Introduction In the process of photosynthesis complex enzymatic reactions are involved particularly in the Benson-Calvin cycle which occurs in the chloroplasts. Some of these enzymes are inactive in darkness but are activated by light, hence, the cycle functions only during the day. Examples of such enzymes are fructose bisphosphatase (FBPase) and sedoheptulose bisphosphatase (SBPase) which are inactivated in darkness. Several mechanisms have been proposed that account for the activation and deactivation of the Benson-Calvin cycle. For example, upon illumination, the pH of the stroma increases from pH 7 to pH 8 owing to the active pumping of protons into the intrathylakoid space. This is accompanied by a transfer of Mg 2+ ions via the thylakoid membrane into the stroma. As the envelope of the chloroplast is impermeable to Mg 2+ ions, the concentration of Mg 2+ in the stroma increase from 1 to 2-5mm. Most of the enzymes of the Benson-Calvin cycle are sensitive to pH and to changes in Mg 2+ concentration but these changes, by themselves, do not wholly account for the activation of the cycle. Two other types of activation mechanisms have been shown: (1) the photoregulation of these enzymes in which light-regulated redox systems such as ferredoxins are involved, and (2) regulation via energy load and via the concentration of metabolites from the cycle.