The color of light plays an important role in plant growth and development. Different wavelengths or colors of light can affect various processes like photosynthesis, photomorphogenesis, phototropism, etc. When it comes to root growth specifically, some colors of light seem to promote better growth than others.
How Light Affects Root Growth
Roots are usually not exposed to light as they grow underground or inside the medium. However, research has shown that light can penetrate through soil/medium and influence underground plant parts. Though roots lack chlorophyll, they are sensitive to certain photoreceptors like phytochromes and cryptochromes which can perceive different light wavelengths. This allows them to monitor light conditions at the surface and make appropriate growth responses.
Blue and red lights are absorbed maximally by these photoreceptors influencing rootarchitecture. Cryptochromes absorb blue light while phytochromes absorb red and far-red light. The light absorption starts a signaling cascade affecting root growth and development.
Effect of Blue Light on Roots
Studies have found that blue light tends to inhibit root growth and development. Cryptochromes are sensitive to blue light and their activation leads to reduced cell elongation and cell proliferation which inhibits overall root growth. Blue light also reduces lateral root formation and root hair growth.
However, blue light is not entirely bad for roots. It enhances nutrient uptake capacity by increasing formation of ATP-binding cassette transporters. Blue light also promotes stomatal opening and transpiration rates. This helps in mass flow of nutrients to the roots. But excess blue light can have detrimental effects on root and shoot biomass.
| Effects of Blue Light on Roots |
|---|
| – Inhibits cell elongation |
| – Reduces cell proliferation |
| – Decreases lateral root formation |
| – Reduces root hair growth |
| – Promotes stomatal opening and transpiration |
| – Increases nutrient uptake |
| – Can reduce root and shoot biomass in excess |
Effect of Red Light on Roots
Red light tends to promote root growth and development. Phytochromes are sensitive to red light, especially the wavelength of 660 nm (R). Their activation leads to increased cell elongation and cell division rates which stimulates overall root growth.
Red light also enhances lateral root formation and root hair development. It modulates expression of many root architecture related genes. However, very high intensities of red light can inhibit root growth.
Red light promotes nutrient uptake by increasing formation of proton pumps and nutrient transporters. It also stimulates carbohydrate metabolism and photosynthate translocation to roots which provides energy for root growth.
| Effects of Red Light on Roots |
|---|
| – Promotes cell elongation |
| – Increases cell division rates |
| – Stimulates lateral root formation |
| – Enhances root hair development |
| – Modulates expression of root architecture genes |
| – Can inhibit growth at very high intensities |
| – Improves nutrient uptake |
| – Provides energy via photosynthates |
Effect of Green Light on Roots
Green light generally has intermediate effects on root growth compared to blue and red lights. It moderately promotes cell elongation and proliferation leading to some increase in root length and growth. But the effects are much less compared to red light.
Green light also causes nominal increases in lateral root density and root hair formation. It does not significantly affect photosynthesis and carbohydrate levels in plants.
Overall, green light has some positive but limited effects on root growth parameters.
| Effects of Green Light on Roots |
|---|
| – Moderately increases cell elongation |
| – Moderately promotes cell proliferation |
| – Slightly increases lateral root density |
| – Marginally enhances root hair formation |
| – Does not affect photosynthesis and carbohydrates |
Effect of Yellow Light on Roots
Yellow light has very little effect on root growth and morphology. It is not strongly absorbed by any known photoreceptors in plants. It does not significantly affect cell elongation or division in roots.
There is no noticeable change in lateral root formation or root hair development under yellow light. It also does not alter photosynthesis, nutrient uptake or gene expression patterns related to root growth.
Overall, yellow light has negligible effects on root growth and development in plants.
| Effects of Yellow Light on Roots |
|---|
| – No significant effect on cell elongation |
| – No noticeable change in cell division |
| – Does not affect lateral root density |
| – No effect on root hair formation |
| – No alteration in photosynthesis and carbohydrate levels |
| – Does not change expression of root architecture genes |
Best Color Light for Root Growth
Based on various studies, red light appears to be the most beneficial for root growth followed by green light, while blue and yellow lights are inhibitory or have minimal effects.
The wavelength of 660 nm red light is absorbed maximally by phytochromes which stimulate cell division and elongation – critical for root growth. It also enhances lateral and fibrous roots and root hairs by modulating gene expression and providing photosynthates.
However, combining some blue light with red light has also been shown to balance the effects and promote optimal root growth. Red light promotes growth while blue light enhances nutrient uptake.
The ideal light recipe can vary between plant species and growth stages. But red light alone or a combination of 90-95% red + 5-10% blue light is generally recommended as the best for root growth based on research.
| Ranking of Light Colors for Root Growth |
|---|
| 1. Red light |
| 2. Red + Blue light |
| 3. Green light |
| 4. Yellow light |
| 5. Blue light |
Using Colored Lights for Rooting and Propagation
The knowledge of how different colored lights affect roots can be useful for plant propagation from stem cuttings or tissue culture. Using appropriate lights can stimulate better root growth during these stages.
Some key tips for using colored lights for propagation are:
- – Use 660 nm red LED lights for rooting of cuttings or plantlets. Can combine with some blue light.
- – Illuminate cut ends or lower plant surface to stimulate root growth.
- – Give short light exposures of 12-16 hours daily.
- – Slowly reduce light intensity as roots develop to avoid excessive biomass.
- – Monitor moisture levels and avoid overwatering under lights.
With optimal use of colored lights, rooting percentage and root quality of cuttings and micropropagated plants can be greatly improved. This allows quicker transplantation into pots and field establishment.
Conclusion
Root growth responds differently to light colors due to the activation of specific photoreceptors. Red light tends to promote better root growth by increasing cell division and elongation, enhancing lateral and fibrous roots and modulating gene expression. Blue light is inhibitory but can improve nutrient uptake.
For getting optimal root growth, red light or a combination of mostly red + some blue light is recommended. These lights can be used strategically during stages like propagation to stimulate stronger roots and improve transplant success.