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.