Conclusion
Plant photosynthesis is the primary producer on earth, making it an essential energy source for the survival of all living things, including humans. In addition, many metabolisms of plants are associated with photosynthesis, including plant growth and morphogenesis, stomatal opening, photorespiration, energy production, enzyme activity, ions transport and cell wall synthesis. 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 apoplast 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. 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. Sucrose-H+ antiporters transport sucrose to vacuole relatively easily. The above results support that sucrose is clearly the main osmotic material for stomatal opening.