RECENT RESULTS AT WAGENINOEN 357 



The curve extrapolates to a finite flash yield for zero flash time 

 (Uo). If the flash time is increased, the flash yield initially rises steeply 

 according to an exponential time course. Further extension of the 

 flashtime beyond 5 to 15 m sec. (dependent upon the algae used) 

 yields a proportional increase of R/m ^vith flash time. This final slope 

 is equal to the saturation rate in continuous light. The described type 

 of results can be most simply interpreted by the mechanism: 



(1) U ^' ^ ) U* (U) + (U*) = (Uo) = 2.10-'O2Chl-i 



(2) U* + E ^U + E* (E) + (E*) = (Eo) = G-IO-" O. Chl-i 



(3) E* ^—^ <^(Oo) + E A-2 Uo = 370 sec.-i 



k, = 92 sec. -1 



U represents a photochemical "unit" or a "holochrome," a complex 

 of (a few hundred) cooperating pigment molecules with a common 

 acceptor. The absorption of a single light quantum within the unit 

 suffices for its excitation; further absorption acts are fruitless until 

 restoration via a subsequent dark reaction (2) has occurred. An- 

 other dark step (3), which may be located anjavhere in the chain of 

 reactions between light and oxygen, limits the restoration of E, 



The four types of measurement discussed above yield more than 

 sufficient information for evaluating the concentrations and time 

 constants involved. The numbers given with the reaction sequence 

 (l)-(3) (derived from an experiment made at 30°C.) may serve as an 

 example. Relatively large variations may occur, depending upon the 

 algae and their pretreatment. 



It must be made clear that only with certain types of algae did 

 the above mechanism suffice for a quantitative and accurate descrip- 

 tion of the observations. The more general formulation of the rate- 

 limiting photosjmthetic reactions probably is more involved. 



III. PHOTOINHIBITION 



B. KOK AND J. A. BUSINGER 



Reaction vessels of a volume as small as 40 lA. are currently used 

 in conjunction with our recording volumeter. Full-scale span of a 

 Brown recorder then corresponds to about 0.05 nl. of oxygen. The 

 area to be irradiated amounts to only 15 mm.^ and extremely high 

 light intensities can be concentrated onto the algae (up to 1000- 

 fold higher than required for photosynthetic saturation). 



