480 W. D. Bonner, jr. 



many laboratories and with a wide variety of plant tissues, there have been 

 no reports that such particulate preparations exhibited control of respiration 

 by adenosine diphosphate (ADP) concentration. It may be, of course, that 

 in plant tissues there is a more complex control of the respiratory process 

 than that we now understand in animal systems (Chance and Williams, 1956; 

 Chance, 1959). 



Many plant tissues exhibit a respiration which is insensitive to the usual 

 ligands, cyanide, azide, and carbon monoxide, a situation analogous to 

 instances recorded in some animal and insect tissues. There are three known 

 types of cyanide-insensitive respiration, viz : 



(a) The so-called 'ground respiration', easily demonstrated through the 

 prolonged washing of many tissues, e.g. carrot. 'Ground respiration' 

 appears after a marked decrease in respiratory rate during washing. 



(b) The development of cyanide-insensitive respiration in 'aged' tissue 

 slices ; the development of the cyanide-insensitive respiration is accom- 

 panied by a marked increase in respiratory rate. 



(c) Tissues in which the rate of respiration is close to maximum and which 

 normally exhibit cyanide insensitivity. These different situations under 

 which cyanide-insensitive respiration appears in plant tissues add com- 

 plexities over and above those already enumerated to the under- 

 standing of the sequence and function of the cytochromes found in 

 plant tissues. 



Cytochromes were first observed in plant tissues by Keilin (1925) who 

 described the reduced spectrum in a wide variety of plant species. Keilin 

 observed that untreated plant tissues showed a two-banded spectrum, a 

 wide diffuse band at 556 m// and a weaker band at 524 m/x and suggested that 

 this spectrum was due to "modified cytochrome" or cytochrome b'. The 

 nature and significance of cytochrome b' remains clouded in obscurity. 

 Following dithionite reduction of the tissue Keihn found that the two-banded 

 spectrum was replaced by a spectrum showing components a, b and c. 

 Curiously, the region of the spectrum between components b and c appeared 

 shaded and frequently was observed as one wide absorption band. 



For fourteen years following Keilin's initial observations the study of 

 cytochromes in plant tissues remained a depressed area. During this time 

 interval, Yakushiji (1935) published on the cytochromes of germinated soy 

 beans in which he described absorption maxima at 640, 603, 560 and 550 mju. 

 The 640 m/< component was erroneously described as cytochrome «2- Mann 

 (1938; see also Hill and Hartree, 1953), made an important contribution. 

 He showed that, in general, the greatest amount of haemoprotein, estimated 

 as pyridine haemochrome, was found in the most actively respiring part 

 of the plant tissue, particularly in the non-vacuolated meristematic region. 



