134 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



DIVISION OF RADIATION AND ORGANISMS 

 (Report prepared by R. B. Withkow) 



In last year's report the developmental responses of seedlings to 

 light were discussed as to the possible biochemical reactions involved 

 in the absorption of the light energy and the subsequent regulatory 

 mechanism responsible for the development of seedlings into normal 

 plants. The indications were that these growth or photomaturation 

 responses were not directly associated with either chlorophyll syn- 

 thesis or photosynthesis, but no strong photomaturation response had 

 been obtained without detectable traces of chlorophyll. The problem 

 remained, therefore, to secure leaf expansion and stem development 

 approaching the normal occurring in sunlight without chlorophyll 

 synthesis or photosynthesis. 



During the past year bean seedlings were grown under a series of 

 seven irradiances respectively of blue (436 m/x) , red (630-700 m/x) , and 

 far red (710-1100 m/x and 725-1100 m/x) radiant energy. The extent 

 of the photomaturation response was measured by taking weights of 

 the leaves, and weights and lengths of the hypocotyl and epicotyl por- 

 tions of the plants. Analysis for protochlorophyll, chlorophyll, 

 carotenoid, and anthocyanin pigments were made in order to obtain 

 a picture of the photosynthesis and photodecomposition of these pig- 

 ments. 



The maximum photomaturation response occurred in the red where 

 the maximum chlorophyll synthesis and anthocyanin synthesis also 

 occurred. With blue of the same energy as the red, the leaf expansion 

 and decrease in hypocotyl length were much reduced. Although the 

 amount of chlorophyll was less in the blue than the red, it was not 

 decreased in proportion to the decrease in photomaturation response. 

 In the far red, very marked photomaturation occurred at the lower 

 irradiances employed without any measurable synthesis of chloro- 

 phyll, which first appeared with this region at 10 2 microwatts per 

 square centimeter. Since all measurements were made with 10-cm. 

 cells in a Beckman spectrophotometer, it is safe to assume that prac- 

 tically no chlorophyll was synthesized under these low-irradiance far- 

 red conditions. Therefore, the evidence is conclusive that the photo- 

 maturation responses are not directly concerned either with chloro- 

 phyll synthesis or photosynthesis, but are due to a regulatory mech- 

 anism associated with a pigment having absorption in the far red. 

 Absorption of this pigment also occurs in the blue, with the maximum 

 in the red. 



In addition, there is clear evidence that the synthesis of anthocyanin 

 is also a photochemical response, separate and distinct from photo- 

 synthesis. Anthocyanin was always present in plants grown in com- 

 plete darkness, but very marked increases occurred with the addition 



