188 CHEMICAL AGENTS AND GROWTH 



DISCUSSION 



Before attempting to interpret the results, one must consider pre- 

 vious views on red light-growth substance relationships. The work of 

 Schneider (1941) was mentioned in the introduction, and will be 

 referred to again below. Galston and Hand (1949) were able to 

 obtain direct and inductive effects of low (white) light doses on excised 

 pea internode sections. Although their results led to the conclusion that 

 "a non-auxin system is responsible for the light growth inhibition," 

 several subsequent papers by Galston and co-workers (cf. Galston and 

 Baker, 1949, 1951 ) concentrated on the photoinactivation of lAA by 

 blue light and riboflavin, and did not pursue the question of red light 

 action. 



Kent and Gortner (1951) and Galston and Baker (1953) showed 

 that pretreatment of intact etiolated pea plants with red light affected 

 the subsequent auxin sensitivity of the tissues. The results of Galston 

 and Baker have been confirmed here, but Thomson (1954) has 

 pointed out the complexities involved in studying light action by treat- 

 ment of intact, growing plants. The light-auxin interactions so 

 demonstrated are perhaps more likely to be consequences of photo- 

 morphogenesis than they are to be the primary mechanism; hence the 

 desirability of studying excised sections rather than intact plants. 



Liverman and Bonner (1953) showed that red light promotes 

 growth in Avena coleoptile sections whether given before or during 

 auxin treatment, while FR reverses the action of red only when given 

 in the presence of auxin but has no effect in its absence. Kinetic treat- 

 ment of the data led the authors to propose that red light increases 

 the production of an auxin-receptive entity, and FR decomposes the 

 active auxin-receptor complex, giving a nonreceptive entity. While 

 the concept is attractive as an explanation for the Avena results, it 

 seems a priori to be inapplicable to stem elongation, which was not 

 considered in the paper in question, and which is promoted by both 

 auxin and FR. The FR promotion of elongation reported for intact 

 plants by Downs et al. (1957) and reported here for sections from 

 red-grown plants can hardly be interpreted in terms of a decomposi- 



