Light Spectrum Effect on Plants: Photosynthesis and the Role of Different Light Colours

People want their plants to grow well, so they often use simple tools to check the light, and sometimes they use a lux meter app on their phone because it is easy and fast, but we must remember that light for a plant is a very important source of energy that gives it orders about growing up or when to make flowers, so we need to understand the colours of light.

The plant uses only a special part of the sun's light for this process, which is known as Photosynthetically Active Radiation (PAR), and this range goes from N to 700 nanometers (nm), so everything outside this range is either not used or used for small things.

Light as a Set of "Keys"

• Chlorophyll takes in blue and red light, which are the main "working" colours.

• Phytochrome reacts to red and far-red light, and it gives orders about when to flower and if the plant is in shadow.

• Cryptochrome and Phototropin react to blue light, and they control the plant's shape and where it grows.

Because different light colours cause different actions, a modern grower should not just turn on a bright lamp, but must give the plant the right "colour orders" at the right time in its life.

Blue Light: Role in Leaf Growth and Stem Shape

Blue light is in the range from about 400 nm to 500 nm, and in the world of growing, we can call blue light the "Builder" and the "Guard".

Strength and Shape (Vegetative Growth)

Blue light is very important in the vegetative stage, which is when the plant is growing its green mass, leaves, and stems.

• Stopping stretching: The main job of blue light is to control the plant's shape, so plants that get enough blue light stay short, strong, and small. If there is not enough blue light, the plant starts to stretch its stems, which happens because the plant thinks it is in shadow under a bigger plant and tries to quickly reach the better light.

• Healthy Leaves: The blue colour helps the plant make chlorophyll, so thanks to blue light, the leaves become thick, strong, and dark green, which makes them much better at photosynthesis.

• Stomata Control: Blue light turns on phototropins, which control how the stomata small holes on the leaves open and close, and these holes are needed for taking in carbon dioxide (CO2​) and letting out water, so blue light directly affects how well the plant breathes and takes in CO2​.

Using Blue Light in Practice

Blue light is needed in these stages:

• Starting Seeds and Seedlings: A lot of blue light makes sure that young plants make a strong, short stem that can hold the weight of the fruit later.

• Leafy Vegetables: For plants like lettuce, spinach, or basil, where the green leaves and thick shape are important, the amount of blue light should be high during the whole growing time.

Red Light: Role in Flowering, Fruit Making, and Growth Speed

Red light is in the range from 600 nm to 700 nm, and it is the most important colour for photosynthesis, so red light is the plant's "Engine" and "Clock".

Energy Efficiency (Photosynthesis)

In the light used for photosynthesis, red light around 660 nm is the most effective colour.

• Speed: Chlorophyll takes in red light with the best success, which means this colour gives the highest speed of changing light into chemical energy, and it gives the plant "fuel" for quick growth and, more importantly, for making big fruits.

• Plant Mass: In the flowering and fruit-making stage, when the plant needs a huge amount of energy to make fruits and seeds, having mostly red light is a must-do.

The Order to Reproduce (Flowering)

Red light is closely connected to far-red light (Far-Red, 700−780 nm) through the phytochrome system, and this system works like a switch or a clock:

• Flowering: The balance of Red (R) / Far-Red (FR) is the main order to start flowering, so when the red light goes up compared to far-red (high R/FR), it helps the plant start flowering early and make many flowers.

• Shade Effect: Far-red light, though it is at the edge of the PAR range, goes deeper into the leaf and tells the plant that it is in the shadow of other plants, so a lot of far-red light (low R/FR) makes the plant stretch very much to "fight" for light, which is usually not good in controlled growing.

Using Red Light in Practice

• Flowering Plants: In the flowering stage, the balance of Red to Blue light should move towards Red for most crops to start the seed-making time.

• Bigger Fruits: For tomatoes, peppers, and berries, a high level of red light is necessary to make the fruits heavy and sweet.

Green Light: Its Role and Why It Is Often Not Considered

Green light is in the range from 500 nm to 600 nm, and it is the most forgotten colour in plant growing.

Why is it not considered?

Plants look green to us because chlorophyll reflects most of the green light, and because of this, people thought for a long time that green light was totally useless for photosynthesis, which is the main reason for the luxmeter's mistake, as it is most sensitive to this "useless" green colour.

The True Job of Green Light (The Submarine)

New science has shown that green light has an important job, which can be compared to a "submarine":

• Going Through the Canopy: Unlike red and blue light, which are almost completely taken in by the top leaves, green light goes deeper into the thick leaves, so it reaches the lower, shadowed leaves, where it is taken in by chlorophyll and still helps with photosynthesis.

• Overall Success: In very thick and tall groups of plants like in vertical farms or greenhouses, adding green light to the spectrum can make the overall success of photosynthesis better for the whole plant mass, because the bottom leaves start to "work".

• Comfort for People: Adding green and yellow light makes the light look "white" to the human eye, which is very important for greenhouse workers, allowing them to look at the plants, find pests or diseases without the colours being wrong like when they have to wear orange glasses under pure pink/purple lamps.

Conclusion about Green Light: Although it is the least effective on one leaf, it is very important for the health of the lower leaves and for the comfort of the people working in the whole farm.

The Luxmeter as a Tool: Checking the Total Light Strength

The Luxmeter measures lux (lx), which is brightness for the human eye, and this is its main limit in modern growing, but it is also its only good point.

The Luxmeter's "Blind Spot"

As we found out, the luxmeter loves green light and does not notice blue and red light.

• If you use a lamp that mostly gives red and blue light like a pink LED grow light, the luxmeter will show a very low number of lux, and this number will be wrong for the plant, because it does not count the useful photons.

• If you use an old yellow sodium lamp, the luxmeter will show a very high number of lux, but most of that light is in the yellow-green range, which is not very good for photosynthesis.

The Right Way to Use the Luxmeter

The luxmeter is still helpful, but only for general, simple checks and when you follow these rules:

• Checking Evenness: The luxmeter is perfect for making a "light map," so you can measure lux in the middle of the light area and in the corners to see if the lamp spreads the light evenly. If the corner lux is half the middle lux, you know that plants there will not grow as well.

• Using Sun or Full White Light: If you are measuring sunlight which naturally has all colours or old white lamps which also give a wide, continuous range of colours, the lux readings can be used for a rough check of the strength, and you can use simple math to change lux → PPFD, but it will never be exactly right.

• Safety and Comfort: The luxmeter is needed for checking the light in work areas and paths in the greenhouse to make sure it is comfortable for people.

The Main Point for the Grower

For exact control of the plant's food, you need a PAR meter (quantum meter), which measures PPFD and only counts the photons that are useful for photosynthesis.

The Luxmeter should only be used as a helping tool for checking light evenness or for working with old light sources. You must never trust the luxmeter when using narrow-colour LED grow lights, or you risk thinking the light is too little when it is too much, which could harm the plant.

To get the best growth, you must balance blue for strength, red for energy and flowers, and a little green for light to go through and for comfort, and you must use exact tools to control the amount of photons (PPFD/DLI).