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). The number of chloroplasts in Fig. 1 was estimated to be 60 to 100 based on the published data. 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).