What is the role of K+ for the stomatal
opening?
Stomata open slowly without lag time under the sun light (Lee & Bowling
1992). Lu et al . (1997) have pulse-labelled the leaflets of broad
bean with 14CO2, then harvested whole
leaf pieces and rinsed epidermal peels for subsequent processing in a
histochemical analysis. There, sucrose–specific radioactivity shows a
peak (111GBq mol-1) in the palisade cells at 20 min.
Therefore, when the plant receives light, sucrose uptake into the guard
cell vacuole could occur firstly. According to the increases of
photosynthetic activities in mesophyll cells over time, stomatal
apertures increase as the synthesized sucrose accumulations into
vacuole. The number of chloroplasts in mesophyll cells is known to be
unexpectedly high. It is expected that the number of chloroplasts in
mesophyll cells may vary depending on the plant species. For the first
time, the number of chloroplasts in mesophyll cells was measured using
an optical microscope and reported to reach up to 70 (Humble & Rashke
1971). Zuzana et al. (2014) applied, for the first time, the
stereological method of an optical dissector based on counting
chloroplasts in stacks of spruce needle optical cross-sections acquired
by confocal laser-scanning microscopy. They reported that, unlike what
was measured with a two-dimensional optical microscope, when measured
with a three-dimensional microscope, the number of chloroplasts
increased by about ten times. Therefore, they estimated that
chloroplasts in mesophyll cells reached hundreds. According to a recent
report, the number of chloroplasts in mesophyll cells was generally
estimated to 100 (Lee 2019). However, the number of chloroplasts in Fig.
1 was comprehensively reviewed and displayed as 60~100.
Except for apoplast, the remaining K+ concentration of
50∼75 mM will be divided into cytoplasm or vacuole. According to the
activity of PM H+-ATPase, K+concentration of the cytoplasm will increase as K+ is
transported to the cytoplasm. Some of the increased K+in the cytoplasm will be transported to the vacuole (Fig. 1). Tonoplast
has with very selective K+/FV-inward and outward
channels (Lebaudy et al . 2007). Since these two-channels are
shaker channels that are activated equally by voltage, it is presumed
that the difference in slope between K+ concentrations
of vacuole and cytoplasm will not be large. TPK/VK channels present in
the tonoplast, which are very selective to K+ ions and
release K+ into the cytoplasm. Tonoplast also have
TPC1/SV channels that transports two ions of K+ and
Ca2+ to the cytoplasm non-selectively (Fig. 1).
Therefore, it is presumed that the transport of K+ to
vacuole from cytoplasm will be limited (Fig. 1).
V-H+-ATPase and H+-ppase are present
in tonoplast, and they transport cytoplasmic H+ into
vacuole. Therefore, vacuolar pH maintains generally around
5~5.5 (Fig. 1). It was also reported that vacuolar pH
was below 3 in 9 plant species. The fruit vacuolar pHs in Citrus
aurantifolia and Prunus cerasus were 1.7 and 2.5, respectively.
The leaf vacuolar pH of the very common Rumex sp. was 2.6, and
the vacuolar pH of the lowest plant, Perpetual begonia , was
0.9~1.4 (Small 1946). This means that tonoplast has a
strong ability to transport cytosolic H+ to vacuole.
In this state, K+ transport by tonoplast-inward
K+ channels must be limited (Fig. 1).