The color of light can have a significant impact on plant growth and development. Plants use light as an energy source for photosynthesis, the process by which plants convert carbon dioxide and water into glucose for growth. Different wavelengths of light contain different amounts of energy, and plants absorb and utilize these wavelengths differently. The effects of light color on plant growth have been extensively studied by botanists and horticulturists seeking to optimize plant growth and crop yields. Here we will review how light color influences various aspects of plant growth and summarize the findings of key studies in this field.
How Plants Use Different Wavelengths of Light
The visible light spectrum that humans see as colors from violet to red corresponds to wavelengths of light from about 400 nm to 700 nm. Plants do not utilize all wavelengths equally. They primarily absorb blue light (400-500 nm) and red light (600-700 nm), which are the most efficient wavelengths for photosynthesis. Green light (500-600 nm) is reflected and not used as much by plants.
Within the optimal blue and red ranges, plants also respond differently to light at specific wavelengths:
– Blue light (400-500 nm): stimulates chlorophyll production and chloroplast development, important for photosynthesis. Also involved in phototropism (growth towards light).
– Red light (600-700 nm): affects phytochrome reactions and can trigger flowering, germination, and shade avoidance mechanisms. Most efficient for photosynthesis.
– Far-red light (700-800 nm): involved in shade avoidance, flowering, and germination responses.
The exact mechanisms by which light color affects plant growth are complex, but involve hormone signaling pathways and expression of genes related to development and metabolism. The effects depend on the plant species as well as environmental factors.
Effects of Blue Light
Research has demonstrated that blue light plays a key role in leaf growth and expansion, chloroplast formation, stomatal opening, and regulation of photoperiodic flowering.
In a study that grew peppers under different monochromatic LED lights, the addition of blue light significantly increased plant height, leaf count, leaf area, and total biomass compared to red light alone. This was attributed to blue light’s role in cell division and expansion.1
Other studies have shown that blue light intensifies chloroplast formation and stomatal function, enhancing photosynthesis and transpiration.2 In the model plant Arabidopsis thaliana, overexpression of a blue light photoreceptor cryptochrome (CRY) has been shown to increase chloroplast development, leaf area, and plant biomass.3
Blue light also activates phototropins that mediate phototropic responses causing stems and leaves to bend towards blue light sources. This allows plants to optimize light absorption.
Effects of Red Light
While blue wavelengths mediate chloroplast development and leaf expansion, red wavelengths generally promote more compact growth and are the most efficient for photosynthesis.
Multiple studies have found that red light alone can increase chlorophyll content, net photosynthesis rate, starch accumulation, and biomass production compared to other monochromatic lights.4,5
Red light also affects flowering and germination processes through the phytochrome system and hormone signaling. Phytochromes exist in an inactive and active form and convert between the two upon absorbing red or far-red light. This system regulates germination, the shade avoidance response, and induction of flowering.6
However, extremely high intensities of red light can inhibit leaf expansion and cause excessive heating and damage in some plants. Balanced red and blue light is optimal for both growth and yield.
Light Color Ratio Effects
Researchers have tested plant responses to different proportions of red and blue light. The optimal red:blue ratio depends on the plant species as well as growth stage.
One study grew lettuce seedlings under red and blue LEDs at different percentages of blue light from 10-50%. They found that 30% blue light produced the highest biomass and leaf count.7 Other researchers have reported optimal blue percentages ranging from 10-30% for various crops.8
Too much blue light can result in compact, stunted plants, while too little causes excessive stem elongation and small leaves. Finding the right balance is key for optimizing growth. The optimal ratio also changes over a plant’s lifecycle.
Effects on Flowering and Fruiting
Light color strongly influences the flowering process in plants. Blue light often promotes vegetative growth while red light can trigger reproductive development by affecting hormone signaling pathways.
Long-day plants that flower in spring or summer usually require more blue light, while short-day plants that bloom in fall require more red light. Supplementing with red light has been shown to induce earlier flowering in short-day crops like rice, chrysanthemum, and poinsettia.9
After flowering, adequate light is still needed for proper fruit and seed development. Studies in tomatoes and peppers have shown that maintaining higher proportions of red light after flowering can improve fruit set and vitamin C content.10
Effects on Plant Chemical Composition
Light color can also alter production of plant metabolites and phytochemicals, some of which have antioxidant or medicinal value.
Blue light has been shown to increase concentrations of phenolic compounds and anthocyanins with antioxidant properties in lettuce, herbs, and sprouts.11,12 Red light can enhance carotenoid and flavonoid levels in tomatoes and leafy greens.13
Managing light color throughout growth can optimize both plant biomass as well as the nutritional quality of edible crops. More research is still needed to characterize responses in different plant varieties.
Commercial Horticulture Applications
The development of LED lighting technologies has allowed growers to control light color in greenhouses and indoor farms. Matching light recipes to a plant’s needs at different growth stages can promote growth, yield, and plant quality.
Some examples of commercial applications include:
– Seedlings and young plants under more blue light to encourage compact, leafy growth
– Transitioning to more red light during fruiting and flowering stages
– Supplementing with red and far-red light to control flowering time
– Adjusting blue light fractions to manage shoot elongation
– Using blue light to increase anthocyanins and other phytochemicals
Growers now have unprecedented ability to manipulate plant growth with light. However, economic viability depends on efficient use of energy. Finding the optimum light color strategies for each crop continues to be an active area of horticultural research.
Conclusion
In summary, light color has a profound influence on plant growth and development through effects on morphology, photosynthesis, photoperiodism, and production of phytochemicals. Blue and red wavelengths are most efficiently utilized by plants, with blue light driving leaf growth and chloroplast formation, and red light maximizing photosynthesis and controlling flowering. The optimal balance of blue and red light depends on the plant species and growth stage. Horticulturists continue to study plant responses to narrow bandwidths and ratios of different wavelengths to optimize growth, yield, and crop quality in a cost-effective manner. The ability to tune light color in indoor farms and greenhouses provides new tools for plant growth management.
| Light Color | Effects on Plants |
|---|---|
| Blue (400-500 nm) |
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| Red (600-700 nm) |
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| Far-red (700-800 nm) |
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Plants require different ratios of red and blue light at various growth stages:
| Growth Stage | Optimal Red:Blue Ratio |
|---|---|
| Seedling/early vegetative growth | 1:1 to 3:1 |
| Flowering/fruit development | 5:1 to 10:1 |
