LIGHT CURVES OF FLUORESCENCE 1047 



(28.48P) p/pmax. = C//V1 + C2/2 



or: 



(28.48Q) p/V(pmax.)2 _ p2 = CI 



To compare the usefulness of functions (27.77) or (28.48P) with that of 

 the rectangular hyperbola, Smith derived, for both tj-pes of functions, 

 equations determining the combinations of the parameters [CO2] and 7 for 

 which the yield P has a certain constant value. Figure 28.23 shows the 

 results: For the lowest value of P used (log P = 0.8), the dashed curve 

 derived from the hyperbolic function gave the better fit; but for three 

 higher values of P (log P = 1.2, 1.6, and 2.0), a very good fit 

 was obtained by means of equations (27.77) and (28.48P). 



B. Light Curves of Fluorescence* 

 1. Relation between Light Curves of Photosynthesis and Fluorescence 



In vitro the intensity of fluorescence usually is proportional to the 

 intensity of illumination. This is so because both fluorescence and the 

 "quenching" processes that compete with it (i. e., energy dissipation, and 

 photochemical reactions) usually are "first order" or "monomolecular" 

 processes with respect to the concentration of excited pigment molecules. 

 In other words, each excited molecule has a certain probability of fluores- 

 cing, a certain probability of being deactivated by energy dissipation and 

 a certain probability of undergoing a photochemical reaction; all these 

 probabilities are independent of the concentration of the excited molecules, 

 and consequently do not depend on the rate of their production and thus 

 also on light intensity. Under these conditions, the rate of photochemical 

 reactions too must be proportional to light intensity. Thus, both the quan- 

 tum yield of fluorescence, <p {= const. X F/I, where F is the intensity of 

 fluorescent light), and the quantum yield of the photochemical change, 7 

 (= const. X P/I, where P is the rate of photochemical change), usually are 

 independent of the intensity of the incident light, 7. 



In photosynthesis, we know already that 7 is approximately constant 

 only within a limited range of low intensities (corresponding to the linear 

 part of the light curves, cf. table 28.11), and then declines gradually. The 

 question arises whether, in this case, <p, too, changes with light intensity. 

 At first, experiments appeared to indicate that the intensity of fluorescence 

 continues to increase proportionally with light intensity (i. e., the yield, 

 (p, remains constant) long after photosynthesis has begun to show light 

 saturation (i. e., 7 has begun to decHne). For example, Wassink, Vermeu- 

 len, Reman and Katz (1938), using a suspension of Chlorella vulgaris, found 



* Bibliography, page 1081. 



