KINETIC ANALYSIS OF PHOTOPERIODISM 447 



What the plant sees then is a very rapid rise in light intensity at 

 sunrise, which within a few minutes reaches saturating values, and 

 then as far as the plant is concerned, the intensity is above saturation 

 until sundown when, within a matter of minutes, the intensity again 

 swings back through the region of photoperiodic saturation and falls 

 to zero. It is almost as if each day, a light is turned on in the morning 

 and off at night as far as the photoperiodic mechanism of the plant is 

 concerned. It is evident, then, that the kinetics of the photochemical 

 reaction are such as to make extremely wide variations in intensity 

 unimportant as far as the ability to measure the duration of the daily 

 light period is concerned. 



PHOTOPROCESSES IN HIGHER PLANTS 



The growth and development patterns of higher plants are regulated 

 in a large measure by the integrated activities of five photochemical 

 reactions that supply the organic nutrient requirements and many 

 control functions. There are undoubtedly many more photochemical 

 reactions in higher plants still to be discovered. The general charac- 

 teristics of these photochemical reactions are given in Table I and 

 their action spectra in Fig. 5. Because of overlapping of the various 

 action spectra and their very different kinetic properties in relation to 

 growth, it is essential that we think in terms of these various processes 

 as an integrated group when attempting to deal with any one process as 

 a singly variant factor. This is especially true at the blue and red ends 

 of the visible spectrum where the curves for photosynthesis, chlo- 

 rophyll synthesis, and phototropism all overlap in the blue, and photo- 

 synthesis, chlorophyll synthesis, and photomorphogenesis in the red. 

 With but few exceptions, it is practically impossible to excite one 

 process without some significant excitation of the others. 



The first two processes given at the top of Table I and Fig. 5 are 

 chlorophyll synthesis and photosynthesis which are shown as related 

 to energy conversion. Whereas chlorophyll synthesis is not directly in- 

 volved in the conversion of radiant energy into chemical bond energy, 

 it is a preparatory step for photosynthesis. The regulatory function of 

 photosynthesis cannot be ignored. The classical work of Kraus and 

 Kraybill (1918) demonstrated many years ago that the overall form 



