Temperature and preillumination dependence of delayed fluorescence of spinach chloroplasts

Delayed fluorescence (luminescence) from spinach chloroplasts, induced by short saturating flashes, was studied in the temperature region between 0 and ?40 °C. At these temperatures, in contrast to what is observed at room temperature, luminescence at 40 ms after a flash was strongly dependent, with period four, on the number of preilluminating flashes (given at room temperature, before cooling). At ?35 °C luminescence of chloroplasts preilluminated with two flashes (the optimal preillumination) was about 15 times larger than that of dark-adapted chloroplasts. The intensity of luminescence obtained with preilluminated chloroplasts increased steeply below ?10 °C, presumably partly due to accumulation of reduced acceptor (Q^?), and reached a maximum at ?35 °C.In the presence of 50 mM NH₄Cl the temperature optimum was at ?15 °C; at this temperature luminescence was increased by NH₄Cl; at temperatures below ?20 °C luminescence at 40 ms was decreased by NH₄Cl. At room temperature a strongly enhanced 40-ms luminescence was observed after the third and following flashes. The results indicate that both the S₂ to S₃ and the S₃ to S₄ conversion are affected by NlH₄Cl.Inhibitors of Q^? reoxidation, like 3-(3, 4-dichlorophenyl)-1, 1- dimethylurea, did only slightly affect the preillumination dependence of luminescence at sub-zero temperatures if they were added after the preillumination. This indicates that these substances by themselves do not accelerate the deactivation of S₂ and S₃.