Chapter 34 



TIME EFFECTS. II. PHOTOSYNTHESIS IN INTERMITTENT 



LIGHT* 



This chapter calls for the same preliminary remark made in chapter 33. 

 When it was first written, "induction" appeared to be merely a gradual rise 

 of photosynthesis, after a dark period, from a low initial rate to a steady 

 final level; and the effects of light intermittency appeared fully expHcable 

 by the combined influence of induction losses (negative intermittency ef- 

 fect), and a continuation in the dark, for a time of the order of 0.03 second, 

 of the hmiting thermal reaction of photosynthesis (positive intermittency 

 effect). This picture appears oversimplified now, after transient bursts 

 and slumps of gas exchange have been shown to follow changes in the in- 

 tensity of illumination. Combined with induction losses, these transients 

 can make the time course of the exchange of carbon dioxide, or oxygen, or 

 both, quite complicated. 



Furthermore, it now seems likely that photosynthesis in intermittent 

 light may be affected by interaction with respiration and other cataboHc 

 processes (which may be one of the causes of the above-mentioned "tran- 

 sients"). Intermediates of the catabolic metaboUsm can be drawn into the 

 photosynthetic process in a subsequent light period; in other words, in 

 intermittent hght, illumination may reverse part-way some of the respira- 

 tion begun, but not completed, during the dark periods (c/. chapter 37D, 

 section 3). 



Still another general remark is appropriate here: In cell suspensions, 

 "intermittency effects" are also inevitable in "steady" hght because of 

 stirring, particularly in the case of dense suspensions illuminated by a nar- 

 row beam of light. Diluting the suspension and spreading the illumination 

 uniformly over the whole surface of the vessel minimizes the intermittency 

 of the light to which each single cell is exposed. However, even in the ex- 

 treme case when practically no mutual shading of the cells occurs, individual 

 chlorophijll molecules still receive variable amounts of light depending on the 

 momentary orientation of the cell (since a single chloroplast absorbs up to 

 50% of incident light in the absorption peaks of chlorophyll) . The closest 

 approximation to uniform illumination can be obtained by using weakly 

 absorbed (e. g., green) light, and a suspension layer containing, on the aver- 

 age, less than one cell in the path of each hght beam. 



* Bibliography, page 1483. 



1433 



