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.