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. 2016). 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 400nm and 500nm and the absorption spectrum of
the red light showed peak between 600nm and 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 single red or blue light. The
seedlings of Salvia splendens F. Sello ex Roem & Schult. were
treated with single blue light only 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 the red light
only. They also observed the induction of flowers from Salvia
splendens which was grown under the blue light alone. Two rose
cultivars, Rosa hybrid ‘Radrazz’ and Rosa 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 of Isochrysis sp. were reported by
Marcetti et al. (2013). The evidences for the complete operation
of the photosynthetic electron transport by blue light and the complete
operation of the Calvin-Benson cycle has been published, and now the
operation of the CO2 assimilations function by single
blue light has gained confidence. The most representative paper is Abidiet al. (2013). They reported that blue light perfectly performs
CO2 assimilation as well as photosynthetic electron
transport. Mesembryanthemum crystallinum was cultured
aeroponically for a 16-h photoperiod at an equal photosynthetic photon
flux density of 350 μmol m-2 s-1under 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).
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.