The decay of delayed fluorescence from chloroplasts blocked with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and uncoupled with gramicidin has been measured in the time range 0.75--45 ms by use of a laser phosphoroscope. 2. The decays have been analysed as the sum of three first-order components of approximate half-lives 0.2, 2.5 and 300 ms by a computer-assisted least-squares fit procedure. 3. The prompt fluorescence yield of the chloroplasts was manipulated by changing the cation concentration of the chloroplast-suspending medium. 4. Analysis of the concentration dependence of the components of the delayed fluorescence decay and of the prompt fluorescence inductions indicates that the emission yield of the intermediate (tau approximately 2.5 ms) component of the decay is equal to the fluorescence yield of a Photosystem II photosynthetic unit with an open trap, and that for the slow (tau approximately 300 ms) component the emission yield is equal to the total Photosystem II prompt fluorescence yield. 5. It is concluded that the delayed fluorescence yield in the time range studied is a complex function of time, which may be due to there being different mechanisms leading to delayed fluorescence production at short and long times after cessation of illumination.
