Photosynthesis by the Etiolated Plant after 

 Exposure to Light* 



N. E. TOLBERT, Biology Division, Oak Ridge National Laboratory, Oak 



Ridge, Tennessee 



Except for chlorophyll synthesis, measurements of the acquisition 

 of photosynthetic ability by the etiolated plant after exposure to light 

 have not been extensive. Oxygen evolution by etiolated barley plants 

 commences about 30 minutes after plants are placed in the light 

 (2,3). However, Irving (4) reported that greening etiolated barley 

 and bean seedlings did not fix CO2 until after they had developed a 

 large part of their chlorophyll. 



Observations have now been made on the fixation of 0^*02 and the 

 products formed during increasing time intervals after an etiolated 

 Thatcher wheat plant has been placed in white light. Except for the 

 initial photo-conversion of protochlorophyll to chlorophyll, chloro- 

 phyll synthesis did not begin until the plants had been in the light 

 for about 2 hours. The rate of Ci^02 fixation was almost zero for about 

 2 hours after rapid chlorophyll synthesis had started, i.e., after 4 to 5 

 hours of continuous light. 



The products formed from the newly fixed Ci''02 were analyzed by 

 paper chromatography. During the first 4 hours after the etiolated 

 plants were placed in the light, the only 0^*02 fixation was CO2 ex- 

 changed into the carboxy groups of malic, aspartic, and glutamic 

 acids. After 4 hours of light, there was no significant photosjmthetic 

 CO2 fixation even though the plants contained appreciable amounts 

 of chlorophyll. Between 4 and 5 hours of light a preponderant amount 

 of the C^'*02 was found in phosphogly eerie acid and alanine. Plants 

 at this stage of greening were apparently not able to reduce the C3 

 compounds, first produced photosynthetically, to sugars. This con- 

 dition in the plant existed for an interval of 1 to 2 hours around the 



* Work performed under U.S. Atomic Energy Commission Contract No. W- 

 7405-eng-26. A more detailed description of this work has been published else- 

 where (1). 



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