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).
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 &
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).