732 S. Tonzig and E. Marre 



and reproducible results. Monodehydroascorbic acid reductase, an 

 enzyme catalyzing the transfer of electrons from the reduced pyridine 

 nucleotide coenzymes to a partially oxidized form of AA, was mark- 

 edly inhibited by concentrations of indole-3-acetic and 2,4-dichloro- 

 phenoxyacetic acids higher than 10'^ M. This provides a good basis 

 for the interpretation of the effect of superoptimal concentrations of 

 auxin on the level of DHA in the intact cells. It also gives a first 

 indication that a pyridine coenzymes-monodehydroascorbic acid 

 reductase-AA system could play a significant role in electron transfer 

 from respiratory substrates to oxygen (8). 



On the whole, these attempts to demonstrate in vitro an action 

 of auxin on AA metabolism, though not yet leading to definitive con- 

 clusions, seem to support the hypothesis that a direct control by 

 auxin of the oxidation-reduction state of the AA system could be of 

 importance in mediating the final physiological effects of the hormone. 



SUMMARY 



AA is considered to be a hormone involved in growth regulation. 

 Almost all of the available evidence indicates that a rise in the AA 

 supply of a tissue (or of its endogenous synthesis) induces a depres- 

 sion of the growth rate. This does not exclude that low amounts of 

 AA could be promotive, or perhaps necessary, for giowth. 



The increase of AA in a tissue seems to inhibit growth essentially 

 through the concomitant larger increase of DHA, which appears to be 

 the real inhibiting substance. The inhibiting effect of DHA on respira- 

 tory enzyme systems in vitro provides a basis for the interpretation of 

 the in vivo effects on respiration and glutathione oxidation-reduction 

 state, as well as those on plasma viscosity and growth. 



On the other hand, the mechanism of action of AA appears strictly 

 connected with that of auxins. In fact, treatment in vivo with AA in- 

 fluences the distribution of auxin in the tissues; and, correspondingly, 

 treatment with auxin affects the oxidation-reduction state of the AA 

 system. 



LITERATURE CITED 



1. Cliinoy, J. J., Giovci, R., and Sirohi, G. S. A study of tlie interaction of as- 

 corbic acid and indole-3-acctic acid in the growth of Avena coleoptile sections. 

 Physiol. Plant. 10: 92-99. 1957. 



2. Forti, G. Studi sulla fisiologia dcH'acido ascorbico. XXIII. Azione inibente 

 dcH'acido deidroascorbico sulla fosforilazione ossidati\a di mitocondri vcgctali. 

 Atti. Accad. Naz. Lincci Rend. 21: 70-70. 1958. 



3. Lona, F., and Porzio Giovanola, E. Ricerche sulla fisiologia dell'acido ascorbico. 

 VII: Contenuto in acido ascorbico dcllc piante in relazionc al fatiore termoper- 

 iodico. Nuovo Gior. Rot. Ital. 58: I(i2-171. 1951. 



