14 AISTNTJAL REPORT SMITHSONIAN INSTITUTION, 19 63 



lems in radiation biology, with specific emphasis on elaborating the 

 intracellular mechanisms involved in regulatory responses of biolog- 

 ical systems controlled by ionizing or nonionizing radiation. 



Emphasis has been centered on the precise determination of the 

 initial processes involved in a number of diverse light-regulated re- 

 sponses. In this division were determined the most precise and de- 

 tailed action spectra that have been reported for photomorphogenic 

 responses, such as bean hypocotyl hook opening, photoreversal of this 

 response, seed germination, interaction of visible light with X-ray- 

 induced chromosome aberrations, and the phototropic response of 

 oat seedlings in the blue and near-ultraviolet spectral regions. From 

 such action spectra, a great deal of significant information has been 

 obtained about the primary photoreceptors responsible for the absorp- 

 tion and transfer of radiant energy in biological systems. 



Kinetic studies have been carried out determining the time course 

 of sensitivity, temperature-dependence of secondary dark reactions, 

 the interaction of photomimetric substances, auxins and antiauxins, 

 with the light-sensitive mechanisms. Descriptions have been educed 

 for some of the physical factors in plant reactions, including optical 

 and mechanical properties of cells. The morphological development 

 of chloroplasts after irradiation has been examined and measured, 

 using cytochemical techniques. 



Investigations have been focused on the intracellular biochemical 

 mechanisms regulated or altered by radiation. These efforts have 

 resulted in a number of published articles on clilorophyll synthesis, the 

 effects of ionizing radiation on chlorophyll synthesis, and the activity 

 and concentrations of various subcellular components isolated after 

 irradiation, such as high energy phosphate compounds, mitochondrial 

 activity, protein synthesis in the photosynthetic apparatus, pigment 

 synthesis, carbohydrate metabolism, and various other enzymatic 

 activities. 



During the past several years, the division staff and facilities have 

 expanded in order to approach radiobiological problems with a wider 

 range of disciplines employing the most advanced tecliniques of bio- 

 chemistry, biophysics, cytology, and plant physiology. A tempera- 

 ture-regulated greenhouse with controlled environment rooms has 

 been constructed with fmids provided by a nonpublic foundation, the 

 Eesearch Corporation. The growth of plants under natural and 

 artificial light conditions has been measured with great accuracy. 

 Concurrently, the construction and acquisition of specialized auto- 

 matic equipment for measuring the spectral distribution of total sky 

 light at frequent intervals have been completed, and long-term correla- 

 tions of daily and seasonal fluctuations with observed plant responses 

 are being made. 



