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 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 recognize K+ as the main osmotic material for stomata opening.
At this time, most stomatal researchers, including me, knew that stomata’s main osmotic material was K+. In this atmosphere, a paper has been reported that the accumulation of K+ concentration beyond the imagination occurs in the guard cell when stomata are opened. In fact, this high concentration of K+ was never observed in plant cells, but it was a time when stomatal opening was believed to be opened by K+. When stomata were opened, a paper was published stating that up to 800mM of K+ was accumulated in the guard cell (Talbott & Zeiger 1996). Even today, many scientists understand that stomatal opening is caused by K+. 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. Even in this situation, many stomatal researchers recognized K+ as the main osmotic material for stomatal openings, but papers that sucrose was actually the main osmotic material for stomatal openings were constantly published (Outlaw 1989, Reckmann et al. 1990, Gautieret al. 1991, Poffenroth et al. 1992, Outlaw 1996, Luet 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. When this atmosphere was created, a paper was reported that sucrose and K+ have similar importance and influence 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. Of course, for stomatal opening, most stomatal researchers recognize that Cl- and malte2- are necessary in addition to K+ and sucrose.
Recently, many papers have been published that the stomatal mechanism is regulated by the sugar-sensing enzyme Hexokinase (HXK), and the function by HXK promotes the decomposition of sucrose, resulting in stomatal closing (Kelly et al. 2013, Li et al. 2016, Hei et al. 2017, Kottapalli et al. 2018, Lugassi et al. 2019, 2020). HXK is an enzyme that catalyzes to fructose-6-phosphate and glucose-6-phosphate from promoting the phosphorylation of fructose and glucose in the glycolysis process. It regulates the concentration of sucrose in the guard cell vacuole. Enzymes that control the concentration of sucrose include sucrose synthase, sucrose phosphate synthase and sucrose phosphate phosphatase. Therefore, enzymes that may be related to stomatal opening may include sucrose phosphate synthase and sucrose phosphate phosphatase, which synthesize sucrose.
Zeaxanthin and phototropins (pho1 and pho2 ), 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. After that, the first and last paper to measure stomatal opening using blue light photoreceptors-deficient mutant plants was published (Talbott et al. 2003). After the blue light receptors-deficient mutant plants were made with Arabidopsis thaliana , the stomatal opening by blue light was observed. In wild type, stomatal opening increased by 0.7 μm when treated with blue light, but stomatal opening of the npq1mutant was suppressed by 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 /photo2mutant. SEM (The standard errors of the mean) of about 20 stomatal apertures repeated twice in the Commelina 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). In addition, photosynthetic activity occurs even with red light alone, but when blue light is added, photosynthetic activity and plant growth are greatly increased. Recently, photosynthesis activity has been known to occur with blue light alone. Therefore, when studying stomata, if red light is continuously added to the background while adding blue light on it, the effect of photosynthesis activity by blue light cannot be blocked. Therefore, the stomatal opening by blue light can partially add photosynthetic effect. 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.