Introduction
Starch-sugar hypothesis was the basic concept of stomatal physiology in the early 20th century. This theory was brought up by Kohl in 1895. When the plant receives light, photosynthesis occurs, the amount of CO2 in the cell decreases, the pH of the guard cell increases. At high pH, starch phosphorylase, which decomposes starch into sucrose, is activated, increasing the osmotic pressure of the guard cell. On the contrary, it was considered that the photosynthesis did not occur in the dark-treated leaves, resulting in an increase of CO2 concentration. As a result, at low pH, the starch does not decompose into sucrose and the stomata close. It is now known that in the distribution of carbon, carbon assimilated by photosynthesis during the day is used for starch synthesis of chloroplasts or transported to the cytoplasm for sucrose synthesis.
Therefore, the initial starch-sugar theory is not perfect, but it is still a partially accepted theory that it was understood as a sucrose as the main osmotic material that opens stomata.
In 1943, Imamura isolated epidermis from the mesophyll cells and cultured epidermal strips in a high concentration of KCl solution. And then, he observed an increase of K+ concentration in the guard cell. Experimented in the same way as Imamura, in 1976, Hsiao announced that the accumulation of K+ occurs when stomata open. From this point on, many stomatal researchers began to see K+ as the main osmotic material for stomata opening. In 1996, when stomata were opened, a paper was published stating that up to 800mM of K+ was accumulated in the guard cell (Talbott et al . 1996). Even today, many scientists understand that stomatal opening is caused by K+. Environmental factors, such as light and CO2 concentration, trigger events, which may result in stomatal opening. However, currently still, how these signals are sensed and how they are transduced into driving osmotic materials, which control stomatal movements, are not fully understood. Some of the stomatal researchers actually measured the K+ concentration of the guard cell to see if it needed so much potassium for the stomatal opening (Travis & Mansfield 1977, Bowling 1987, DeSilva et al . 1996). When the K+concentration of the guard cell was measured, the total concentration of K+ ions presents in the cytoplasm, apoplast, and vacuole was 100~150 mM, and most K+was known to exist in the apoplast (50~75 mM).
The above results showed that the concentration of K+for stomatal opening was not higher than expected. In this confused state, the osmotic material needed for stomatal opening was considered to be sucrose, as in early theory (Outlaw 1989, Reckmann et al . 1990, Gautier et al . 1991, Poffenroth et al . 1992, Outlaw 1996, Lu et al . 1997, Asai et al . 2000, Outlaw & De Vleighere 2001, Lawson et al . 2002, 2003, von Caemmerer et al . 2004, Outlaw 2003, Kang et al . 2007).
Currently, according to stomatal researchers, K+ or sucrose is believed to be the main osmotic material, so two types of theories are compatible. Of course, for stomatal opening, most stomatal researchers recognize that Cl- and malte2- are necessary in addition to K+ and sucrose. It has been found that K+ and sucrose can act similarly for stomatal opening (Tallman & Zeiger 1988). They reported that stomata were opened by k+ in the early morning and sucrose acts as an osmotic material in the afternoon. Zeaxanthin and phototropins (pho1 andpho2 ), blue light photoreceptors for stomatal openings, have been identified. Blue light has been shown to promote regulatory 14-3-3 protein, as the activity of PM (plasma membrane) H+-ATPase by IAA is mediated by regulatory 14-3-3 protein (Eigo & Kinoshita 2018). However, despite the discovery of a mechanism for stomatal opening by blue light, stomata are also opened by red and white light. The size of the stomatal apertures caused by white light was about 18μm in Commelina communis , but increased by about 6μm stomatal aperture by single blue light and stomatal aperture of about 7.3 μm by red light (Schwarz & Zeiger 1984, Lee & Bowling 1992). The stomatal aperture by blue light was estimated to be the sum of the stomatal opening by chlorophyll and carotenoid and the stomatal opening mediated by blue light photoreceptors. Indeed, experiments with stomatal aperture measurements associated with blue light receptors have been described by Talbott et al . (2003) is the only one. After the blue light receptors-deficient mutant plants were made inArabidopsis thaliana , the stomatal opening by blue light was observed. In wild type, stomatal opening increased by about 0.7 μm when treated with blue light, but stomatal opening of the npq1 mutant was suppressed by about 0.3 μm. The photo1 /photo2 mutant had a rather increased stomatal opening of about 0.3 μm. In the experiment using the blue photoreceptor mutation, the wild type increased about 0.4 μm compared to the photo1 /photo2 mutant. SEM (The standard errors of the mean) of about 20 stomatal apertures repeated twice in theCommelina communis was ± 0.89 μm (Lee & Bowling 1992). Therefore, it is difficult to see that the effect of the distinct blue light receptor appeared in Talbott et al ’. (2003)’s experiment.
Recently, stomatal researchers who studied stomata in relation to blue light photo-receptors were difficult to find, but review papers were available (Inoue & kinoshita 2017, Matthews et al . 2020). Therefore, in this paper, the environmental characteristics of ion and sucrose transport between the guard cell cytoplasm and vacuole are examined, and attempts are made to clarify the opinions on stomatal opening by blue light.