Light is one of the most essential environmental factors for plant growth. It is not only the basic energy source for photosynthesis, but also an important regulator to plant growth and development. Plant growth and development is affected not only by the light intensity (photon flux density, PFD), but also by the quality of light which means different wavelengths and radiation, and their different composition ratios.
Plants are able to detect subtle changes in light quality, light intensity, the duration and direction of the light in plant growth environment. Blue, red and far-red light play a key role in the control of plant morphogenesis of light. Some light receptors such as Phytochrome (Phy), cryptochrome (Cry) and phototropin (Phot) receive light signals and initiate changes in plant growth and development via signal transduction.
Here we discuss the effects from some single color light (with a specific wavelength band) to the plant growth.
Red light can suppress the elongation between plants’node, promote lateral branching and tillering, delayed flower differentiation, increase anthocyanin, chlorophyll and carotenoids. However, far-red (FR), in many cases, can offset the effects by red light. In the growth chamber with white fluorescent lamp as the main light source, far-red LEDs supplementary radiation (emission at 734nm) can decrease content of anthocyanins, carotenoids and chlorophyll, leaving the plant stem length, leaf length and leaf width increases.
Blue light is need in higher plant chlorophyll and chloroplast synthesis. Photosynthetic rate of umbrella algal cells decreased when under red light. But it recover rapidly after being under blue light or blue light added.
Obviously, for plant photosynthesis and growth, only red light is not enough. For example, wheat can complete the life cycle under a single red LEDs light. But in order to obtain a large number of seeds and tall plants, you must add the right amount of blue light. However, excess blue light inhibit plant growth, lead to shorter internodes, decreased branching, and smaller leaf area. There are obvious differences between plant species in terms of need for blue light.
Tomato seedlings grown under white light (including red, blue and green) have significantly lower dry weight than that grown under the red and blue light. Spectral test results in tissue culture growth inhibition suggests that the most harmful light quality is green with a peak at 550nm. In the growth of marigold in light that get rid of green light, the height, fresh and dry weight increased of 30% to 50% higher than plants grown under full-spectrum light.
However, there are also studies about green light promoting plant growth. In 2006, scientists concluded that supplementary green light to a combination of red and blue light (LEDs) results in inhibition of plant growth when the green light accounts for more than 50%, and while strengthen to plant growth when the ratio of green less than 24%. Effects of green are usually as opposed to the blue effect in stomatal opening. However, treatment with a green laser to the seeds of radish and carrots can grow to twice the size of counterpart. based on the past 50 years Plant photobiology studies, scientists think green (580 ~ 600nm) sensing system and blue-red sensors regulate plant growth and development in harmony.
Yellow light (580 ~ 600nm) inhibits the growth of lettuce. People compared the chlorophyll contents of lettuces grown under different light proportions of red and dry weight, far-red, blue, yellow and UV. The result shows only yellow light (580 ~ 600nm) can inhibits the growth even more than green light.
UV radiation can reduce plant leaf area, inhibit hypocotyl elongation, reduced photosynthesis and productivity, to make plants susceptible to attack by pathogens, but can induce the synthesis of flavonoids and defense mechanisms. UV-B can reduce the amount of ascorbic acid and β- carotene, but can be effective in promoting anthocyanin synthesis.