RECENT RESULTS AT WAGENINGEN ?)bd 



only after a certain time has elapsed {cf. Fig. 4). This decay appeared 

 to have first-order character and was only slightly influenced by 

 temperature. These experiments suggest that a component that is 

 usually present in excess under conditions of light saturation is 

 photochemically destroyed. The rate in weak light (the quantum 

 jdeld), on the other hand, decays immediately upon addition of too 

 strong light. 



finishing 

 r*4ction$ 

 \.rate limiting) 



n 



Fig. 5. Light and dark reactions in photosynthesis (bold arrows) and photo- 

 inhibition (thin arrows, left side). A complex of pigment molecules with its stabili- 

 zation center U is excited by absorption of a single quantum and may be inacti- 

 vated by a double absorption act. In a fast dark reaction (kiUo) the photo- 

 synthetically activated complex is restored by enzyme E. A slower dark reaction 

 (A-3) in turn limits the restoration of E. 



The maximum flash yield {R/m for tf-^0,ld-^ °= ) also decays and 

 does so in close parallelism with the decay of the quantum yield. 

 From these observations we concluded that photoinhibition involves 

 a photochemical inactivation of the pigment system and more 

 specifically the inactivation of U or of the link between U and its 

 group of pigment molecules (cf. our chapter on flashing light) . 



Further kinetic studies in continuous and flashing light indicated 

 that a two-quanta process is involved and that photosynthetically 

 excited units (U* in the preceding chapter) are only slightly (or not) 

 susceptible to this photoinactivation; i.e., photoinhibition may result 

 from the coincidence of two light quanta in a non-excited photo- 

 synthetic pigment complex. The action spectrum of photoinhibition 



