The Photoinhibition Site of Photosystem I in Isolated Chloroplasts under Extremely Reducing Conditions

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Abstract
Under anaerobic atmosphere where the gas phase was simply replaced by N2, photo-inhibition of PS I of isolated spinach chloroplasts was insignificant. However, when dithionite was included in the irradiation mixture, severe photoinhibition of the NADP+ and the MV photo-reduction occurred. Neither P700 determined by continuous illumination-induced difference spectroscopy, Fe-S centers determined by EPR under cryogenic temperatures, nor vitamin K-l determined by HPLC analysis were significantly decreased under these photoinhibition conditions. Although photobleaching of antenna chlorophylls occurred to more or less extent, NADP+ photoreduction activities were markedly depressed even under saturating actinic light. The maximal amplitude of the flash induced absorbance changes of P700 in ms range decreased almost in parallel with the loss of NADP+ photoreduction activity. These results indicate that although the Fe-S centers of the photoinhibited chloroplasts were reducible by continuous illumination, to almost the same extents as that of the control chloroplasts, the efficiency of reduction by each flash was much lower than that of the control chloroplasts. The site of photoinhibition in PS I under extremely reducing conditions is between A0 and Fe-S X.

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