108 THE FLOWERING PROCESS 



difficulty in obtaining it could prove to be a clue to its action in 

 flowering. 



6. The Nature of Phytochrome 



The protein nature of the pigment is especially interesting. Physio- 

 logists have known for many years that some very important pigments 

 are attached to protein. Chlorophyll is apparently attached to a 

 protein when it absorbs the light used in photosynthesis. When 

 chlorophyll is extracted, however, the protein is left behind, and only 

 the relatively small colored molecule goes into solution. The red 

 colored heme is also easily separated from the protein globin of the 

 hemoglobin in blood. The phycobilin pigments of the red and blue- 

 green algae also absorb light used in photosynthesis. These pigments 

 are so tightly attached to protein that removal of them results in a 

 drastic change in their properties, including the color. In the case 

 of chlorophyll and hemoglobin, molecular structures of the relatively 

 small, colored molecules have been determined. This has never been 

 successfully accomphshed with the phycobilin pigments, and it 

 appears that it will not be an easy task with phytochrome. Studies 

 so far have failed to separate the colored portion of the phytochrome 

 molecule from the protein; indeed we might anticipate that it is 

 attached even more strongly than the chromatophores of the 

 phycobilin molecules which it resembles closely in absorption spectrum 

 (e.g. allo-phycocyanin). At any rate the fact that phytochrome is a 

 protein is encouraging, because this could well imply that it has 

 enzymatic activity. So the next problem, even if we are unable to 

 further determine the structure of phytochrome, might be to deter- 

 mine its enzymatic role in photoperiodism. 



The Beltsville workers have now had a chance to study a number 

 of phytochrome's properties by studying the isolated phytochrome. 

 Work with whole plants had shown that the light driven conversions 

 were independent of temperature, for example, and it has been shown 

 that photo-conversion in the test tube will take place at temperatures 

 as low as -78°C (but not below). The pigment is rather easily 

 denatured by temperatures of 50°C, repeated freezing, or simply 

 allowing it to stand at room temperature for several hours. Color 

 changes occur upon denaturation, but the dark shift so essential to 

 phytochrome's control of physiological processes does not occur in 

 the test tube. 



