482 W. D. Bonner, jr. 



carbon monoxide, these workers deduced that the respiration of these tissues 

 was mediated by cytochrome oxidase. However, mature carrot leaves did not 

 show any appreciable inhibition with azide, cyanide, or carbon monoxide. 

 From these widely varying approaches to plant respiration substantial 

 evidence was available already in 1939, showing that cytochrome oxidase 

 does mediate the reaction of plant tissues with oxygen and that plants possess 

 a full complement of other cytochromes. In spite of this fact, progress in 

 understanding the cytochromes of plant tissues has continued to be slow for 

 the reasons outlined above. 



THE CYTOCHROME COMPONENTS THAT HAVE BEEN 

 DESCRIBED AND NAMED IN PLANT TISSUES 



As mentioned above, both Okunuki (1939) and Hill and Bhagvat (1939) 

 and Bhagvat and Hill (1951) described components a, b and c, and prior to 

 this, Keilin had described 'modified cytochrome' or cytochrome b' as well as 

 components a, b and c. Hill and Scarisbrick (1951) described a b type 

 cytochrome present in the leaves of a wide variety of higher plants and also 

 extracted from the leaves of Vicia faba. The soluble component, called 

 cytochrome b^, was auto-oxidizable and did not combine with carbon 

 monoxide or cyanide. The presence or absence of cytochrome b^ has neither 

 been confirmed nor denied, although Martin and Morton (1955, 1957) have 

 described a similar component in the microsomal fractions derived from 

 chlorophyll-free portions of silver beet petioles and from wheat roots. The 

 absorption maxima given by these two groups of investigators are sufficiently 

 different to warrant the view that Martin and Morton were not dealing with 

 the same component described and extracted by Hill and Scarisbrick (see 

 Martin and Morton, 1957). 



While studying the cytochrome components in the plastids of etiolated 

 barley leaves, Davenport (1952) observed a b cytochrome which was later 

 observed in Chlorella and named cytochrome b^ by Hill (1954). 



There is an additional /)-type cytochrome component which has been 

 described in the tissues of Arum macidolum and called 'cytochrome b^"" 

 (Bendall and Hill, 1956). The term 'cytochrome 6,' has also been used by 

 Bendall (1958), by Hackett and Haas (1958) and by Chance and Hackett 

 (1959). Evidence that cytochrome h.^ differs from previously described b 

 cytochromes rests on the visual spectroscopic determination of oxidation- 

 reduction potentials in tissue preparations. This determination is based on 

 the absorption characteristics of preparations that are in equilibrium with 

 various solutions which differ in their oxidation-reduction potentials. 



The cytochrome A-type components which have been described in plant 

 tissues are summarized in Table 1. 



All observations point to the fact that the cytochrome c of higher plants is 

 very similar to that of other organisms. Goddard (1944) was the first to 



