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.