366 



Kouchkovsky et al. 



B'/tgi, D' and E' are concerned with the second burst. The regeneration 

 of the initial complex XHOHY (reaction IV) can then be evaluated as the ratio 

 of, for instance, B'/tgt to B/tg. When the value of this ratio was plotted as 

 a function of the dark period between illuminations, a curve was obtained indi- 

 cating that reaction IV is second order '^'. This is consistent with a reaction 

 between two components (i. e. , X and Y). 



For the purpose of determining the kinetics of reaction II, the logarithra of 

 the amount (p) of the oxygen precursor was plotted as a function of time. 

 However, the first order rate constant previously reported ' ' is certainly 

 too small, because of the time response of our apparatus. 



The rate of reaction III was analyzed by plotting the logarithm of the amount 

 (q) of reducing power as a function of time. This reaction appeared to be 

 first order, with a rate constant of '^ 1.7' 10""^ sec" ^ , 



This reaction is probably bimolecular, but gives a first order kind due to the 

 relatively high concentration of O2 in aerobic conditions .Then, ki»k2- 



Several of our observations indicate that, in addition to the Mehler reaction 

 there was another mechanism of light-dependent oxygen uptake. Firstly, 

 CMU and ortho-phenanthroline, which are known to inhibit photosynthetic 

 reactions, suppressed the oxygen burst, but did not entirely suppress all 

 oxygen uptake in the light steady- state. Secondly, we found that net absorp- 

 tion of 1 mole O2 was accompanied by the formation of only 1. 5 mole H2O2. 

 instead of the two moles anticipated in the Mehler reaction. Thirdly, when 

 we plotted the rate of photoabsorption as a function of illumination, the result- 

 ing curve had a sharp break indicating two contributing reactions (in contrast, 

 the illumination curve for the burst was a smooth curve rising to saturation). 

 These findings are most simply explained assuming that a ' 'photo - oxidation ' ', 

 as well as the Mehler reaction, contributes to oxygen uptake in the light ^^'. 



500 

 <00- ' 

 300- 

 200- 

 100' 



m^MOp/s 



^. 



X. 



300 



200- 



100 



-pH 



VO^{L) ,., mf^l^O^/^ 



••400 



200 



Fig. 3. pH effect on burst, photoabsorption and extra-absorption. 

 Chloroplasts : 300 u g chlorophyll /ml in TMS. At left : initial 

 rate of the burst (B/tg, Fig. 2). At right : -•- = steady-state 

 rate of the photoabsorption (C/tc. Fig. 2); -o- = steady-state 

 rate of oxygen evolution (G/tQ, Fig. 2); -x- = steady-state rate 

 of oxygen uptake (F/tF. Fig. 2). 



