156 



Bacon Ke 



Because the 430 mp absorption change in aged chloroplasts was enhanced 

 and its decay was accelerated by ascorbate, and because the absorption 

 change was abolished by ferricyanide, it may be inferred that an oxidation 

 reaction is responsible for the absorption decrease. Since the absorption de- 

 crease occurred at both 430 and 703 m^j, and the slowly decaying portion of 

 the 430 nryj absorption change and the 703 m^ absorption change have the same 

 kinetics, it is reasonable to assume that these absorption changes are caused 

 by the oxidation of chlorophyll, probably the far- red absorbing pigment 

 P-700 ^^^ 



The breadth and peak position of the difference spectrum in Fig. 2 indi- 

 cate that cytochrome oxidation, probably that of cytochrome f, may also be 

 partly responsible for the 430 m>i absorption change. Thus, "the initial ab- 

 sorption decrease may be interpreted as being due to photooxidation of chloro- 

 phyll, part of which rapidly extracts electrons from cytochrome f in the pres- 

 ence of ascorbate and PMS at the stated concentrations. The light intensity 

 dependency of the composite curve suggests that the latter reaction occurs 

 only when the light intensity exceeds a certain level. Cytochrome f then re- 

 reduces rapidly in the presence of PMS and chlorophyll re-reduces~more 

 slowly (by reduced cytochrome, PMS, ascorbate, or endogenous reductants). 

 A similar reaction route has recently been proposed by Witt and co-workers 

 for PMS concentrations less than 10"^ M, but no experimental details were 

 given ^^'. 



Absorption decreases at 430 m^ with rapid decay times of ^ 10"* sec 

 have previously been reported by Moraw and Witt ^^^ The so-called "type O" 

 change has been attributed to the tt -tr* triplet state. The so-called "type 1" 

 absorption decrease has been observed in many types of algae containing 

 chlorophyll a and consequently described as due to the formation of a chloro- 

 phyll a derivative. The "type 1" signal was observed only at high illumination 

 intensities and no saturation could be reached even at extremely high inten- 

 sity. Judging from these characteristics, it can be concluded that the 430 mp 

 signal observed in the present work is not identical with either the "type O" 

 or "type 1" 430 m^ signals reported. Furthermore, the difference spectra 

 for the various types of absorption changes are entirely different. 



In CMU- treated chloroplasts with ascorbate and PMS at concentrations 

 similar to those used here, Jagendorf and Margulies concluded from a high 

 ATP/TPNH ratio that a cyclic electron flow must also have occurred in 

 addition to photoreduction of TPN. It is not known whether a similar situation 

 exists here, and if so, what effect it might have on the transient absorption 

 change. However, experiments on the light-induced 430 mji absorption 

 changes in the presence of ascorbate and PMS were conducted under both 

 aerobic and anaerobic conditions, and practically identical results were ob- 

 tained. Under anaerobic conditions PMS should exist exclusively in the re- 

 duced state in the presence of excess ascorbate. More extended and detailed 

 experiments on these absorption changes will be reported elsewhere. 



