Studies on chloroplast development in Chlamydomonas reinhardtii II. Effects of interposed darkness on chlorophyll synthesis

Effects of an inserted dark incubation on light-induced chlorophyllsynthesis in dark grown Chlamydomonai reinhardtii y-1 cellswere studied. Chlorophyll synthesis in cells with the interposeddark incubation proceeded faster than that in cells withoutthe dark incubation when it was inserted within 2.5 hr afterthe onset of illumination. Within this limit, the longer theinitial illumination given, the shorter was the length of darkincubation required to obtain a maximum rate of chlorophyllsynthesis. However, when the dark incubation was provided laterthan 2.5 hr, the rate of subsequent chlorophyll synthesis wasreduced. Since cells responded to the dark treatment in differentmanners before and after the 2.5 hr point, this time was designatedas the transition point. This 2.5 hr period corresponds to thelength of the regular lag phase in chlorophyll synthesis undercontinuous illumination. Based on these results, the nature of the previously postulatedpromoting factor (P-factor) in chlorophyll synthesis is discussed. (Received June 13, 1972; )