Ascorbic Acid As a Growth Hormone 731 



terns. The observation that — besides growth — plasma viscosity, res- 

 piration, and glutathione are also influenced by auxin, although in 

 an opposite direction than by AA, indicated a close connection be- 

 tween the mechanisms of action of these substances. This hypothesis 

 found further support in experiments showing that lAA markedly 

 influences in vivo the oxidation-reduction state of AA (4, 9, 25). When 

 added to the medium at concentrations up to the optimal for growth 

 stimulation (10 p. p.m. for the pea internode segments), lAA increased 

 the AA content and decreased the DHA content in the tissues. At 

 higher, relatively growth inhibiting concentrations, lAA induced an 

 opposite effect, that is, a decrease of the AA:DHA ratio (Table 4). It 

 was also found that in etiolated pea seedlings the removal of the 

 apex, a center of production for native auxin, causes a rapid in- 

 crease of DHA and a decrease of AA. In all of these cases, the de- 

 crease of DHA paralleled growth stimulation, and its increase paral- 

 leled growth inhibition (13). 



The possibility that auxin could affect growth and metabolism at 

 least in part through a direct effect on enzymes controlling the 

 oxidation-reduction state of the AA system prompted a series of in 

 vitro investigations on this aspect of the problem. A relatively modest 

 but significant inhibition of AA-oxidase activity of cell-free extracts 

 or of partially purified preparation from pea internodes was observed 

 in these experiments (5), which is in agreement with the previous re- 

 stdts of Wagenknecht et al. (26) on different materials. Some inhibit- 

 ing action of auxin on AA oxidase from lettuce seeds has also been 

 recently reported by Mayer (15). However, in other experiments this 

 in vitro effect of auxins on AA oxidation appeared poorly reproduc- 

 ible. It appears that further investigations on the mechanism of oxi- 

 dation of AA in the intact tissues are needed before a fruitful studv 

 of a possible direct effect of atixins on AA oxidase can be demonstrated. 



On the other hand, some investigations on the enzyme systems in- 

 volved in the reductive metabolism of the AA system led to interesting 



Table 4. The internal concentration of as- 

 corbic acid (AA) and of dehydroascorbic acid 

 (DHA) in segments of pea internodes treated 

 with ascorbic acid. 



