THE CYTOCHROMES OF PLANT TISSUES 



By W. D. Bonner, jr. 



Johnson Research Foundation, University of Pennsylvania, 

 Philadelphia 4, Pennsylvania 



INTRODUCTION 



Knowledge concerning the cytochrome components and their function in 

 plant tissues has not developed as rapidly as has been the case in animal 

 tissues. This slow development is not entirely due to the fact that, until 

 recently, it was not always considered respectable to work on plant tissues. 

 Visual observation of cytochromes in plant tissues is difficult because of the 

 presence of plastid pigments and the low haem concentration. Plant physio- 

 logists themselves, because of their preoccupations with the copper-protein 

 'oxidases' have been partially responsible for the lack of knowledge of plant 

 cytochromes. 



The development of techniques for the extraction and concentration of 

 cytoplasmic inclusions, coupled to the development of rapid and sensitive 

 spectrophotometers, has resulted in considerable progress toward under- 

 standing the nature of some of the plant cytochromes. In spite of this progress 

 our present understanding of plant cytochrome components is inadequate 

 and our knowledge concerning the path of electron transport is negligible. 



Plant cells possess a rigid cell wall, an immediate barrier to rapid and 

 effective homogenization. This difficulty can be partly overcome by using 

 dark-grown material, the absence of hght preventing the large-scale pro- 

 duction of lignin. The cytoplasm represents only a modest fraction of the 

 total cell volume, the major portion of a mature plant cell being vacuole. 

 Frequently the vacuolar contents are very acid and on rupture of the cell this 

 acid can produce a considerable and undesirable drop in the pH of the 

 homogenate. This high acidity that is frequently encountered presents an 

 obstacle to the extraction and fractionation of the cytoplasmic constituents 

 of the cell. A completely suitable isotonic extracting medium with low salt 

 content and the necessary buffering capacity remains to be found. 



The isolation of the 'mitochondrial fraction' from plant tissues has been 

 more or less stereotyped since the methodology was introduced in 1951 

 (Millerd, Bonner, Axelrod and Bandurski, 1951; Axelrod, 1955). This 

 method involves homogenization of the tissue in isotonic sucrose together 

 with a rather high concentration of phosphate butfer. In spite of the fact that 

 active, phosphorylating particulate preparations have been reported from 



479 



