LIGHT AND THE PIGMENT 111 



fat, and protein metabolism as well as in some other reactions. There 

 are other biochemical reasons for suspecting this reaction, most of 

 which come from some of the best work which has so far been done 

 with the phytochrome system. The synthesis of anthocyanin pigments 

 in apple skins, turnip skins, red cabbage leaves, and other plant 

 materials has been studied intensively by the Beltsville workers. Such 

 a process offers an excellent opportunity for the study of biochemical 

 mechanisms, since we already know something about the biochemical 

 steps involved. 



Other Photomorphogenic Pigment Systems 



It is quite easy to become so impressed with studies on phytochrome 

 that one overlooks the possibility that other important pigment 

 systems might exist, which could also control growth. Of course it 

 is well known that phototropic stem bending towards the light occurs 

 in response to a pigment (probably carotene and/or riboflavin) which 

 absorbs blue light. Some other known blue light effects could involve 

 other pigment systems, or perhaps they represent effects due to weak 

 absorption by phytochrome in the blue region of the spectrum. 



Recently, H. Mohr (65), in Freiburg, Germany, has emphasized 

 the importance of a photoreaction which could be unrelated to 

 phytochrome. A number of plant responses are controlled, most of 

 which are also controlled by phytochrome. The pigment system 

 might be different, however, since relatively high intensity illumina- 

 tion is required for relatively long periods of time, and no signs of 

 reversibility have been detected. Furthermore, the action spectra 

 show fairly sharp peaks in the blue (440 m/x) and in the far-red 

 (725 miLt). 



Mohr feels that the response under these conditions occurs through 

 the action of a pigment system which circumvents phytochrome 

 control. Phytochrome might also initiate the process in the absence 

 of Mohr's pigment system, and the two could also act synergistically 

 with each other. The Beltsville workers, on the other hand, feel that 

 the responses observed by Mohr are exclusively phytochrome con- 

 trolled, but that they come about while phytochrome conversion is 

 taking place in both directions under the steady-state conditions 

 which might be expected in high intensity light {both forms of 



