Regulation of Photosynthetic Carbon Assimilation

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Abstract
When whole tissues or isolated chloroplasts are brightly illuminated they do not immediately commence to assimilate carbon at maximal rates. Instead there is an initial lag or induction period which may persist for several minutes (1). This is perhaps the best known and most readily observed example of photosynthetic regulation. Clearly the chloroplasts are potentially capable of rapid photosynthesis because they soon begin to evolve O2 and fix CO2 at high rates. Equally clearly this potential ability is slowed or regulated during the first few minutes of illumination. What is the nature and function of this regulation? Osterhout and Haas (2), who first observed induction at Woods Hole in 1918, suggested two possible causes. Either the lag represented a period during which substrates were built up to the level required for full activity or else the catalysts concerned might be activated in the light. Sixty years later there is little that can be added to these statements in principle, except of course the inevitable notion that it might be both. This article deals with these possibilities in regard to ribulose bisphosphate (RuBP) carboxylase and in particular to the role of ortho-phosphate in metabolic regulation.