364 



Kouchkovsky et al. 



uptake of oxygen (a-b), which we call "endoabsorption", because it differs 

 from typical respiration V"'. When illumination begins, a " burst" of oxygen 

 (b-c) appears, similar to bursts observed with Chlorella cells ^ '. The burst 

 is followed by a long-lasting, light-induced oxygen absorption (c-d) or "photo- 

 absorption. " On turning off the light, the rate of oxygen uptake abruptly in- 

 creases (d); this increase in rate we refer to as "extra- absorption". There- 

 after, the rate gradually declines, finally approaching asymptotically a con- 

 stant value, corresponding to the rate of endoabsorption. When, after a dark 

 period, illumination is repeated, a new, but smaller, burst appears. The 

 amplitude of this second burst increases ("regeneration") with the length of 

 the preceding dark period and may finally attain the amplitude of the first 

 burst. These observations are similar to those of Fork'*'. 



These results, along with others to be described below, appear to be con- 

 sistent v/ith a modified cyclic mechanism: 



4 XHOHY i^l— > 4 XH + 4 YOH (I) 



kl 

 4 YOH ->■ O2 + 2H2O + 4Y (II) 



kz 

 4 XH + 2 O2 > 2 H2O2 + 4 X (III) 



4 X + 4 Y + 4HOH --->■ 4 XHOHY. (IV) 



The mechanism supposes that there exists a limited amount of a complex 

 (XHOHY) which disappears in light, giving an oxidant (YOH) and a reductant 

 (XH). The oxidant would decay, giving O2 and the "carrier" Y. The reduc- 

 tant would reduce molecular oxygen, giving hydrogen peroxide and generate 

 the "carrier" X. Finally, X and Y would react with water regenerating the 

 original complex. These different reactions have different rate constants 

 (k*, kl, k2 and k3) and indicate that the sums of the different states of X and 

 Y are constant and equal. 



The present mechanism appears to be able to explain all the features 

 observed in Fig. 1. First, the Mehler reaction'" J - equation III - along 

 with the low activity of catalase in chloroplasts ^^' "' would explain the steady- 

 state, net, consunnption of oxygen in the light (hydrogen peroxide, the product 

 of the Mehler reaction, was demonstrably formed in our experiments). 

 Secondly, the transient net evolution of oxygen in the first seconds of illumi- 

 nation, and the later net consumption of oxygen, are readily explained if one 

 supposes that reactions III and IV are slow compared with reactions I and II. 

 Thirdly, the comparative slowness of reaction III means that a pool of XH 

 must be maintained in light; it is the continuing oxidation of this pool which 

 appears as extra-absorption (because at the end of illumination, reactions I 

 and II, and hence oxygen evolution, stop). As the pool of XH is depleted dur- 

 ing darkness, the oxygen consumption returns to that characteristic of endo- 

 absorption. Fourthly, the gradual regeneration in darkness of a capacity to 

 show a burst of oxygen in subsequent illumination would reflect the slow 

 nature of reaction IV. 



