CHAPTER 7 



LIGHT AND THE PIGMENT 



Two paramount aspects of the flowering process are important in 

 any photoperiodic response, and they form the roots from which the 

 term photoperiodism was derived : response to light and the measure- 

 ment of time. This chapter discusses what is known about the way 

 that the plant senses when the lights are on or off, and the next 

 chapter considers the manner in which the plant might measure the 

 length of light and dark periods. 



The Pigment System in Flowering 



1. Pigments in General (8) 



A basic rule of photochemistry is that light must be absorbed in 

 order to be effective in any process involving chemical change. If 

 the light is not absorbed, no response can result. If a molecule 

 absorbs light, then it will appear to our eyes to be either colored or 

 black, and we refer to it as a pigment. If all visible wavelengths are 

 absorbed, it appears to our eye to be black. If any wavelength in the 

 visible spectrum is not absorbed, then the compound will appear to 

 be that color. If the red, yellow, and blue wavelengths of light are 

 absorbed and green wavelengths are not, then the compound appears 

 green (e.g. chlorophyll) but if green is absorbed and blue and red are 

 left, then the mixture is sensed as purple. If only blue is absorbed the 

 remaining wavelengths give us the impression of yellow. The pigment 

 to be discussed in this chapter absorbs mostly red and thus appears 

 to be bluish-green. 



In the flowering process, there is a response to light, meaning that 

 the light must be absorbed by a pigment. Finding and characterizing 

 the pigment involved in photoperiodism would be an important 

 initial step in understanding the process. Knowing the color should 

 help us find the pigment, and knowing the wavelengths absorbed 

 should tell us the color. The approach is to measure the relative 



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