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 minus 40 degrees 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 minus 35 degrees 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 minus 10 degrees C, presumably partly due to accumulation of reduced acceptor (Q minus), and reached a maximum at minus 35 degrees C. In the presence of 50 mM NH4Cl; at temperatures below minus 20 degrees C luminescence at 40 ms was decreased by NH4C1. 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 minus 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.