Plant growth and photosynthesis could occur under single blue
light
In the 1960s, mechanisms for photosynthetic electron transport have been
revealed, which is the fact that electron transport is centered around
photosystem II (P680) and photosystem I (P700). Since then, it was
recognized that photosynthesis was caused mainly by red light. In 1882,
Engelmann used a prism to disperse sunlight into a rainbow, then
illuminated aquatic filaments, Spirogyra sp. and observed the
migration of the O2-seeking bacteria population. As a
result, O2-seeking bacteria gathered at the sites that
generate the most oxygen, where the similar numbers of bacteria gathered
at blue and red light regions. This experiment was surprisingly the
first to show that photosystem II works perfectly by single blue light
only. Recently, many papers were reported that photosynthetic electron
transport by blue light occurred perfectly (Evans et al . 2017,
Gruszecki et al . 1997, Miao et al . 2015). The absorption
spectrum regions of chlorophyll a and b are much larger and have a wider
range than red light region. The absorption spectrum of chlorophyll b
showed the peak between 400~500nm and the absorption
spectrum of the red light showed peak between 600~700nm.
The sizes of blue light absorption spectrum region are twice higher than
that of red light in chlorophyll b. The highest absorption spectrum of
chlorophyll a showed a larger and broader absorption spectrum from 300
to 450 nm than the highest red light absorption spectrum in 670nm
(Comar
&
Zscheile
1942). β-carotene absorbs only the regions of blue light band. These
facts imply that the main absorption spectrum of chlorophyll is blue
wavelength, which is very important for photosynthetic activities. It
was reported that blue light increased net leaf photosynthetic rates
when those are compared to red light alone in wheat (Goins et al .
1997). Only 50 % blue light doubled the photosynthetic activities over
red light alone (Hogewoning et al . 2010). The most surprising
facts are that plants could grow only with red or blue light. The
seedlings of Salvia splendens F. Sello ex Roem & Schult. were
treated with only blue light for 4 weeks and then growths were observed
for the first time (Runcle 2017). In the absence of blue radiation, the
plants had purplish leaves. However, in outdoors, the plants had green
leaves. Seedlings grown indoors with blue light were often shorter and
had smaller leaves than grown under only red light. They also observed
the induction of flowers from Salvia splendens which was grown
under the only blue light. Two rose cultivars, Rosa hybrid ‘Radrazz’ andRosa chinensis ‘Old blush’, were cultivated under blue or white
light. While plant development was totally inhibited in darkness, blue
light could sustain full development from bud burst until flowering
(Abidi et al . 2013). These mean that the plants operate
photosynthetic electron transports and the Calvin cycles even though
they were only treated by blue light. It was reported that blue light
was more essential than red light for the activities of photosystem II
and I in cucumber leaves (Miao et al . 2016). The similar effects
of blue light on biochemical composition and photosynthetic rate ofIsochrysis sp. were reported by Marcetti et al . (2013).Mesembryanthemum crystallinum was cultured aeroponically for a
16-h photoperiod at an equal photosynthetic photon flux density of 350
μmol m-2 s-1 under red and blue
light. Compared to plants grown under red light condition, blue light
treated plants had similar but higher total chlorophyll contents,
carotenoid contents and higher Chl. a /b ratios
(He et al .
2017). Blue light perfectly performs CO2 assimilation as
well as photosynthetic electron transport (Abidi et al . 2013).
The study of stomatal opening by blue light began with the idea that
photosynthetic activities were induced only by red light. In the studies
of the stomatal opening mechanism, those were recognized that the
treatment of red light as a background could eliminate all
photosynthetic effects and observed the stomatal opening reactions by
blue light. Surprisingly, the studies of the stomatal opening mechanisms
caused by the blue light originated from this experimental prerequisite.
Nevertheless, some stomatal researchers did not think that blue light
was essential for the activities of photosynthesis.
Fig. 1. The possible mechanisms of stomatal opening. The explanations of
the figure are below.