PHOTOSYNTHESIS BY THE ETIOLATED PLANT 225 



fifth hour of light. After exposures of the plant to light for longer than 

 5 hours, quantities of C^* began to appear in the sugar phosphates 

 and sucrose. The percentage of C^* going into sucrose increased 

 gradually during further greening until it had reached a constant 

 value after the etiolated plants had been illuminated for 9 to 12 hours. 

 However, after 12 hours of light, the rate of C'''02 fixation was still 

 only 15% to 20% that of a fully greened plant after 32 hours of light 

 exposure. Thus, by the end of 12 hours, the reactions in the reduction 

 of phosphoglyceric acid were functioning at about equilibrium con- 

 ditions of normal steady-state photosynthesis. The increase in total 

 fixation rate after 12 hours might be the result of other, more slowly 

 developing processes, such as formation of more chlorophyll and of 

 the CO2 acceptor, ribulose diphosphate. 



Only traces of C^^ activity were present in the ribulose diphosphate 

 or sedoheptulose phosphate from the fifth to the twenty-fifth hour 

 analyses after illumination of the plant. Thus one reason for the lag 

 in CO2 fixation rate during the greening period and for the low effi- 

 ciency of the newly formed chlorophyll was the failure of the plant 

 to adapt to synthesis of the ribulose diphosphate needed to drive 

 the photosynthetic carbon cycle. When etiolated plants were sprayed 

 with ribose prior to illumination, the amount of C^^02 incorporation 

 into the various compounds was increased, but the time sequence 

 when the greening plant could incorporate C^'*02 into these com- 

 pounds was not greatly changed. 



One of the last pathways to be activated during the greening process 

 was that for synthesis of serine. About 20 hours of light was required 

 before large amounts of C^'*02 began to be incorporated into serine. 

 This was at a markedly later time than that for labeling alanine. 

 It has already been shown that glycohc acid oxidase activity in- 

 creased greatly in etiolated plants after about 20 hours of light 

 (5) and that glycolic acid is readily converted to serine in plants 

 (6,7). Thus a pathway from the sugars of the photosynthetic carbon 

 cycle, through glycolic acid to serine, appears to be a major pathway 

 for serine synthesis, which occurs late in the greening process. 



The very short lag phase in CO2 fixation (seconds to a few minutes) 

 by a green plant, when transferred from dark to light, indicates the 

 rate at which steady-state photosynthesis is reached in a system 

 already containing chlorophyll and all the enzymatic components for 

 photosynthesis. That can be compared to the >24-hour lag in photo- 



