496 W. D. Bonner, jr. 



In short, there are no cytochrome components unique to cyanide-insensitive 

 tissues, a fact that requires a further reinterpretation of the alternate pathway 

 hypothesis. Such a reinterpretation has been made by Bonner and Smith 

 (1961) who propose that all tissues possess the capacity for exhibiting cyanide- 

 insensitive respiration. Such a hypothesis presumes two pathways of electron 

 transport to oxygen, perhaps as illustrated in Fig. 14. 



Some possible consequences of such a proposal are discussed by Bonner 

 and Smith (1961). However, considerable effort is needed to provide proof 



CONTROL 



SUBSTRATE 



'Q 



CYAMDE 



"sensitive * 2 



C_YAJ^LDE_ _ . ^ 



insensitive 2 



Fig. 14. Hypothetical pathways of electron transport in plant tissues. 



that plant tissues can, or cannot, transport electrons to oxygen through a 

 cytochrome system working along with, or alternately to, the c, a, a^ system. 

 Although there has been considerable progress in the last few years in 

 information concerning the cytochromes of plants, there remain enormous 

 gaps in our knowledge. 



SUMMARY 



1. A survey of the cytochromes in various plant tissues has been carried 

 out utiUzing the remarkable sensitivity and resolution of the low temperature 

 divided-beam spectrophotometer. 



2. Previous observations, from other investigators showing that plants 

 contain cytochromes a -f a^ and c have been confirmed and extended. 



3. Plant tissues do not appear to contain cytochrome q, as defined in 

 animal tissues, nor cytochrome b. 



4. The cytochromes 'f of plants are unusually auto-oxidizablc as shown 

 by substrate steady-state reduction of the pigments of mitochondria in the 

 presence and absence of cyanide. 



5. Plant mitochondria, when reduced with DPNH in the presence of 

 HOQNO or antimycin-A, show three well-defined absorption maxima at 

 555, 558 and 564 m//. 



6. The microsomal fraction, derived from various plant tissues, show, on 

 DPNH reduction, a two-banded spectrum of maxima 552 and 558 m//. 

 Reduction of this same fraction with dithionite results in an asymmetric 

 band of maximum 559 m/^. 



