430 



PLANT PHOTOPERIODISM 



The fractional conversion F of the pigment is also: 



where w is the absorption coefficient of the pigment (cm2/gram mole 

 weight), ^ is the quantum efficiency for conversion (dimensionless), X 

 is the fraction of the incident energy E (Einsteins/cni") reaching the 

 site of the reaction. Thus, ojc^X can be obtained from measured values 

 of the incident energy giving a particular response. The molar extinc- 

 tion c is 



e = co:10-3:logelO 

 Values of e^X at the absorption maxima are given in Table 11, 



Again, it is to be emphasized that the values of i<^X at the absorp- 

 tion maxima as recorded in Table II do not depend upon assumptions. 

 From these values a number of pertinent deductions can be made. 

 These are that the values of the molecular absorption coefficients at the 

 absorption maxima for both forms of the pigment are greater than 1.0 

 X 10'', that the quantum efficiencies for conversion are near 1.0, and 

 that reaction (2), the photohydrogenation, is the correct one. 



The extinction coefficients e are molecular constants for the red- 

 absorbing and far-red-absorbing forms of the pigment and accordingly 

 are invariant for the three responses shown in Table II. Since X is 

 necessarily less than 1.0 and </> is probably 1.0 or less, the minimum 

 possible values of e are: 



€r at 6500 A (red-absorbing form) 1.15 X 10'^ 



err at 7300 A (far-red-absorbing form) 0.44 X 10"^ 



