Photon trapping in photosystem II of photosynthesis. The fluorescence rise curve in the presence of 3-(3,4-dichlorophenyl)-1,1-dimetnhylurea.

Using isolated chloroplasts in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an analysis was made of the rise of the fluorescence yield effected by weak light. Depending on the pretreatment, the time-course of the rapid photochemical part of the rise varied between nearly first-order and quadratic kinetics, i.e., reflected either a one-quantum or a two-quantum conversion. We consider the occurrence of two photoreductants per system II unit, which are reoxidized in different dark reactions. The data further showed that the "first-order process" is also inhomogeneous.