SECRETARY'S REPORT 71 



spectrum of this response in oat plants to have two maxima in the blue. 

 However, because of poor resolution of the reaction in this region, 

 subsequent disagreement has arisen and it has not yet been clearly 

 established what pigment absorbs the incident energy and initiates 

 the response. There are several blue-absorbing pigments, including 

 the carotenoids and flavins, with absorption characteristics that might 

 qualify them as candidates for the role of the photoactivated pigment. 

 For example, beta-carotene has absorption maxima in the blue at 

 435 and 470 mp and riboflavin at 445 and 475 m/*. Both pigments are 

 commonly found in plants. 



However, the flavins and the carotenoids have very clear-cut spectral 

 differences in the near ultraviolet. Kiboflavin has a strong absorp- 

 tion maximum at 370 rn.fi, but the carotenoids do not absorb in this 

 region. Therefore, the effectiveness of various ultraviolet wave- 

 lengths in promoting phototropic curvature might be used as an 

 indicator as to which of these two pigment types is involved. When 

 the pigment system responsible is identified, it will be possible to 

 resolve the initial steps of the chemical reactions leading to curvature. 



In order to establish the effectiveness spectrum, a large-grating 

 monochromator, for irradiation in the near ultraviolet and visible, 

 has been built in the Observatory shops. Calibration of the equip- 

 ment has been completed and Walter Shropshire has standardized 

 bioassay technics, using curvatures of the oat and barley coleoptiles 

 to measure the effectiveness spectrum. Although positive phototropic 

 curvatures have been obtained in the near ultraviolet in preliminary 

 studies, a complete monochromatic analysis of the action spectrum in 

 the entire visible and near ultraviolet appears necessary before the 

 photoactivated pigment system can be clearly identified. 



Photochemical synthesis of plant pigments. — Dr. J. Wolff and L. 

 Price have found that the complete chlorophyll molecule is not formed 

 immediately on irradiation of leaves of plants grown in the dark, as 

 heretofore postulated. Instead, protochlorophyll, the green pigment 

 present in very low concentrations in leaves of dark-grown seedlings, 

 is rapidly converted by light to chlorophyllide a. This pigment is 

 subsequently linked to the long chain alcohol, phytol, by the action of 

 the enzyme chlorophyllase in a strictly nonphotochemical thermal re- 

 action. Chlorophyllide a and chlorophyll a have identical absorption 

 spectra in the visible, but differ in chemical properties. Protochloro- 

 phyll has a different absorption spectrum from chlorophyllide a, yet 

 the two pigments have similar chemical properties. These facts indi- 

 cate that what has been commonly termed "protochlorophyll" is actu- 

 ally protochlorophyllide. 



Modification of X-ray damage by visible radiant energy. — The 

 damaging effects of X-rays and other forms of ionizing radiation to liv- 



412575—57 6 



