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Why Do Plant Leaves Sometimes Turn Red Under Artificial Grow Lights?

When plants are grown under artificial lighting, such as LED grow lights, the leaves might turn red in colour, due to a phenomenon known as anthocyanin accumulation.

Anthocyanins are water-soluble flavonoid plant pigments that produce the major red, blue and purple colours in flowers, fruits and vegetables to attract pollinators and seed dispersers. An interesting fact about anthocyanins is that under acidic conditions (pH < 7) they typically appear as red pigments, but in more neutral to slightly alkaline conditions (around pH 7-9) anthocyanins can appear as blue or purple pigments.

Another purpose of anthocyanins is protective. These compounds have potent antioxidant properties and are involved in protecting plants against both abiotic (environmental) stresses such as ultraviolet (UV) light, cold temperatures and drought, as well as biotic stresses, in the defence against pathogens and herbivores.

Reasons Why Plants May Accumulate Anthocyanins Under Artificial Grow Lights

Plant cuttings being propagated under LED grow lights, the Chinese Star Jasmine leaves (also pictured at top of article) have turned a deep red colour

Plants might accumulate anthocyanins and turn a red colour when grown under artificial lighting conditions for various reasons, such as:

1. Protection Against Excessive Visible Light

When light intensity is too high, it can lead to the production of harmful reactive oxygen species (ROS) compounds within plants, which can cause damage to plant cells.

Anthocyanins can function as a shield against excessive light by absorbing and dissipating excess light energy, reducing the risk of photodamage.

Action: Make sure that grow lights aren’t overly bright, are the right distance above plants, and aren’t on for too long.

The instructions/user manuals for grow lights specify the height to hang the light above the plant canopy, the number of hours to run the light each day, and brightness levels for the various stages of plant growth.

As an example, from a product I reviewed, the ViparSpectra VA600 Dimmable 600W LED Plant Grow Light, here are some of the recommendations which vary between the stages of growth of plants:

VA600Height above plant canopyHour of lightingBrightness
3-5 Day Acclimation Period65-75cm (26-30”)10 on / 14 off40%
Seedling/Young Vegetative55-65cm (22-26”)12 on / 12 off60%
Vegetative Stage45-55cm (18-22”)18 on / 6 off100%
Flowering Stage35-45cm (14-18”)12 on / 12 off100%

The manufacturer of these LED grow lights instructions on the use of this product state:

The Dimmable 600W LED grow light is perfect for 75cm x 75 cm (2.5’x2.5’) vegetative coverage at 50cm (20”) above the plant canopy and 60cm x 60cm (2’x2’) flowering coverage at 40cm (16”) above the plant canopy .

We recommend hanging it at 60cm – 80cm (24″-32″) above the plant canopy and running the light 18 hrs per day during the vegetation stage. During the flowering stage, hang it at 35cm – 45cm (14″-18″) above the plant canopy with 100% brightness while running 12 hrs per day.

Remember to properly adjust the height and brightness accordingly to your plants desired level for optimal growth.

When setting the height of the light above plants, remember the inverse square law for light from physics – when we halve the distance to the light, the plant receives four times more (not double) the amount of light!

2. Response to Red and Blue Light

Artificial grow lights appear purple in colour because they mainly produce red and blue light. This is because chlorophyll, the green plant pigment involved in the process of photosynthesis to produce energy from sunlight, primarily absorbs the blue and red wavelengths of light from white light, while green light, which is not as efficiently absorbed and therefore contributes less to photosynthetic processes, is partly reflected away giving plants their characteristic green color.

The columnar cells of the palisade mesophyll layer contain many chloroplasts (the organelles within cells that contain chlorophyll where photosynthesis occurs), and captures most of the sunlight to carry out most of the photosynthesis, while the spongy mesophyll layer below has fewer chloroplasts.

Chlorophyll absorbs red light more efficiently than blue light, so red light is essential for photosynthesis and plant growth. The purpose of chlorophyll is to make energy for plants to grow, but it doesn’t tell the plant how to grow, there are other compounds in plants that carry out that task.

The Relative Quantum Efficiency (RQE) curve of photosynthesis shows how efficiently plants use different wavelengths of light between 300nm and 800nm for the process of photosynthesis. Green light is the least efficiently used colour of the visible light spectrum.

Plants are able to measure the amounts of red and blue light they receive and respond to alter their growth and development. They contain several types of specialized light-sensitive proteins for this purpose, known as phytochromes and cryptochromes, that change shape when they absorb specific colours of light.

Both phytochromes and cryptochromes contribute to photomorphogenesis, the growth and developmental changes that occur in response to light. The role of these compounds is to tell plants how to grow.

Phytochromes are chromoprotein photoreceptors that are only sensitive to red light and far-red light. Their functions include:

Cryptochromes are flavoprotein photoreceptors that are only sensitive to blue light and ultraviolet (UV-A) light. Their functions include:

In many cases, both phytochromes and cryptochromes work together in coordination to regulate leaf development and plant morphology (shape, form, and structure) based on the prevailing light conditions.

The key point here is that plants use light as an environmental indicator to respond appropriately to their surroundings. For example, short day length and reduced temperatures would imply that winter is coming.

