Introduction to flowering with far-red light and Phytochromes

Plants have evolved over thousands of years and utilise the natural light spectrum in different ways. The most common way is photosynthesis where the plant uses light and converts it into growth. However there are other functions of light as well, mainly to trigger plants and provide guidance. The receptors that are responsible for this are Phytochromes and they mainly respond to light in the 600 to 800nm area. Many people refer to this light as far-red, although it is far red and some parts of other red. By Jille Kuipers

What does far-red do to plants?

The human eye can barely see far-red light. However it can trigger the plant to elongate and stretch. It also influences the size of leaves, the length of stems and the shape of plants.

What is far-red light used for?

Far-red in horticultural applications is used to stimulate the plant development or development of plant organs. It is widely used in greenhouses to trigger or delay flowering and to stretch the plant height. This light is used as supplemental light and far-red in itself is not effective to support plant growth and photosynthesis. 

In several studies with HortiPower LEDs the effect of far-red on flowering is visible. It is also found in other public studies such as above from Park, Y., and Runkle, E., from Michigan State University. 

What is the difference between red and far red?

With Red we often mean Red for the human eye. Red for the human eye has a peak at around 630nm. For far red we often mean far red for plants which has a peak at around 730nm.

Does far-red light inhibit flowering?

Far-red can delay flowering for short-day plants such as chrysanthemums and poinsettias. These plants grow when the nighttime length is long enough. Using far red at the middle of the night will “break” the nighttime length and therefore inhibits flowering. 

Is infrared the same as far-red?

No, infrared ranges from 700nm to 800nm. Far-red for plants has a peak at 730nm and is within the infrared range. 

What light is best for flowering?

If you’re using LED, red LEDs are the best choice to promote flowering buds and flowering. Far-red and deep red have the greatest impact on Phytochromes receptors which regulate flowering functions in plants.

Are far-red LEDs included in regular plant lights?

Most likely not. The most commonly used definition for plant centric lighting ranges from 400nm to 700nm. Therefore if lights use light at 730nm it is not counted as plant-light and reduces the fixture efficiency on paper. In most applications Far-red should not be used for the same amount of lighting hours as other plant-centric LEDs. Therefore manufacturers such as HortiPower use either flexible spectrum fixtures which allows the farmer to selectively turn on LEDs or separate far-red flowering lights. 

What are Phytochromes?

Phytochromes regulate functions related to flowering, chloroplast development, leaf-aging, and leaf fall. These functions are also known as photoperiodism. Phytochrome pigments perceive the day length throughout the year and control plant development functions other than growth and photosynthesis. 

What are the types of flowering plants?

Flowering plants are sensitive to the uninterrupted night length or day length. For easier classification they are split into three main groups: 

• Short-day plants (flower after a period of long nights)
• Long day plants (flower after a period of short nights) and 
• Day-neutral plants (flowers that are largely unaffected by day or night length)

Day-neutral plants are often originally from the tropics where day and night are about 12 hours.

Short-day plants flower in the summer when they sense that the length of the night is decreasing.

Long-day plants flower in the spring when they sense that the length of the night has been increasing. Plants combine their internal sense and together with other factors such as increasing or decreasing temperatures they can trigger a flowering response. There are certain thresholds and they won't be fooled by a cloudy day. A cloudy day will reduce the intensity of the sunlight, for example a bright sunny day can be up to 1000 micromole, whereas a cloudy day can be 150 to 300 micromole. therefore plants will not be influenced by the weather. 

The length of darkness matters to flowering plants

Since this response from flowers is mainly about the length of the darkness, interruptions at night with the right type of light can therefore reduce the critical night-time length. Knowing how to use this will help farmers and plant-lovers to trigger their plants into flowering at the right time. The most obvious example is Poinsettias which are popular during Christmas or chrysanthemums which are popular during the Spring festival in Asia. Poinsettias need to bloom during Christmas so they can be sold at that time. Note that bursts of darkness will not have a meaningful impact on the flowering response of plants. however, bursts of light at night will delay plants' flowering response. In the case of Chrysanthemum, the right light at night can delay bud production and flowering so that growers can grow a stronger, taller plant which will fetch a higher price once it blooms and make the nutrient intake more efficient. 

How greenhouse growers can use far-red

Greenhouse growers use especially far-red light to trigger flowering. In research that universities conducted with HortiPower grow lights, it was found that the intensity of the light and the frequency of the light waves are very important. A small amount of light but larger than 10 micromole can have an effect on plants if it is in the right spectrum. The right spectrum consists of light in the 600 to 800nm category which features deep red and far-red. Normal red was not able to trigger any response.

Phytochromes do not stimulate growth directly but indirectly. It also influences seed germination and fruit development.

Note that plants will use temperature and darkness length to understand if winter has arrived or departed and that energy should be spent on the photosynthesis process.

Fruit development and light

Fruit development happens after flowering, so managing the flowering rate can help increase the fruiting yields for farmers of crops such as Dragon Fruit. These crops will develop fruit when their natural sense will tell them to do so during the safest period of the year. e.g. if they would start to develop fruits too early because of a relatively warm summer, they might not survive when there are normal winter days. 

Phytochromes help regulate the biological clock

Phytochromes also help the plant develop an internal biological clock. The approximate twenty-four-hour cycle is known as the "circadian rhythm" and is calibrated by the sun. The sense of time is important for plants as it helps them to readjust the position of leaves and flowers in the evening and night so that they can be in an optimal position during the next sunrise. During the night the plants will also relax their leaves and reduce the surface area for photosynthesis. Sunflowers can predict the direction of the sun and adjust their heads to make sure they are oriented in the best possible way. Other plants such as orchids use the circadian rhythm as a way to release scent during the night or day, depending on the animals that pollinate them. 

Temperature also provides additional information to the plant, but when plants get conflicting information it throws them off balance and they are creating new leaves when they shouldn’t or dropping leaves when they shouldn’t. 

Note that normal light for humans is often not enough for plants to grow and photosynthesize. However it is also not little enough for plants to ignore at night. In their natural setting, plants ignore moonlight for example (which does not contain much red). Plants should rest and respire at night to support their biological rhythm, so they can develop and photosynthesise during the day. Continuous lighting therefore is not recommended, also not in indoor farming applications. Continuous lighting can cause affect growth and flowering. Plants may experience more air pollution and water stress as the stomata remain open.

Benefits of understanding far-red in the horticulture sector

Using the understanding of Phytochromes will help growers to improve the quality of the plants and support their development in the best possible way. For example you can make plants stretch longer so you have taller plants that fetch a higher price. You can trigger plants into earlier fruiting so you can get the harvest ready just before the peak season and get a peak price for your harvest instead. You can maximise plant growth and nutrient efficiency by focusing on separate stages of plant growth and development. 

What products are made for Phytochromes impact?

HortiPower has products such as the Bloomer 1 and the Nurser 3 FR which contain only far red and deep red. These light sources do not emit any blue, and green and therefore can be used as steering light. Farmers use these lights for e.g. 2 hours at the end of the day to stretch their plants and mimic a natural sunset for plants light. Other farmers use these lights to break the night (length) so they can get more flowers and a bigger fruit harvest. The lights are only 5w and 20w and therefore a very energy efficient and affordable solution to get more control of your plants and crops.