552 J. A. Lockhart 



Participation of the endogenous gibberellin hormone in control 

 of stem growth by radiation cannot be ruled out in any of the cases 

 listed above (see discussion in reference 18). It may be suggested here 

 that the gibberellin hormone in various plants differs chemically (31). 



Interaction Between Light and Auxin 



Meijer (24) showed that when young tomato plants are exposed 

 to far-red radiation a strong epinastic response occurs, just as when 

 the plants are treated with auxin. Furthermore, the radiation-induced 

 epinasty can be prevented by anti-auxin treatment. It must be kept 

 in mind that responses such as this may well be due to a balance be- 

 tween factors. Thus, de Zeeuw and Leopold (44) showed that the epi- 

 nastic response of tomato to auxin can be prevented by ultraviolet 

 irradiation. Since auxin (1-naphthaleneacetic acid) was added after 

 irradiation, radiation must reduce the response of the plant to auxin. 

 Perhaps far-red radiation increases the responsiveness of petioles to 

 endogenous auxin. Then epinasty would occur as a result of irradia- 

 tion and could be prevented by added anti-auxin. 



One of the few reports of an actual promotion of stem growth in 

 intact plants as a result of added auxin is due to Meijer (24). He 

 found a 60 per cent promotion of growth of irradiated gherkin {Cu- 

 cumis) seedlings as a result of tryptophol treatment as well as a 

 somewhat lesser response to lAA (indole-3-acetic acid). However, the 

 growth elicited by tryptophol was only a small fraction (< 40 per cent) 

 of the growth of these seedlings in darkness. In any case, these results 

 are for hypocotyl length only. Thus, it is impossible to distinguish 

 between true effects on stem elongation and hypocotyl inhibition due 

 to an increase in epicotyl growth. Lockhart (18) found no effect of 

 added lAA on another variety of gherkin, either with or without si- 

 multaneous gibberellin treatment. 



van Overbeek (36) studied the relation of auxin to inhibition of 

 stem elongation by red radiation in Rnphaniis. Added auxin would 

 partially restore growth of irradiated stems, but a large proportion of 

 the inhibition had to be attributed to "the responsiveness of the cells 

 to auxin," presumably gibberellin (?). 



Galston and co-workers have observed many correlations between 

 irradiation and auxin metabolism, but have found no evidence for a 

 causal relationship between auxin and the effects of radiation on 

 stem growth in intact plants (9, 10, 13). 



Other Hormonal Factors 



Brian and Hemming (4) have suggested that gibberellin acts to 

 promote stem growth by counteracting an inhibitor from some other 

 part of ilie plant. They reach this conclusion by demonstrating that 



