Does GA Act Through Auxin-mediated Mechanism? 597 



mains virtually unchanged by these treatments (Figures 3 and 4), a 

 result which seems to render any direct relationship between GA and 

 auxin action unlikely, although it cannot disprove it. 



Alternative Views 



If present evidence leads to a rejection of the concept that gib- 

 berellin action is auxin-mediated, it may still be useful to consider 

 ways in which auxin and gibberellin might interact in growth phe- 

 nomena. It has been proposed (2) that GA and auxin interact through 

 some third factor. If this factor is conceived of as an auxin-destruction 

 system, or a direct inhibitor of auxin action, such an hypothesis ap- 

 pears inilikely in view of the above evidence. If, however, it is re- 

 garded as some unspecified complex of unknown reactions, or, to ex- 

 pand it further, the plant tissue itself, the hypothesis is of course 

 perfectly reasonable, if unspecific. 



It was suggested in the experimental section that the increment 

 of elongation induced by GA in a section is relatively independent of 

 the presence of lAA or PCIB, although it is affected by the presence or 

 absence of sucrose. Similarly, the increment (positive or negative) in 

 elongation caused by lAA is about the same in the presence or ab- 

 sence of GA. Such results, in which the absolute effects of various 

 treatments appear to be independent of each other, are frequently 

 encountered in pea section growth tests, at least in this laboratory, 

 and merit some consideration here. A digression into light physiology 

 is necessary to provide further background. 



In 1941 Schneider (11) reported that the absolute magnitude of 

 the inhibition of elongation caused by red light in dark-grown Avena 

 first internode sections was more or less constant, even when the total 

 elongation of the sections was varied over a wide range by changing 

 the auxin level. Hillman (5) extended these observations in studying 

 the red light inhibition of the elongation of pea sections from dark- 

 grown plants and the far-red promotion of sections taken from plants 

 grown in red light. The absolute magnitudes of the red light inhibi- 

 tion or far-red promotion were unaffected by any but high levels of 

 lAA, and were also independent of GA, although both growth sub- 

 stances affected total elongation. The conclusion was reached that a 

 portion of "endogenous" growth was light-sensitive while GA- and 

 lAA-induced growth was not. These results were obtained with long, 

 subapical pea sections. Since then, Bertsch (1) has shown that in SI 

 sections, in which elongation is greatly promoted by sucrose, only 

 that increment of elongation attributable to sucrose is labile to red 

 light; the red light inhibition is the same in absolute units as the 



