The Cytochromes of Plant Tissues 495 



In spite of the apparent lack of 'cytochrome f in plant tissues we are still 

 presented with the problem of finding a home for the cytochromes b that have 

 been specifically named in plant tissues, viz., cytochromes ^3, b^ and b^. 

 The name 'cytochrome b^ has been given to the microsomal pigment (Martin 

 and Morton, 1953, 1957) wliile cytochrome b^ has been localized in plastids 

 (Hill, 1954). Of the named Z> cytochromes in plants we are left with cytochrome 

 b-, and yet one has to account for the three banded spectrum that char- 

 acteristically appear on reduction of plant mitochondria in the presence of 

 HOQNO and Antimycin A. It is felt that any attempt to name specifically 

 any of the components b that have been presented in this paper would be 

 premature. 



There are two characteristics of the /?-type cytochromes of plant tissues 

 that differ markedly from the cytochrome b component of heart-muscle 

 preparations. Steady state substrate reduction studies of substrate reduction 

 in the presence of cyanide both show that plant b cytochromes possess a high 

 degree of auto-oxidizability. 



The plant tissues and preparations also show an unusually high amount of 

 dithionite-reducible b. An important question concerning this dithionite- 

 reducible material is whether it is the same as one of the substrate-reducible 

 components already described, or whether it represents a separate, non- 

 enzymic component. Chance and Hackett (1959) found that the dithionite- 

 reducible component differed in its light absorption characteristics from the 

 substrate-reducible component. Not only were the y- and a-bands at different 

 positions but the ratio of their intensities also was different, a situation 

 analogous to mammalian tissue preparations (Chance, 1958). The answer 

 to the nature of the dithionite-reducible material is not available at the 

 present time nor is it known if this material represents one or more than one 

 component. 



Mention has been made concerning the cytochrome components known 

 to function in electron transport to oxygen in cyanide-sensitive plant mito- 

 chondria. "While this is not the place for a detailed discussion of cyanide- 

 insensitive respiration, this topic should be considered briefly. Bendall and 

 Hill (1956) have suggested that cytochrome b-, could act in electron transport 

 to oxygen in the cyanide-insensitive particles extracted from the spadix of 

 Arum macula turn. Chance and Hackett (1959), from their study of skunk 

 cabbage, concluded that the time was still premature to consider alternate 

 pathways of electron transport. On the other hand, Bonner and Yocum 

 (unpublished) feel that their evidence with the skunk cabbage does support 

 the alternate pathway hypothesis; although they do agree with Chance and 

 Hackett (1959) that really definite support for an alternate pathway is still 

 lacking. 



The evidence presented in this paper has shown the cytochrome components 

 of cyanide-insensitive and cyanide-sensitive tissues to be exactly comparable. 



