786 PLANT GROWTH lO 



Fluoride was first found to inhibit the enzyme enolase, which converts 2-phos- 

 phoglyceric to phosphopyruvic acid, but has since been found to inhibit phos- 

 phatases and also phosphoglucomutase. Perhaps for these reasons the poisoning 

 of growth by fluoride appears to be complex. In coleoptile sections a careful study 

 of the relations between fluoride concentration and inhibition yielded no evidence 

 that enolase is involved but indicated that several enzymatic steps participating 

 in cell enlargement are probably sensitive to fluoride (Bonner and Thimann, 

 1950). Curiously enough, it has little effect on root growth. 



A third group would include substances directly inhibiting specific steps of 

 oxidation. So far fluoracetate is the only effective one of this type to be studied; 

 it interferes with the Krel^s cycle, being converted therein to fluorocitrate. Fluor- 

 acetate inhibits cell enlargement in coleoptiles, pea stems, potato disks etc. ; in 

 young coleoptiles low concentrations give a very marked growth promotion which 

 has not yet been explained (Bonner and Thimann, 1950). A feature of the fluor- 

 acetate inhibition is its almost complete prevention by a slight excess of acetate or 

 pyruvate. This provides good evidence of the role of Krebs cycle oxidations in the 

 metabolism of cell enlargement. 



All of these inhibitors show a similar relationship between their concentration 

 and the resulting growth. At low levels there may or may not be a growth promo- 

 tion; then in the effective range the growth is about linearly proportional to the 

 logarithm of the inhibitor concentration (see Fig. 9C) ; finally at very high con- 

 centrations the inhibition flattens out, or else secondary effects supervene. 



Of a different type are those inhibitors which interfere with the terminal oxi- 

 dase. The first of these to be used was cyanide which Bonner in 1933 found to 

 inhibit both growth and respiration of Avena coleoptile sections. Cyanide com- 

 bines with, and thus inactivates, virtually all enzymes containing iron, copper or 

 manganese, such as catalase (Fe), peroxidase (Fe), cytochrome oxidase (Fe), poly- 

 phenol oxidase (Cu) and ascorbic acid oxidase (Cu). More specificity is provided 

 by carbon monoxide, for although CO combines with several of these, only in the 

 case of cytochrome oxidase is the CO-compound dissociated by light, especially 

 in the violet end of the spectrum at 430 m[j,. It is of great importance, therefore, 

 that the elongation of pea stem and Avena coleoptile sections is inhibited strongly 

 by CO in the dark, while in light the elongation is nearly restored to normal 

 (Hackett and Scheiderman 1952). The enlargement of potato tuber slices behaves 

 similarly but in this case light reduces the growth by nearly 50% so that the restora- 

 tion of growth in CO cannot be complete; it is, however, brought up to the level 

 of the controls without CO in light (Hackett et al., 1953). Furthermore, a charac- 

 teristic of cytochrome oxidase is that the inhibition by CO is dependent on the 

 ratio between CO and O, ; it can therefore be prevented by increased oxygen ten- 

 sions. Such a competitive effect has been shown for all the three tissues mentioned. 

 It can safely be concluded, therefore, that the terminal oxidase which fimctions 

 in cell enlargement, i.e. the system through which electrons must pass in order to 

 yield energy for cell enlargement, is cytochrome oxidase. Needless to say, other 

 oxidases may be present, particularly in potato, which is a classical source of 

 polyphenol oxidase, but evidently their function is not connected with cell enlarge- 

 ment. 



