Light plays an important role in plant growth and development. It affects almost all stages of plant growth.
The effect of light on plants is mainly shown in two aspects:
One is to provide radiant energy for photosynthesis.
Second, as a signal to regulate many physiological processes throughout the life cycle of plants.
Effects of light on plant growth - photosynthesis and photosensitive pigments
Usually, the growth and development of plants will depend on sunlight, but the factory production of vegetables, flowers and other commercial crops, tissue culture and the reproduction of in vitro seedlings, etc. also need artificial light source to supplement light, in order to promote the photosynthesis.
Photosynthesis is the process by which green plants use light energy through chloroplasts to turn carbon dioxide and water into energy-storing organisms and release oxygen. A key player in this process is the chloroplasts within plant cells. Under the action of sunlight, chloroplasts transform carbon dioxide entering the leaf through the stomata and water absorbed by the roots into glucose, releasing oxygen at the same time.
The photosystem in which light reactions occur is composed of various pigments, such as Chlorophyll a, Chlorophyll b and Catotenoids. The main absorption spectra of chlorophyll a, chlorophyll b and carotenoids are concentrated at 450nm and 660nm. Therefore, in order to promote photosynthesis, 450nm deep blue LED and 660nm super red LED are mainly adopted, and some white LED are added to achieve efficient LED plant light supplement, as shown in figure 1:
In order to be able to perceive the light intensity, light quality, light direction and photoperiod of the surrounding environment and respond to its changes, plants have evolved the light-sensing system (light receptor).
Photoreceptors are the key for plants to sense changes in the external environment. The most important photoreceptors in plants are phytochrome, which absorbs red/far-red light.
Photosensitive pigments are a group of pigment proteins that reverse the absorption of red and far-red light, participate in the photomorphogenesis and regulate plant development. They are extremely sensitive to red light (R) and far red light (FR) and play an important role in the whole growth and development process from germination to maturity.
Photosensitive pigments in plants exist in two stable states: red light absorption type (Pr, lmax=660nm) and far-red light absorption type (Pfr, lmax=730nm). The two types of light absorption can be reversed in red and far-red light.
Studies on the correlation of photosensitive pigments, the effects of photosensitive pigments (Pr, Pfr) on plant morphology include seed germination, desulphurization, stem elongation, leaf expansion, shade avoidance and flowering induction.
Therefore, the complete LED plant scheme needs not only 450nm blue light and 666nm red light, but also 730 nm far-red light. Deep blue light (450nm) and ultra red light (660nm) provide the spectrum needed for photosynthesis, while far red light (730nm) controls the process from germination to vegetative growth to flowering.
As shown in figure 2, a suitable combination of deep blue (450nm), ultra red (660nm) and far red (730nm) provides better chromatographic coverage and optimal growth patterns.
There are two effects of 730nm far-red leds on plants
1. Shadow avoidance of far-red light at 730nm
One of the most important effects of 730nm far-red light on plants is shade avoidance (FIG. 3).
If a plant is exposed to only 660nm of deep red light, it will feel as if it is in direct sunlight and will grow normally. If the plant is mainly exposed to the far red light of 730nm, the plant will feel as if it is blocked by another higher plant from direct sunlight, so the plant will work harder to get out of the shade, which helps the plant grow taller, but does not necessarily mean that there will be more biomass (bio mass).
2. Flowering induction effect of 730 nm far-red light
Another important role of 730nm far-red light in horticulture applications is that it can control flowering cycle through 660nm and 730nm without relying solely on the influence of seasons, which is of great value for ornamental flowers.
The conversion of photosensitive pigment from Pr to Pfr is mainly induced by the deep red light of 660nm (representing the sunlight during the day), while the conversion of Pfr to Pr usually occurs naturally at night, and can also be stimulated by the far-red light of 730nm, as shown in figure 4.
It is generally believed that the flowers of plants controlled by photosensitive pigments mainly depend on the ratio of Pfr/Pr, so we can control the Pfr/Pr value by 730 nm of far-red light, and thus control the flowering cycle more precisely.
3. Prescription of LED plants for fixed light
Leds are used in horticulture and can boost plant growth by up to 40 per cent or in flexible florescence. Because the single LED is independent of each other, it can control performance easily in the greenhouse.
The Photosynthetic Photon Flux (PPF) of the LED itself is very effective, and the typical PPF of deep blue (450 nm) and far red (730nm) LED is 2.3. Mol /J, ultra red (660nm)LED with typical PPF photoactivity of 3.1? About mol/J, and the wavelength of these LED is well matched with chlorophyll a/b, carotenoid and photosensitive pigment Pr/Pfr absorption spectrum, which can achieve high efficiency and significantly reduce energy consumption.
Leds do not emit heat in the direction of the plant and do not damage the plant and are suitable for top, interior and multi-layer cultivation. The R/FR ratio is the ratio of red light (660 nm) to far-red light (730 nm). The R/B ratio is the ratio of red light (660 nm) to blue light (450nm). Through the control of R/FR ratio and R/B ratio, the optimal light prescription can be achieved for various plants.