The Role of Extracellular Space (Apoplast) in The Regulation of Physiological Processes in Plants

The paper summarizes the results of studies of the role played by the plant leaf extracellular space in the regulation of mesophyll cells water exchange, photosynthesis and export of assimilates to consuming sink organs. When comparing different methods of assessing water exchange rates, the participation of the plasmalemma in the formation of the leaf water potential was shown. Using the in vivo method of extracting the contents of the apoplast, the dynamics of the exit and movement of labeled photosynthetic products through the extracellular space was traced. It was shown that there is a circulation of assimilates in the plant. As the sugars move downwards along the phloem, they partially escape into the extracellular space and are carried away upwards by the transpiration stream, where they re-enter the leaves that have completed growth and are re-exported. This creates a common pool of photosynthetic products, which serves as a test of the balance between photosynthesis and the use of assimilates by consuming organs. Analysis of the metabolism of 14C-labelled photosynthetic products in genetically transformed plants with increased or decreased apo plastic invertase activity showed the key importance of this enzyme in the regulation of photosynthesis and export of 14CO2 assimilation products from the leaf. Simultaneous measurement of photosynthesis and transpiration in leaves of these plants under rapid changes in light exposure showed a regulatory link of photo processes in chloroplasts, dark reactions of carbon dioxide assimilation by mesophyll cells and transport of assimilates into phloem, the actuator of which is the leaf stomata. If the balance between photo-processes and the reduction of CO2 to sugars in the cell is disturbed, the ratio of sugars to organic acid changes, which leads to a pH shift in mesophyll cells, and then in the apoplast, which alters apo plastic invertase activity. Accordingly, depending on the degree of sucrose hydrolysis in the apoplast, the osmolarity of the extracellular medium changes (1 mole of sucrose → 2 moles of hexoses), increasing with approaching to the stomatal aperture, which affects the stomata and leaf resistance to CO2 diffusion. As a result, the ratio between light and dark processes is normalized.

Keywords: Photosynthesis; Apoplast; Cell water exchange; Nitrates; Nitric oxide; Invertase; Transport of assimilates; Photo processes of chloroplasts