Kinetic Evidence for New Participants in the Hill Reaction

Abstract

A SIMPLE method for obtaining non-stationary kinetic data from photo-reactions is to use periodic illumination. The photosynthetic process of Chlorella and other algæ has been extensively studied in this way. The results have been confusing in that different relationships between the photosynthetic yield per flash and the interval between flashes (t _d) have been observed. They can be divided into two categories depending upon whether the period of illumination was short (10 µsec. per flash) or long (1 m.sec. per flash or greater). Under conditions of short flashes, the yield per flash reached a maximum value when t _d was about 0.03 sec. ; further increase of t _d did not change the yield, nor was the maximum yield altered by variation of temperature^1–3. With long flashes at high light intensities, greater values of t _d were required to secure greater maximum yields per flash than observed with short-period illumination^4–6. There has been controversy as to the source of the latter observation ; some authors believing that the carbon dioxide-reducing system of Chlorella is involved, while others single out reactants common to both photosynthesis and the Hill reaction^3–7. The latter reaction, named after its discoverer, Dr. Robert Hill⁸, consists of the photolysis of water by isolated chloroplast preparations or whole Chlorella cells upon illumination in solutions of certain oxidizing agents. There is no participation of carbon dioxide.