The article presents the literature review in the field of photosynthesis, photosynthetic carbon metabolism and assimilates transportation in plants in vivo under changing conditions. The sequence of the data presented leads the reader to understanding as to the principle of photosynthesis regulation in the system of the whole plant. The essence of the proposed conception is to coordinate the light stage, during which the energy from sunlight is absorbed and converted to be stored in the form of ATP and NADPH+, and the dark stage, during which (at CO2 fixation) organic acids are formed. Sugars (which are completely exported to the organs-acceptors of photosynthesis product) are formed at a balanced state of these two stages of photosynthesis. If the conditions are changed (reduced illumination, inhibition of sugars outflow from the leaf with a reduction in the mass of the organs-acceptors or an increase in nitrate supply), the export of sugars from the leaf becomes inhibited, and the number of products of the dark stage is relatively greater than that of the light one, then:1) acids accumulate in the mesophyll cell (including resulted from photorespiration and the formation of glycolate with its metabolic products); 2) excess acids come out of the mesophyll and acidify the aqueous medium in the apoplast, which activates apoplast invertase; 3) as a result of hydrolysis of sucrose (from the sucrose molecule → two moles of hexose) osmoticity of the extracellular aqueous medium increases, which increases also due to evaporation of water when moving to the stomata 4) stomata are closed osmotically, the diffusion of CO2 into the leaf and the intensity of photosynthesis decreases. As a result, the disturbed ratio of dark and light processes in chloroplasts is normalized. All these processes in the leaf are changed in the opposite direction in case of an increase in illumination, demand for the products of photosynthesis with the reduction of the leaf surface or nitrate reduction.
