VI CELL ENLARGEMENT 787 



An inhibitor of a different type again is canavanine, which is an antagonist of 

 arginine. This compound inhibits growth of coleoptiles at i mg/1 and above, 

 which provides evidence that arginine is needed for growth (Bonner, 1949). 



The evidence from the inhibitor studies can be summed up by the statement 

 that cell enlargement is dependent on one or more sulfhydryl enzymes, probably 

 more than one phosphate transfer, possibly some action of arginine, organic acid 

 oxidation by way of the Krebs cycle and cytochrome oxidase. 



(g) Relation between cell enlargement and metabolism 



{i) Respiration. From the evidence just given, it is not surprising that a number of 

 attempts have been made to elucidate the metabolic processes which accompany, 

 or perhaps cause, cell enlargement. The relationships, however, are far from 

 simple. 



The application of auxin, in growth promoting concentration, to isolated shoot 

 tissue nearly always increases oxygen consumption. Curiously enough, it is in the 

 most-used tissue, Arena coleoptile sections, that the data are conflicting. The first 

 researchers in this field agreed that lAA by itself did not stimulate oxygen uptake, 

 and indeed Commoner and Thimann in 1941 showed that only if the sections 

 were pretreated with sucrose or potassium malate would there be an increase in 

 respiration when auxin was added. Later, however, Bonner (1949; cf. Ordin et al., 

 1956) found, contrary to his own previous report, that lAA increased the respira- 

 tion about 20%, and Anker (1951) obtained the same result in several different 

 media with and without sucrose. At that time, however, the writer (in unpublished 

 experiments) had no difficulty in duplicating his previous finding of no effect due 

 to lAA added alone. Looking back on all these data it now appears that the posi- 

 tive results were all obtained in the presence of potassium salts or potassium-con- 

 taining buffers; the negative results had no potassium. Since K"^ ions do have a 

 large effect on growth, this difference is significant and it may mean that K"^ is 

 required for auxin to exert its full effect on respiration. 



In other shoot tissues there can be no doubt that added auxin increases oxygen 

 consumption. In pea stem sections the increase is about 20%; in potato slices it is 

 about 40% bvit is delayed at least a day, while cell enlargement in this tissue shows 

 a similar lag before the sections in auxin show an increase over the controls 

 (Hackett and Thimann, 1953). NAA has to be used in order to obtain both effects 

 in potato. In artichoke sections the increase is the most spectacular, averaging 

 some 400% and taking place within a few h. (Hackett and Thimann, 1952b). 



As might be expected, inhibitors of cell enlargement usually cause a decrease 

 in respiration. This has been observed with iodoacetate, arsenite, dinitrophenol, 

 organic mercury salts, cyanide and carbon monoxide. Fluoride, however, in 

 growth-inhibiting concentrations, did not decrease the respiration of pea stem 

 sections but even slightly increased them (Christiansen and Thimann, 1950b), 

 while arsenate had no effect on respiration of coleoptile sections (Bonner, 1950; 

 Audus, 1952). Table 4 summarizes the data of a number of experiments in which 

 the effects of the same inhibitor on respiration and on growth have been measured, 

 under parallel conditions. It will be seen that the influence of the inhibitor on 

 respiration is always much smaller than that on growth. An exception is provided 



lAteralure p. 8t6 



