78 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



Reports from other laboratories have indicated that chemicals such 

 as gibberellin and kinetin will, to some extent, replace red irradiation 

 in promoting some photomorphogenetic processes such as seed germi- 

 nation and flowering. These materials have been tested on seedling 

 growth during the past year by Dr. W. H. Klein and Victor Elstad. 

 It was found that gibberellin and kinetin could not substitute for red 

 irradiation in this case. Both inhibited the growth induced by a red 

 exposure and produced results similar to those of the growth-regu- 

 lating hormone, indoleacetic acid. Gibberellin and red radiant 

 energy initiate separate and distinct growth responses and, when 

 added together, produce a resultant of the two reactions. 



The induction of growth by red radiant energy (660 nut) can be 

 blocked by far-red energy (710-730 m/x). Dr. Klein, Dr. R. B. 

 Withrow, and Victor Elstad have found that the efficiency of the far- 

 red is markedly increased by interposing a dark period of 60 to 90 

 minutes between the red and far-red treatments. This suggests that 

 a thermochemical step intervenes between the absorption of red radi- 

 ant energy and maximum capacity for inactivation by far-red. Re- 

 ducing the temperature from 25° C. to 2° C. during the light treat- 

 ments has no measurable effect on the induction process, but the 

 lowered temperature reduces the maximum photoinactivation by 50 

 percent when compared to photoinactivation at 25° C. This sub- 

 stantiates the thesis of an intervening thermal step. 



Phototropism. — There is a wide range of growth reactions acti- 

 vated by blue radiant energy, including cytoplasmic streaming, 

 changes in cell-membrane permeabilities, the regulation of respira- 

 tory enzymes, changes in bioelectric potentials and phototropism or 

 bending of plants toward a light source. The late Dr. E. S. John- 

 ston of this laboratory became interested in phototropism in 1934 and 

 made the first precise quantitative measurements of the spectral char- 

 acteristics of the phototropic response in oats. From these data it 

 was postulated that the pigment system activating the response ab- 

 sorbed chiefly in the blue and was likely to be a carotenoid or a flavin. 



At the present time Walter Shropshire is conducting experiments 

 to resolve the question of the identity of the photoreceptor by deter- 

 mining if the response occurs in the near-ultraviolet where the ab- 

 sorption of flavin is markedly greater than that of carotenoids. The 

 results of ^.-vena-tip-curvature experiments indicate that both pig- 

 ment systems may be involved. The action spectrum in the visible 

 has the peaks characteristic of carotenoid absorption, while in the 

 near-ultraviolet the response is characteristic of a flavin system. 



Experiments are in progress to ascertain the function of each of 

 these pigment systems, using straight growth measurements of intact 

 Avena seedlings and the curvature of carotenoid-deficient Phyco- 

 myces. An attempt is being made to correlate all the blue photoreg- 



