Sucrose is the main osmotic material for the stomatal opening and most of sucrose comes from the mesophyll cells
Sucrose is much more efficient osmotic material than K+ ions. The sucrose in increasing the spacing of the water solution was mainly responsible for osmotic potential; this contrasted with K+ and Cl ions where their spacing effects were only a little higher to that of water held to those ions (Cochrane & Cochrane 2007).
During the day, sucrose synthesized in the cytoplasm of mesophyll cells is actively transported to the guard cell by the H+-sucrose symport through the plasma membrane (Fig. 1). All plant cells usually have plasmodesmata. However, guard cells initially are coupled symplastically to adjoining epidermal cells. With time, however, their plasmodesmata become truncated and eventually nonfunctional, eliminating intercellular communication between mature guard cells and surrounding epidermal cells (Roberts & Oparka 2003). It has now become a general theory that light-induced stomatal openings are made from metabolites or signals synthesized from mesophyll cells (Mottet al. 2008, Mott 2009, McAdam & Brodribb 2012, Fujita et al. 2013, Sibbernsen & Mott 2010, Fujita et al. 2013, Lawsonet al. 2014). In recent years, it was reported that the sucrose produced in the mesophyll cells can be transported to the vicinity of the guard cells via the transpiration stream (Lima et al. 2018). It has been reported that starch decomposition in guard cells can be decomposed in a very short time, and some of sucrose may be sourced from starch decomposition (Sabrina et al. 2020).
When the pH of the vacuole is acidic, Cl- and malate2- may be transported to the vacuole for the charge balances (Fig. 1). Humble and Raschke (1971) reported that only 5% of K+ was balanced by Cl- inCommelina communis . In the same species, the accumulation of malate2- could account for half of the K+ uptake (Allaway 1973). At that time, the stomatal researchers believed that the stomatal opening was caused by K+. However, if the main osmotic material was supposed to be sucrose, the importance of Cl- and malte2- as the osmotic materials needed for stomata to open will be lower.
Assuming that sucrose is the main osmotic material of the stomatal opening, sucrose has to be transported from mesophyll cells. It is generally accepted that all Calvin-Benson cycle enzymes are present and functional in guard cells, but their activities of the chloroplasts definitely low, and they cannot supply all the osmotic materials to guard cells (Lawson et al. 2002, 2003). If the guard cell itself could not supply all the requirements of the energy, then imports from the mesophyll cells must occur (Outlaw 1989, Reckman et al.1990). It has been observed that pulse-labelling solutes are actively transferred from labelled mesophyll cells to the epidermis (Outlaw &d Fisher 1975, Outlaw et al. 1975). There were rapid exchanges of photosynthetic products between the mesophyll and epidermis. These metabolites include glucose, sucrose, sugar phosphates, malate, glycine and serine (Thorpe & Milthorph 1984). Guard cells usually contain from 10 to 15 chloroplasts. In case of Selaginella , the number of guard cell chloroplast were 3∼6. Erigeron annuus (L.) PERS. had 9 chloroplasts per guard cell: Sedum sarmentosum , 7;Chamaesyce supina MOLD, 8; Trifolium repens , 7;Persicaria tinctoria , 9; Portulaca oleracea L., 8) (Lee & Park 2016). In the early days, plant without chloroplasts in the guard cells was firstly known in Paphiopedilum insigna var. (Nelson & Mayo 1975). The succulent plant, Pelagonium zonale cv.Chelsia gem . have no chloroplasts in guard cells (Avrill & Willmer 1984). Slow photosynthetic induction and low photosynthesis inPaphiopedilum armeniacum are related to its lack of guard cell chloroplast and peculiar stomatal anatomy (Zhang et al. 2011). Although these plants do not have chloroplasts in the guard cell, stomata work normally, meaning that these guard cells are sink cells that must receive photosynthetic products from mesophyll cells.
The thickness of the guard cell wall can be reach about 5μm and the width of mesophyll cell wall is only under 100nm (depicted in Fig. 1). It is estimated that about 90% sucrose of the total amount needed by the guard cell comes from mesophyll cells. 80% of which may be transported to vacuole, and 10% of which can be used for maintenances and repairs of guard cells including the wall structures (Fig. 1). Sucrose synthesized by guard cell chloroplasts may be under 10%. It could be estimated that about 5% sucrose may be transported to vacuole, and the rest can be used for maintaining the structure of the guard cell wall and metabolism in the guard cell (Fig. 1).