Bluelight has one of the largest effects in the development of aplant. Blue light is usually have a wavelengths between 425 nm to 490 nm(Runkle, 2017). This wavelength is within the visible spectrum. It has relativelyhigh energy, and have given effects on plant growth and flowering. Multiple studies have shown that exposing a plant to this colourwill give many benefits to it. Blue light also has an effect on photosynthesis,and more exposure to this light can increase the opening of stomata andmaturity rates.
This process is called as photomorphogenesis (Runkle,2017). Photomorphogenesis is defined aslight-mediated development, and is regulated by the light perception network.Photomorphogenesis is consist of three classes of photoreceptors, which isphytochromes, cryptochromes, and phototropin as shown in Figure 1 (referappendix) (Briggs and Olney, 2001). Phytochromes are sensitive to red andfar-red radiation, and cryptochromes are sensitive to blue and ultraviolet.
Cryptochromes are not only found in plants but also inanimals, including humans (Lin, 2002). While, phototropins are sensitiveto blue, green and ultraviolet-A. Proteins thatare related to phototropins play an important role in regulate responsestowards environmental stimuli, such as light and oxygen.
It can beconclude that, cryptochromes work together withphytochromes to regulate photomorphogenic responses, including the regulationof cell elongation and photoperiodic flowering; phototropins, on the otherhand, mediate movement responses (Lin, 2002). In addition, the opening of stomata, which are the tinyopenings on leaves that control both water loss through evaporation during hotand dry days is induced by blue light. The stomata also control theuptake of carbon dioxide (CO2) during photosynthesis. High intensityof blue LEDs can promote plant growth by controlling the integrity ofchloroplast proteins that optimize photosynthetic performance in the naturalenvironment (Muneer et al., 2014). However,from the research done, it shown that blue light will also suppresses theextension growth.
Plants grown under blue light are usually shorter and havesmaller, thicker and darker green leaves but blue light will resulted in leafexpansion and unrolling that allowed earlier light interception compared to plants grown in red light as shownas in Figure 2 (refer appendix). It also have been proved by study of someresearchers , it showed that lettuce plants grown under red LEDs alone had moreleaves and longer stems than plants grown under blue LEDs only (Massa et al., 2008). According toSamuoline et al., 2010, the result from the their research shows that the redlight alone give an influenced to the elongation of flowering stem compared tothe strawberries grown under combination of red and blue light.
Thestrawberries that treated with red light alone were 30% taller. This provedthat the addition of blue light spectrum will suspended the elongation process(Samuoline et al., 2010). Inaddition, under combination of blue and red LEDs also will result to the plantto have a greater specific leaf area and more compact (Samuoline et al., 2010).
According to the previous research done by Lin et al., (2013), it shows that blueLEDs is important for leaf expansion and enhances the leaf area and biomassproduction of lettuce compared to the red LEDs (Lin et al., 2013). Stutteet al. (2009) and Yorio et al. (2001) also came with a result that biomass oflettuce plants grown under mixed blue and red LED lights was higher comparedwith plants grown under monochromatic red LEDs.
When red light is used as theexclusive lighting source, the photosynthetic rate and chlorophyll wererecorded as declining for a variety of crops ( Son & Oh, 2013). Moreover, blue LEDswill increases the production of healthful compounds in some leafy green crops,such as vitamin and antioxidants that play an important roles in protectionfrom photooxidative. It increased the polyphenol, carotenoid and anthocyanincontents that affected leaf coloration (Olle & Virsile, 2013). Delivery ofblue LED radiation also increase the crop quality attributes such as nutrition(Runkle, 2017). The content of total phenolic compounds both in leaves andstems was significantly increased when blue LEDs was engaged as shown in Figure3 refer appendix (Kim et al., 2013).
However,synergetic effect was observed when mixtures of blue and red LEDs were used.Mixed light conditions enhance more the growth of various vegetables, includinglettuce, compared with red LEDs alone. The blueand red LEDs ratio will also altered the total phenolic concentration in bothcultivars as shown in Figure 4 (refer appendix). Increased blue LED ratiosstimulated the accumulation of total phenolics.
For example, ‘Sunmang’ lettuceplants grown under 47 B had 1.4 and 2.4 times significantly have higher totalphenolic concentration compared with those grown under the control and 0 B, at 4 weeks after the onset of LED treatment.The total phenolic concentrations under 26 B to 59 B for ‘Grand Rapid TBR’ were2.2 to 2.7 times significantly higher compared with the control.
The lettuceplants grown under 0 B and 13 B had similar total phenolic concentrations asthe control (Son & Oh, 2013)