Since the light provided by grow lights is artificial, it may be interpreted wrongly by plants. For example, elevated levels of blue and red light, especially without sufficient green light, can be interpreted by plants as a sign of stress, causing them to produce anthocyanins as a protective mechanism.

Blue light itself strongly induces anthocyanin production and promotes the accumulation of anthocyanin pigments. With red light, an increase in the ratio of red light (~667 nm wavelength) to far-red light (~730 nm wavelength) can stimulate phytochrome-mediated responses that include anthocyanin production.

Action: Use good quality full spectrum growing lights, these provide all the required wavelengths (colours) of light, including UV, in the right proportions. Run the grow lights for the recommended number of hours, which is usually 12 or more hours per day.

3. Protection from Cold Temperatures and Frost

The Indian Guava pictured here, which is an evergreen subtropical tree, changes leaf colour to a deep purple-red over winter to protect the leaves from cold damage

Anthocyanins can improve cold tolerance in plants by functioning like a form of antifreeze to protect plant cells from cold temperatures and frosts.

Cold temperatures can alter the fluidity and permeability of plant cell membranes, increasing the risk of damage due to ice crystal formation within the cells, making them more susceptible to frost damage. This can disrupt cellular processes and lead to cellular damage when the plant is exposed to freezing temperatures.

The presence of anthocyanins can help reduce this damage by stabilising the cell membranes and reducing the risk of freezing-induced injury.

Here’s the long explanation for those interested in the plant science:

Plant cells have a thick cell wall that is semi-permeable and serves as a structural and protective outer layer. Within the cell wall is the cell membrane (plasma membrane), which separates the cell wall from the contents (cytoplasm) of the plant cell. The cell membrane helps provide the shape of the cell, and also plays a role in the transport of molecules. It controls the passage of organic molecules, ions, water into and out of the cell, allows for passage of and the gases oxygen and carbon dioxide, and also enables waste products such as ammonia to leave the cell.

Plant cell membranes are composed of a lipid bilayer (double-layer), which needs to maintain its fluidity for various cellular processes to function. Cold temperatures can affect its fluidity. At lower temperatures the lipid molecules in the cell membrane tend to pack together more closely, reducing the overall fluidity of the membrane, which can impact the movement of proteins and other molecules within the membrane that are essential for various cellular functions.

Additionally, when the cell membrane’s fluidity decreases due to cold temperatures, it can lead to changes in membrane permeability (how easily substances, such as water and ions can pass through it).

Cold temperatures can cause the lipid molecules in the lipid bilayer to become less flexible, which can create gaps or disruptions in the membrane’s structure that can allow substances to leak into or out of the cell more easily, disrupting the balance of ions and water within the cell.

When plant cells are exposed to frost, if the water inside and outside of the cells freezes it can form ice crystals, which expand on freezing and can physically damage the cell membrane, further compromising its integrity and permeability, essentially making it leakier. As a result, the cell will lose water and essential ions (minerals), resulting in damage to the plant cells and ‘frost burn’.

That’s the end of the technical explanation!

Action: Ensure that grow lights are switched on for a sufficient length of time each day so that plants don’t mistakenly sense short day-length, especially when temperatures are cooler, otherwise they will respond for the coming of winter weather.

If plants are grown under grow lights in a cold, unheated indoor environment such as a garage or basement during colder seasons, they may accumulate anthocyanins and turn red simply due to temperature stress. Electric plant heat mats can also be used to raise the temperature around the plants if necessary.

4. Protection Against UV Light Damage

Anthocyanins play an important role in protecting plants leaves from environmental stresses such as exposure to high levels of ultraviolet (UV) light by a mechanism known as light screening or UV screening.

Leaves function like solar panels, to collect light, but too much light can actually damage them. To protect leaves from overexposure to UV light, anthocyanin concentrate more on the leaf side facing the sun.

The anthocyanins reflect the UV light before it can reach the chloroplasts within the plant cells, reducing the levels of UV light absorption by chlorophylls, thereby reducing light exposure damage.

In new shoots, the chloroplasts within the plant cells (which contain the green pigment chlorophyll for photosynthesis) are not yet developed. To protect the new growth, plants replace the green chlorophyll with anthocyanins as a protective ‘sun block’, which is why the new growth is bright red on some plants.

The delicate new pomegranate leaves emerging in spring are a fiery red colour

Action: Seedlings (young plants grown from seed) have delicate leaves, so avoid intense light by dimming grow lights a little or elevating them higher above the plants.

Keep in mind that the inverse square law of light also works in reverse. Doubling the distance of the light above the plants reduces the light to a quarter of the strength, not half (it’s 1/2 squared = 1/2 x 1/2 = 1/4)!

It’s important to note that plants will respond differently to artificial light depending on their species and environmental factors.

If plants growing under artificial lighting turn red, it’s possible that the light spectrum (mix of colours) or the intensity of the grow lights is influencing the production of anthocyanins. We can alter this by adjusting the intensity or duration of the lighting, or modifying the quality of light in terms of the colour spectrum to help regulate this natural protective response.


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