Plants require light throughout their whole life-span from germination to flower and seed production. During the grow process, they do not absorb all wavelengths of light (solar radiation), but selective in absorbing the proper wavelength according to their requirements.
Chlorophylls (chlorophyll a and b) play an important role in the photosynthesis but they are not the only chromophores. Plants have other photosynthetic pigments, known as antenna pigments (such as the carotenoids β-carotene, zeaxanthin, lycopene and lutein etc.), which participate in light absorption and play a significant role in photosynthesis.
LED is a type of semiconductor diode which allows the control of spectral composition and the adaptation of light intensity to be matched to the plant photoreceptors in order to furnish better growth and to influence plant morphology as well as different physiological processes such as flowering and photosynthetic efficiency. Several reports have confirmed successful growth of plants under LED illumination.
For example, biomass yield of lettuce increased when the wavelength of red LED emitted light increased from 660 to 690 nm. Blue LEDs (440 and 476 nm) used in combination with red LEDs caused higher chlorophyll ratio in Chinese cabbage plants. Positive effects of blue (400-500 nm) LED light in combination with red LED light on green vegetable growth and nutritional value have been shown in several experiments. Red LED (640 nm) light as a sole source and results showed increase in anthocyanin contents in red leaf cabbage. Green (495-566nm) and yellow (566-589nm) light contributes to photosynthesis, orange (589-627 nm) will optimize for maximum photosynthesis and red light (627-770 nm) enhances flowering, stem elongation. Several horticultural experiments with potato, radish and lettuce have shown the requirement of blue (400-500 nm) light for higher biomass and leaf area.
The most important part of the light spectrum is 400 to 700 nm which is known as photosynthetically active radiation (PAR), this spectral range corresponds to more or less the visible spectrum of the human eye.
Far-red also important during the growing process. Application of far-red (730 nm) with red (640 nm) caused increase in total biomass and leaf length while anthocyanin and antioxidant potential was suppressed. Addition of far-red (735 nm) to the red (660 nm) LED light on sweet pepper resulted in taller plants with higher stem biomass than red LEDs alone .
The solar radiation can be divided into three wavebands:
- the ultra -violet (UV) corresponds to the wavelengths less than 400 nm and can cause skin damage because of their high energy.
- the visible light, within the 380-770 nm waveband, and contains the PAR (400-700 nm) waveband. The different colors of the visible light, which corresponds to different waveband, may not have the same function towards plant’s development.
- the infrared (IR), greater than 770 nm and have an heating effect. Red: Far-red (R:FR) ratio is very important for plants because it influences plant growth response.
Red (630-720 nm) light is required for the development of the photosynthetic apparatus and photosynthesis. It is essential for the growth of stems, as well as the expansion of leaves. This wavelength also regulates flowering, dormancy periods, and seed germination.
Blue (400-520 nm) light is important for the synthesis of chlorophyll, chloroplast development, stomatal opening and photomorphogenesis. Blue light needs to be carefully mixed with light in other spectra since overexposure to light in this wavelength may stunt the growth of certain plant species. Light in the blue range also affects the chlorophyll content present in the plant as well as leaf thickness.
Green (500 – 600 nm) penetrates through thick top canopies to support the leaves in the lower canopy. Green light alone is not enough to support the growth of plants because it is least absorbed by the plant but when used in combination with red, blue, and far-red, green light will certainly show some important physiological effects. Supplementation of green light enhanced lettuce growth under red and blue LED illumination. Green LEDs with high PPF (300 µmol/m-2/S-1) are most effective to enhance the growth of lettuce.
Far-red LED light
Far-red LED light (700-725 nm) which is beyond the PAR has been shown to support the plant growth and photosynthesis . Far Red Light also passes through dense upper canopies to support the growth of leaves located lower on the plants. In addition, exposure to IR light reduces the time a plant needs to flower. Another benefit of far red light is that plants exposed to this wavelength tend to produce larger leaves than those not exposed to light in this spectrum.
Different wavelengths of red (660, 670, 680 and 690 nm) and blue (430, 440, 460 and 475 nm) light might have uneven effects on plants depending on plant species.
Green + Red+ Blue
The effect of green (525 nm) LED light on germination of Arabidopsis seedlings and results showed that seedlings grown under green, red and blue LED light are longer than those grown under red (630 nm) and blue (470 nm) alone.
Red and far-red light have been shown to affect photomorphogenesis, thus, the ratio of red and far-red light also plays an important role in regulation of flowering. Experiments with different wavelength of green, red, blue, and far-red lights (provided by LEDs) would be beneficial in determining the species specific optimal wavelength for plant growth. The findings of the light response spectrum studies could be used to design an energy efficient tailored light response spectrum for specific plant species.
As plants mature and go through their growth cycle from seedling, to adult, and then flowering and fruiting they use different color spectrums so the ideal LED light is different for each stage of growth. The best color spectrum also depends on the type of plant you are trying to grow. This can get very complicated and is important for commercial growers where they want to maximize results.
It also suggest that lights can increase nutritional value and enhanced antioxidant status in green vegetables: increased carotenoid, vitamin C, anthocyanin and polyphenol. In future more and more research will help us to better understand how lights shape growth of plant.