Blue light is an important part of the light spectrum that affects plant growth and development. While plants need some blue light for healthy growth, getting too much can actually be detrimental. There are a few key reasons why excessive blue light can be bad for plants.
Blue light effects on plant morphology
Blue light primarily impacts the vegetative growth of plants. It affects processes like leaf expansion, stem elongation, and phototropism or the directional growth of plants towards light. While moderate amounts of blue light promote compact, sturdy growth, getting too much blue light can lead to excessive elongation and leggy, weak growth as the plants over-stretch to reach the light source.
Studies have shown that tomato and cucumber seedlings grown under 100% blue LED lighting had 50-100% more hypocotyl elongation compared to seedlings grown under a mix of red and blue light. Eggplants grown under 100% blue light were taller with larger leaves but had weaker stems and were more prone to disease. Too much blue also causes abnormal chlorophyll development, giving leaves a yellowish cast.
The optimal blue light level for compact plant growth is around 10-20% of the total light spectrum. Going above 30-50% blue can result in undesirably leggy growth.
Effects on flowering and fruiting
While blue light is important for vegetative growth, it can be detrimental for plants transitioning to the flowering/fruiting stage. Exposure to blue wavelengths keeps plants in an active growth mode and delays flowering. This is because blue light activates cryptochromes that suppress flowering by inhibiting production of florigen proteins that induce flowering.
Tomato plants receiving 17% blue light took 13 days longer to flower compared to plants getting only 2% blue light. Too much blue light can also cause flower and fruit drop in some plants. For example, orchids getting a higher percentage of blue light during the flowering stage had up to 40% flower abortion.
Thus, the blue light percentage should be reduced once plants transition from the vegetative to reproductive stage. A 90:10 ratio of red to blue light is optimal for flowering versus the 60:40 ratio used for leafy growth.
Blue light effects on plant physiology
While blue light is beneficial for leaf expansion, getting excessive amounts can disrupt other plant physiological processes and stress plants. Blue wavelengths stimulate stomatal opening more than other colors, which can lead to excess water loss through transpiration if stomata remain open for too long under intense blue light. This is especially problematic for seedlings and transplants that cannot handle rapid moisture loss.
| Light Treatment | Stomatal Conductance |
|---|---|
| 100% blue | 0.4 mol/m2s |
| 100% red | 0.25 mol/m2s |
| Blue + red | 0.3 mol/m2s |
Higher transpiration under excessive blue light also cools leaf surfaces. Cooler leaves mean less efficient photosynthesis and carbon gain for plants.
In addition, blue light activates enzymes involved in anthocyanin pigment production. Anthocyanins act as sunscreens to protect plant tissues from intense light. Too much blue light prompts plants to synthesize more anthocyanins as a defense response, which can drain resources away from growth and reproduction.
Blue light and photoinhibition
Photoinhibition refers to the decrease in photosynthetic efficiency when plants are exposed to excessive light. The chlorophyll molecules that carry out photosynthesis become overexcited and damaged under intense light, decreasing their photosynthetic capacity.
Plants are more susceptible to photoinhibition under blue wavelengths compared to other colors. One study showed that spinach leaves under 100% blue LEDs had 76% lower photosynthetic rates versus leaves receiving only red light due to severe photoinhibition and chloroplast damage from the high-energy blue photons.
Too much blue light can also lead to oxidative stress and photooxidation as excess absorbed light energy reacts with oxygen to generate reactive oxygen species that damage cells. Maintaining the right balance of blue light is therefore important to minimize photoinhibition and allow unhindered photosynthesis.
Blue light effects on crop quality
Alterations in plant morphology, physiology, and metabolism under excessive blue light can affect the visual appearance, flavor, and nutritional quality of crops. As mentioned earlier, too much blue light can make plants stretchy and leggy with weaker stems, which reduces visual appeal for ornamental horticulture crops.
Higher anthocyanin and lower chlorophyll levels under blue light make leafy greens and vegetables look less vibrant green. Blue light also lowers carotenoid levels responsible for orange and yellow pigments in tomatoes, peppers, and other fruit crops. This results in paler, less colorful produce.
Studies show blue light affects tissue firmness, dry matter content, and concentrations of sugars, organic acids, and bioactive compounds that influence crop flavor and nutrition. For example, blue light made strawberries, blueberries, and grapes softer while increasing sugars for better sweetness. But it lowered beneficial polyphenols in lettuce and vitamin C levels in tomatoes.
Optimizing blue light levels is therefore critical for balancing plant growth with improved visual and edible quality for commercial crops.
Conclusion
In summary, while blue light is an indispensable part of the light spectrum for plants, excessive amounts can negatively impact morphology, flowering, physiology, photosynthesis, and crop quality. As a rule of thumb, the blue light percentage should be kept between 10-20% of total light during vegetative growth and reduced further during flowering/fruiting. Carefully regulating blue light exposure allows growers to maximize plant performance and quality.
