Intensity and Decay Half-Time of the Electron Paramagnetic Resonance Signal I in Bean Chloroplasts at Various Stages of Greening

The size of EPR signal I and its decay half-life were measuredon chloroplasts isolated from primary bean leaves after illuminationof 7 d old dark-grown seedlings. The results indicate that the ratio of P700/total chlorophylldecreased approximately 2-fold only between the 6th and 24thhour of illumination. The relative size of the signal increasedwith increasing light intensity up to 360 W m^–2 in allstages of greening studied. Addition of HgCl₂ (ImM) caused anincrease in the magnitude of the signal, especially at lowerlight intensities. Treatment of isolated chloroplasts with DCMU(20 µM) did not have any effect on the intensity of thesignal at the early stages of greening, that is 6 or 12 h ofillumination, but it caused a slight enhancement after 24 or48 h of Illumination. The higher intensity used for the signalproduction the smaller the enhancement observed. The addition of HgCl₂ caused an increase in decay half-lifeof the signal 20-fold or 35-fold after 6 h or 48 h of greening,respectively. In DCMU-treated chloroplasts, an enhancing effectbecomes visible after 12 h of greening and subsequently it graduallyincreased. The data obtained were interpreted in terms of an alterationin efficiency of the re-reduction of P700⁺ under different conditionsand different stages of greening.