Salinity‐dependent regulation of starch and glycerol metabolism in Dunaliella parva

The glycerol and starch metabolism of synchronized Dunaliella parva cells as a function of the salinity of the medium has been investigated. The higher the salinity of the medium the higher is the rate of glycerol synthesis and the endogenous glycerol concentration, whereas starch content and salinity of the medium are inversely related. Upon transfer to a hyperosmotic NaCl‐medium cells respond by an immediate increase in glycerol synthesis and an inhibition of starch formation in the light. Under corresponding conditions in darkness, starch degradation is stimulated. In both light and darkness hyperosmotic shocks are followed by a rapid increase in the endogenous pool of inorganic phosphate (P i ). It is suggested that in the light the increase in the endogenous phosphate level inhibits the chloroplast ADPG‐pyrophosphorylase (E.G.2.7.7.27), and thereby starch synthesis, and promotes starch phosphorolysis. Photosynthetically produced triosephosphates and triosephosphates derived from starch degradation are converted to glycerol. Also, in the dark the increase in the P i ‐level stimulates phosphorolytic starch degradation and thereby synthesis of glycerol. Reasons for the salt stress induced increase in the endogenous P i ‐level are discussed.