INTERPRETATION OF LIGHT CURVES OF FLUORESCENCE 1071 



between the two yields without the narcotic.) Franck suggested that 

 "self-narcotization" may be a protective device of major importance for the 

 preservation of plants from destructive photochemical reactions (such as 

 photoxidations) that are likely to occur whenever the photosynthetic ap- 

 paratus is for some reason or another prevented from working on its normal 

 substrates. 



It is obvious from this discussion that we should not be astonished to 

 find rather complicated changes in the yield of fluorescence when we accel- 

 erate or retard photosynthesis by changing external factors such as hght 

 intensity or the concentrations [CO2] or [RH], or by adding various inhibi- 

 tors. 



As discussed previously on page 1013, one would expect, in general, 

 that, whenever photosynthesis is limited or slowed down by "starvation" 

 {i. e., by the slo\vness of a preparatory dark reaction), the composition of 

 the chlorophyll complex will change, the photosensitive form will be used 

 up, ki will therefore go down and ^ will increase correspondingly— unless 

 ka increases simultaneously and so strongly that its increase overcompen- 

 sates the decrease of k^ {c.J. equation 28.51). On the other hand, if photo- 

 synthesis is limited or slowed down by "constipation" i. e., by the slowness 

 of a "finishing" dark reaction, the composition of the chlorophyll complex 

 will remain unaffected and fluorescence yield will show no change, unless 

 one is produced indirectly by the "self-narcotization" postulated by Franck. 



It should be recalled here that, according to the criterion we have es- 

 tablished to distinguish preparatory from finishing dark reactions (page 

 1013), slow return of the photochemically changed form of the chlorophyll 

 complex into the normal photosensitive form must be considered a prepara- 

 tonj reaction (since its slowness reduces the rate of the primary photochemi- 

 cal process) . This remains true whether this restoration occurs through a 

 "forward" reaction with the oxidant { ACO2} or the reductant ({ A'H20| , or 

 H2R in purple bacteria); or by reaction with an intermediary catalyst 

 such as (28.41b), or by "primary back reaction" such as (28.41a'). Many 

 of the mechanisms of light saturation which we have considered involved 

 the transition of the chlorophyll complex in strong light into a changed 

 (tautomerized, oxidized, reduced, denuded or narcotized) form. The ac- 

 cumulation of this form — which we assume to be photochemically inert 

 (-y = 0) — is held responsible for changes in the fluorescence yield observed 

 in strong light. We will designate this inactive form generally as {Chl|. 

 If the quantum yield of fluorescence of the photosensitive form of the chloro- 

 phyll complex is ^1, and that of the inactive form {Chl| is <pi, and the ab- 

 sorption coefficients (and the spatial distribution) of the two forms are the 

 same, then the observed yield of fluorescence {<p = F/I; number of quanta 

 emitted per quantum absorbed) is : 



