Blue
light has one of the largest effects in the development of a
plant. Blue light is usually have a wavelengths between 425 nm to 490 nm
(Runkle, 2017). This wavelength is within the visible spectrum. It has relatively
high energy, and have given effects on plant growth and flowering. Multiple studies have shown that exposing a plant to this colour
will 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 and
maturity rates. This process is called as photomorphogenesis (Runkle,
2017).

            Photomorphogenesis is defined as
light-mediated development, and is regulated by the light perception network.
Photomorphogenesis is consist of three classes of photoreceptors, which is
phytochromes, cryptochromes, and phototropin as shown in Figure 1 (refer
appendix) (Briggs and Olney, 2001). Phytochromes are sensitive to red and
far-red radiation, and cryptochromes are sensitive to blue and ultraviolet. Cryptochromes are not only found in plants but also in
animals, including humans (Lin, 2002). While, phototropins are sensitive
to blue, green and ultraviolet-A. Proteins that
are related to phototropins play an important role in regulate responses
towards environmental stimuli, such as light and oxygen. It can be
conclude that, cryptochromes work together with
phytochromes to regulate photomorphogenic responses, including the regulation
of cell elongation and photoperiodic flowering; phototropins, on the other
hand, mediate movement responses (Lin, 2002).

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            In addition, the opening of stomata, which are the tiny
openings on leaves that control both water loss through evaporation during hot
and dry days is induced by blue light. The stomata also control the
uptake of carbon dioxide (CO2) during photosynthesis. High intensity
of blue LEDs can promote plant growth by controlling the integrity of
chloroplast proteins that optimize photosynthetic performance in the natural
environment (Muneer et al., 2014).

            However,
from the research done, it shown that blue light will also suppresses the
extension growth. Plants grown under blue light are usually shorter and have
smaller, thicker and darker green leaves but  blue light will resulted in leaf
expansion and unrolling that allowed earlier light interception compared to plants grown in red light as shown
as in Figure 2 (refer appendix). It also have been proved by study of some
researchers , it showed that lettuce plants grown under red LEDs alone had more
leaves and longer stems than plants grown under blue LEDs only (Massa et al., 2008). According to
Samuoline et al., 2010, the result from the their research shows that the red
light alone give an influenced to the elongation of flowering stem compared to
the strawberries grown under combination of red and blue light. The
strawberries that treated with red light alone were 30% taller. This proved
that the addition of blue light spectrum will suspended the elongation process
(Samuoline et al., 2010).

 

            In
addition, under combination of blue and red LEDs also will result to the plant
to 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 blue
LEDs is important for leaf expansion and enhances the leaf area and biomass
production of lettuce compared to the red LEDs (Lin et al., 2013). Stutte
et al. (2009) and Yorio et al. (2001) also came with a result that biomass of
lettuce plants grown under mixed blue and red LED lights was higher compared
with plants grown under monochromatic red LEDs. When red light is used as the
exclusive lighting source, the photosynthetic rate and chlorophyll were
recorded as declining for a variety of crops ( Son & Oh, 2013).

 

                Moreover, blue LEDs
will increases the production of healthful compounds in some leafy green crops,
such as vitamin and antioxidants that play an important roles in protection
from photooxidative. It increased the polyphenol, carotenoid and anthocyanin
contents that affected leaf coloration (Olle & Virsile, 2013). Delivery of
blue LED radiation also increase the crop quality attributes such as nutrition
(Runkle, 2017). The content of total phenolic compounds both in leaves and
stems was significantly increased when blue LEDs was engaged as shown in Figure
3 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, including
lettuce, compared with red LEDs alone. The blue
and red LEDs ratio will also altered the total phenolic concentration in both
cultivars as shown in Figure 4 (refer appendix). Increased blue LED ratios
stimulated the accumulation of total phenolics. For example, ‘Sunmang’ lettuce
plants grown under 47 B had 1.4 and 2.4 times significantly have higher total
phenolic 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’ were
2.2 to 2.7 times significantly higher compared with the control. The lettuce
plants grown under 0 B and 13 B had similar total phenolic concentrations as
the control (Son & Oh, 2013)

           

 

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