LIGHT AND THE PIGMENT 113 



1. The Law of Reciprocity and the Principle of Saturation 



The law states that in a photochemical process it is the total amount 

 of light energy which is effective, as discussed in Chapter 3. There is 

 a reciprocal relationship between time and intensity. Thus a long 

 exposure at low intensity may produce the same effect as a short 

 exposure at high intensity providing the same amount of energy is 

 supplied in both cases. If reciprocity holds, other kinds of experi- 

 ments become much simpler, so the problem was investigated at an 

 early stage of the research. Nevertheless we still lack complete 

 information on the topic as it relates to flowering. 



The method of study is to interrupt the dark period with light of 

 different intensities applied for different durations. A problem arises 

 in how to interpret the data. The Beltsville team approached this 

 problem in the early 1940's by determining the amount of light (time 

 and duration) required to just barely inhibit the flowering of cockle- 

 burs or soybeans when light interruption was given in the middle of 

 the dark period. Their results indicated that during the middle 2 hr 

 of a 16-hr dark period, the reciprocity law held. Then in a given 

 experiment (e.g. with the spectrograph) it was possible to vary only 

 the time of illumination at constant light intensity. This has been the 

 experimental procedure used by most researchers for many years now. 



One problem of interpretation concerns the saturating amount of 

 light (see Chapter 3) required for the pigment system. As shown in 

 Fig. 7-5, the amount of light required to completely inhibit flowering 

 in the middle of a long dark period (with cocklebur) is less than the 

 amount of light required to saturate the pigment system a few hours 

 before or after the middle of the dark period. It is probable that the 

 amount of light required to saturate the pigment system cannot be 

 determined for the most sensitive part of the dark period. It appears 

 that something less than the saturating amount inhibits flowering 

 completely. The best way to study the reciprocity relationship would 

 be to determine at the appropriate time the saturating light quantity 

 (intensity times duration), by determining the saturating light dura- 

 tion at various intensities. Such an approach is required to accurately 

 evaluate the reciprocity law in flowering. 



Surely reciprocity would hold over a range of times and intensities 

 broad enough to validate the early work done at Beltsville, but most 

 studies with biological materials have indicated that when the 

 intensities become extremely high or the times extremely long, the 



