As an important branch of modern agriculture, the concept of the plant factory has become hot. In the indoor planting environment, plant lighting is an essential energy source for photosynthesis. LED plant lights have overwhelming advantages that traditional fill lights do not have, and will become the first choice for main light or fill light in large commercial applications such as vertical farms and greenhouses.

Plants are among the most complex life forms on the planet. The cultivation of plants is both simple and difficult. In addition to plant lighting, many variables affect each other, and balancing these variables is a masterful art that growers need to understand and master. But as far as plant lighting is concerned, there are still many elements that need to be seriously considered.

Let’s start with the spectrum of the sun and the absorption of the spectrum by plants. As can be seen from the figure below, the solar spectrum is a continuous spectrum, in which the blue and green spectra are stronger than the red spectrum, and the visible spectrum range is about 380~780 nm. There are several key absorption elements in plant growth, and there are significant differences in light absorption spectra of several key auxin factors affecting plant growth. Therefore, the application of LED plant growth lights is not a simple matter, but very targeted. Here it is necessary to introduce the concept of the two most important photosynthetic plant growth elements.

photosynthesis in plants depends on the chlorophyll in chloroplast. It is the most important pigment related to photosynthesis. It has to be found in all organisms that can create photosynthesis. These include green plants, prokaryotic blue-green algae (cyanobacteria) and eukaryotic algae. Chlorophyll absorbs the energy of light, hydrating carbon dioxide and forming carbohydrates (hydrocarbon).

Chlorophyll a is blue-green and mainly absorbs red light. Chlorophyll b is yellowish-green and mainly absorbs blue-violet light. Mainly to distinguish shade plants and sun plants. Shade plants have a small ratio of chlorophyll b to chlorophyll A, so shade plants can strongly use blue light and adapt to shade growth. The strong absorption of chlorophyll a and chlorophyll b has two: a red region with a wavelength of 630~680 nm, and a blue violet region with a wavelength of 400~460 nm.

carotenoids are a group of important natural pigments, commonly found in animals, higher plants, fungi, algae yellow, orange-red or red pigments. More than 600 kinds of natural carotenoids have been discovered so far. Carotenoids produced in plant cells not only absorb and transfer energy to help photosynthesis, but also have the function of protecting the cell from the damage of excited single-electron bonded oxygen molecules. Carotenoids absorb light covering the range of 303~505 nm, which provides the color of food and affects the human body’s food intake; In algae, plants, and microorganisms, their color is not visible because it is covered with chlorophyll.
In the design and selection process of LED plant lights, several misunderstandings need to be avoided mainly in the following aspects.

The red-blue wavelength ratio of light wavelengths
As two main absorption areas of plant photosynthesis, the spectrum emitted by LED plant lamps should be mainly red and blue light. However, it cannot be measured simply by the ratio of red to blue, such as the ratio of red to blue is 4:1, 6:1, 9:1 and so on.

Plant species are very different and have different habits, and different stages of growth have different light requirements. The spectrum required for plant growth should be a continuous spectrum with a certain distribution width, and the light source made of two specific wavelength chips with a very narrow spectrum of red and blue is obviously inappropriate. In the experiment, it was found that the plant will be yellow, the leaf stem is very light, the leaf stem is very thin and so on. There have been a lot of studies on the response of plants to different spectra abroad, such as the effect of infrared part on photoperiod, the effect of yellow and green part on shading effect, and the effect of purple part against pests and diseases and nutrients.

In practical applications, seedlings are often burned or wilted, so the design of this parameter must be targeted according to the plant type, growth environment and conditions.

2. Ordinary white light and full spectrum

The effect of light that a plant “sees” is different from that of the human eye. The white light we commonly use is not able to replace the sun’s light, such as the three primary white light tubes widely used in Japan, etc., the use of these spectra has a certain effect on the growth of plants, but the effect is not as good as the light source made by LED.

For the fluorescent tubes commonly used in previous years with three primary colors, although the synthesis of white, but its red, green, blue spectrum are discrete, and the width of the spectrum is very narrow, the spectral intensity of the continuous part is relatively weak, while the power relative to LED and so on is still large, 1.5 to 3 times the energy consumption. The full spectrum of leds designed for plant lighting is optimized for the spectrum, and although the visual effect is still white, it contains an important part of the light required for plant photosynthesis.

3. Light intensity parameter PPFD

Optical quantum flux density (PPFD) is an important parameter for measuring plant light intensity. It can be expressed in terms of both light quanta and radiation energy. It refers to the effective radiation flux density of light in photosynthesis, representing the total number of light quanta incident on the plant leaf stem in the wavelength range of 400~700 nm per unit time and per unit area. Its unit is μE·m-2·s-1 (μmol·m-2·s-1). Photosynthetic active radiation (PAR) refers to the total solar radiation in the wavelength range of 400 to 700 nm.

The light compensation satiety point of plants, also known as the light compensation point, means that PPFD needs to be higher than this point, its photosynthesis can be greater than respiration, plant growth is greater than consumption, and plants can grow. Different plants have different light compensation points, and it cannot be simply considered that a certain index can be reached, such as PPFD greater than 200μmol·m-2·s-1.

The light intensity reflected by the illuminometer used in the past is brightness, but because the spectrum of plant growth changes due to the height of the light source from the plant, the coverage of the light and whether the light can pass through the leaves, etc., it is not exact enough to be used as an indicator of light intensity in the study of photosynthesis, and most of the current use PAR.

Generally positive plants with PPFD > 50 μmol·m-2·s-1 can initiate the photosynthetic mechanism; Shade plant PPFD only needs 20 μmol·m-2·s-1. Therefore, when installing LED plant lights, they can be installed and set according to this reference value, select the appropriate installation height, and achieve the ideal PPFD value and uniformity on the leaf surface.

4. Light formula

Light formulation is a new concept proposed recently, which mainly includes three aspects: light quality, light quantity and duration. Simply understood, light quality is the spectrum most suitable for plant photosynthesis; The amount of light is the appropriate PPFD value and uniformity; The duration is the cumulative value of exposure and the ratio of day to night. Dutch agriculturalists have found that plants judge the change of day and night by the ratio of infrared to red light, and the infrared ratio increases sharply at sunset, and the plants respond quickly to sleep. Without this process, the plant takes hours to complete the process.

In practical applications, it is necessary to accumulate experience through testing and choose the best combination.