production
Last, but not least, some recent research shows that blue light may effect cannabinoid production in cannabis. One study looked at the effect of light quality on the yield of THC and other cannabinoids in cannabis cultivation. Plants grown using LEDs (which had 6 – 16% more blue wavelengths than HPS lamps) had about 38% more THC compared to those grown under HPS lamps . Cannabis plants grown under LED lights also had higher concentrations of CBD, THCV, CBG, and cannabinoids . The authors suggested that UV-A and blue wavelengths might cause the plant to produce more CBG (a precursor of THC and other cannabinoids). The mechanisms underlying the effect of blue wavelengths on the cannabinoid pathways will also require further research.
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