The Cytochromes of Plant Tissues 499 



oxidation of ferrocytochrome b^ is partly cyanide resistant, suggesting a possible in- 

 volvement of this cytochrome in the cyanide-resistant respiration of certain plant cells 

 (Martin and Morton, 1955, loc. cit.). Lundegardh (1958, loc. cit.) has also detected 

 cytochrome b^ in plant cells, and Crane {Plant Physiol. 32, 619, 1957) has observed 

 cytochrome b^ in plant microsomes. We considered that cytochrome b^ of plant 

 microsomes is closely analogous to cytochrome b^ of animal microsomes (cf. Martin 

 and Morton, 1955, loc. cit.; Bailie and Morton, Nature, Land. 176, 111, 1955; 

 Morton, 1958, loc. cit.) The observations reported here by Bonner have all been made 

 at — 190°C and it is therefore difficult to relate the various absorption bands detected 

 by him to the components observed by other workers, and especially to the components 

 which Martin and I designated as cytochrome Cj and cytochrome b (in mitochondria) 

 and cytochrome b^ (in microsomes). This difficulty arises because of the variable 

 shifts and variable splitting of the a-absorption bands of the different ferrocytochromes 

 when cooled to — 190°C (see Estabrook, this volume, p. 436) and because Bonner's 

 results give difference spectra (reduced minus oxidized component) whereas our 

 results give absorption bands of the reduced component. The broadening and in- 

 tensification of the absorption at about 560 mjj, (at room temperature) on treatment 

 of plant mitochondria with dithionite after addition of substrate under anaerobic 

 conditions is readily observed. We attributed this to modification, probably involving 

 denaturation, of the labile cytochrome b of plant mitochondria by dithionite. In our 

 experience, dithionite readily produces denaturation of labile haemoproteins such as 

 yeast cytochrome c and yeast cytochrome b^ even when the system is adequately 

 bufTered against changes of pH. 



Finally, in agreement with Slater, we have suspected that CN" may combine with 

 cytochrome b of plant mitochondria, thus accounting for the loss of absorption near 

 560 mn in the presence of this ligand and substrates. 



Bonner: As regards the microsomal component I think that this is a single pigment which, 

 like cytochrome 65 shows (at — 190°C) a^ and aj bands. It differs from cytochrome 65, 

 as shown in Fig. 10 in my paper, in that, on reduction with DPNH with cyanide 

 present, it no longer retains the double peak but shows a single a-band. 



The component with an a-band at 552 m^t (at — 190°C) in plants is not reduced by 

 ascorbate. Since cytochrome Cj of animal tissues is reduced by ascorbate, it does not 

 appear to be identical with cytochrome c^ 



Estabrook : A further point to be considered in assigning the term cytochrome Cj to the 

 pigment of plant mitochondria, in addition to its inability to be reduced by ascorbic 

 acid, is the fact that an absorption band characteristic of this pigment appears in the 

 presence of antimycin A. Thus applying the functional definition descriptive of 

 cytochrome c^ of mammalian systems (cf. Estabrook and Sacktor, Arch. Biochem. 

 Biophys. 76, 509, 1958) one must conclude that this absorption band is not associated 

 with a pigment analogous to cytochrome c^ of mammalian systems. 



The Cytochromes of Roots 

 Chance: In the writing of a review on cytochromes in roots with Lucille Smith (Smith and 

 Chance, Ann. Rev. Plant Physiol. 9, 49, 1958), some aspects of Lundegardh's experi- 

 mental data on cytochromes b kinetics appeared to be inconsistent with those of other 

 components of the respiratory chain. For example, alternating oxygen and nitrogen in 

 the wheat root bundle causes the absorption bands of cytochromes a, c and b to appear 

 in less than a minute after the aerated medium is replaced by oxygen-free medium. 

 Thereafter large and slow absorbency changes occur in the region of cytochrome b. 

 Similarly in his important experiments on activation of anion respiration, there is a 

 rapid change of cytochrome 03 and a slower change of cytochrome b which is maximal 

 in about 10 min. The latter change is in fairly good agreement with the time course 

 of the chloride absorption. 



In the course of the work on plant cytochromes, it has occurred to Bonner and me 

 that the large amounts of accessory b pigments (see Chance, this volume, p. 476) 

 found in other parts of the plant might also occur in the roots and thereby afford an 



