474 RADIATION BIOLOGY 



energy supplied; it is likely that this base light-growth response causes 

 the phototropic oscillations described by Burkhardt (1926). 



Whereas, at most, only a small amount of auxin destruction can be 

 caused by the light that is absorbed, there are three other auxin effects 

 that are produced by light. One is the effect of the unilateral light on 

 the lateral transport of auxin. As just discussed, this may come about 

 by a potential difference induced by the light, causing electrophoretic 

 redistribution of auxin. If we assume that this potential is concentrated 

 in the cell interfaces, its magnitude may be sufficient to account for the 

 auxin movement. If this auxin, which is most likely auxin-a, is responsi- 

 ble for most of the actual cell elongation, its redistribution will demon- 

 strate itself in a differential growth rate of the two sides of the coleoptile. 

 The lag period between unilateral illumination and onset of curvature is 

 slightly longer than the rate of auxin movement [about 15 mm/hr (Went 

 and White, 1939)] would lead one to expect. This difference is probably 

 due to the fact that the lateral auxin transport only gradually reaches a 

 maximum value. 



A third effect of light which has received theoretical consideration with- 

 out much experimental support is that on auxin synthesis. Oppenoorth 

 (1941) and Galston (1950) hypothesize that small amounts of light 

 decrease auxin production in the Avena coleoptile tip, giving an increase 

 at light energies causing negative curvatures and a decrease again at 

 higher light-energy levels. The evidence for this is very meager and 

 need not be considered at present. 



A fourth effect of light has, surprisingly enough, hardly been con- 

 sidered: the reduced response of cells to auxin during and immediately 

 after light exposure. Through extraction experiments we know that little 

 or no auxin is destroyed by light; yet there is a considerable decrease in 

 growth rate during and after illumination, as shown in the light-growth 

 response, e.g., in Helianthus hypocotyls (Blaauw, 1915), Raphamis hypo- 

 eotyls (van Overbeek, 1933), or in stems in general during the day. 

 Therefore it must be concluded that cells respond less to auxin during 

 and immediately after light exposure than they do in darkness. This 

 may be due to blocking of the auxin or to inactivation of some other 

 factors that, in conjunction with auxin, are needed to produce growth. 



From all the facts presented it is clear that a unitarian theory of pho- 

 totropism cannot account for all of them and therefore serves no practical 

 end. As Table 9-2 shows, almost all the facts are consistent with a dual 

 theory. It is very tempting to assume that the two completely different 

 phototropic mechanisms are based on the effects of light on two different 

 auxins. This would also tie in with the two pigment systems, each of 

 which would be connected with a different auxin. 



Of the extensive literature on phototropism in other plants, only a few 



