Vernalization and Photoperiodism — 76— A Symposium 



This could be due to an excess of the active substance even in continuous 

 light or failure to reach the threshold concentration after the maximum per- 

 missible dark period. At the present time examples of such cases cannot 

 be sought with any confidence because photoperiodic conditions are by no 

 means the only ones to be satisfied in floral initiation. Experiments with 

 sugar beets {22) and biennial Hyoscyamus (11), however, suggest either 

 that adequate concentrations are not attained in such plants in the first 

 year or that some other controlling factor has not then been removed. 



Information about the leaf pigment responsible for photoperiodic control 

 of flowering is given by the action curve, irrespective of any hypothesis 

 about active substances and mechanisms of control. For a number of 

 reasons, however, the action spectrum cannot simply reflect the absorp- 

 tion curve of a leaf pigment. Chlorophyll and carotenoids, the principal 

 leaf pigments are present in sufificient amounts greatly to reduce the in- 

 tensity of most wave lengths of visible radiation deep in the leaf (21). 

 An action curve thus will reflect the screening action of such pigments even 

 though they do not take part in the effective photochemical reactions. 



At first sight it would appear that photosynthesis, despite constant 

 quantum efificiency across the spectrum, would be greatly reduced in a 

 leaf in regions where chlorophyll has a low absorption coefficient. Actually 

 most leaves contain sufficient chloroplasts to make about equal photosyn- 

 thetic use of all visible radiation. Consequently photosynthetic use of 

 strongly absorbed radiation, as in the blue, chiefly takes place near the 

 surface of chloroplast-containing tissue. Weakly absorbed radiation, as 

 in the green, also is most effective at the surface but increases in impor- 

 tance, relative to more strongly absorbed radiation, and at sufficient depth 

 it may become the most effective. This condition is illustrated by Seybold 

 (20). As a natural phenomenon the absorption process and leaf structure 

 give efficient use of the incident light, since in photosynthesis it is probably 

 unimportant, over small distance, exactly where the reaction takes place. 



If chlorophyll is involved either directly or as a screening pigment in 

 some other photochemical process in a leaf, in which control of a process 

 has to be effected throughout the absorbing tissue then the self screening 

 becomes most important. In fact the paradoxical situation can be met 

 where the most strongly absorbed radiation is least effective. 



Four features of the photoperiodic action curve indicate that chlorophyll 

 may be playing an important part in the process. These are : 



a) All regions of the visible spectrum are effective. 



6) The limit of effectiveness in the red is the same within 

 limits of experimental measurement as the absorption limit of 

 chlorophyll in leaves. 



c) The region of maximum response is near the position of 

 red absorption maximum for chlorophyll. 



d) The region of greatest change in response, 4900 to S400 

 A., is the region in which the absorption coefficients of chlorophyll 

 change most rapidly. 



Low photoperiodic effectiveness for radiation in the blue is not in ap- 

 parent agreement with chlorophyll being the effective pigment, even when 

 screening by high absorption in the chloroplasts is considered. According 

 to present concepts the red fluorescence of chlorophyll irradiated in the blue 



