Conclusion
Algae does not have guard cells. Intercellular space is well developed
for gas exchange and supporting action in leaves of aquatic plants, and
intercellular space is connected to guard cells. Leaves submerged in
water have no stomata, whereas leaves floating on water have guard cells
in the upper epidermis. Evolutionally, guard cells are a new apomorphy
from algae. The evolution of the stomata was derived from the process in
which algae evolved into terrestrial plants. Guard cells are present in
the leaves of bryophytes, fern and almost all vascular plants. The basic
role of stomata is to minimize water stress and to maximize the
efficiency of photosynthesis activity through photosynthesis and
transpiration. Photosynthesis plays a central role in the physiology of
plants and the understandings of its response to light are, therefore,
critical to any discussion of how plants sense and respond to light. It
is likely that many responses exhibited by plants to light are in fact
mediated by the response of photosynthesis (Lee 2019). The stomatal
mechanism could be controlled by the command and the operation. This
suggests the idea that the stomata should be controlled according to the
demand of the mesophyll cells. In this hypothesis, the command
originates in the mesophyll cells and the operation in guard cells (Lee
& Bowling 1995). About 95% of total photosynthesis in plants occurs in
mesophyll cells of the leaves (Lee 2016). Therefore, it is assumed that
the photosynthesis products, inorganic ions and hormones, are very
likely to be transported between the guard cells and the mesophyll
cells. Chloroplasts in mesophyll cells have evolved optimally for
photosynthesis for hundreds of millions of years.
However, chloroplasts in guard cells have evolved to be less optimized
for photosynthesis. This may be seen as a reduction in the role of
chloroplasts in the guard cell or a decrease in photosynthetic activity.
There were much lacks of understanding of guard cell vacuole when
explaining the stomatal opening mechanism. The guard cell vacuole is
normally maintained at pH 5~5.5. It is regulated by the
high activities of V-H+-ATPase and
H+-PPase in apoplast. Therefore, due to the activity
of V-H+-ATPase and H+-PPase in the
guard cell vacuole, vacuole has a high positive charges by
H+, so the transport of K+ into
vacuole through the inward vacuolar K+ channels can be
limited. Tonoplast has outward-K+-channels/FV,
outward-TPK/VK-channels and TPC1/SV channels that release vacuole
K+ into the cytoplasm. These channels are responsible
for balancing the positive charges as well as the K+concentration in the vacuole. Therefore, sucrose-H+antiporters transport sucrose to vacuole relatively easily.