50 DELAYED LIGHT PRODUCTION 



Therefore, the cross section for the reaction can be found by dividing 

 the initial slope by the saturation value. 



From the data given in Figure 2 it can be seen that 



^ = ^-^ ^^^^~'' = 3.1 X 10-1^ cm2 



Since the cross section for the absorption of light by chlorophyll in 

 the living plant has been given (Arnold and Oppenheimer, 1950) as 



0.5 - 1.4 X 10-i« cm2 



it can be seen that the reaction K -^ KB has a cross section 200-600 

 times larger than a single chlorophyll molecule. 



This large cross section, as well as the early saturation of the de- 

 layed light, can be understood in the following way. Let 



ch + hp -^ ch* 

 ch* -\- A ^ B + ch 

 K + B^KB 



be the reactions involved in the early steps of photosynthesis and 

 delayed light production (here ch stands for chlorophyll and c/i* for 

 excited chlorophyll ) . The large cross section is due to the ability of any 

 B to combine with one of the K's, since the ratio of chlorophyll to K 

 is supposed to be very large and the reaction K -» KB is thought to 

 be very fast. If it is now believed that either the amount of B or 

 the amount of B plus KB determines the rate of photosynthesis, it is 

 seen that the saturation of photosynthesis can take place at a much 

 higher light intensity than the saturation of the delayed light. 



References 



Arnold, W. A., and J. B. Davidson. 1954. The identity of the fluorescent and 



delayed light emission spectra in Chlorella. J. Gen. Physiol, 37, 677-84. 

 Arnold, W. A,, and J. R. Oppenheimer. 1950. Internal conversion in the 



photosynthetic mechanism of blue-green algae. /. Gen. Physiol., 33, 



423-35. 

 Strehler, B. L., and W. A. Arnold. 1951. Light production by green plants, 



;. Gen. Physiol, 34, 809-20. 



