Ageing of chloroplasts in vitro II. Changes in absorption spectra and the DCPIP Hill reaction

Changes in absorption characteristics and in Hill activity werestudied during ageing of isolated chloroplasts in the dark andlight. Ageing of chloroplasts brings about significant reductionin the peak absorption in the blue and red bands and resultsin a blue shift in the red band. Incubation of chloroplastsin the dark prevented the early appearance of a light-inducedblue shift at pH 7 and pH 8 but not at pH 6 of the storage media.Addition of BSA or DTT to chloroplast suspensions of pH 6 andpH 7 significantly delayed the blue shift both in the lightand dark, but the chemicals had no effect at pH 8. Ageing-induced loss in DCPIP reduction is restored considerablyby the addition of exogenous electron donors such as DPC orMn²⁺, which suggests loss of the oxygen evolution system. Mn²⁺loses its ability to donate electrons for DCPIP photoreductionduring ageing faster than DPC. This indicates that there aretwo sites between water and the PS II reaction centre for theentry of electrons from these two donors and that ageing causessequential loss of these sites. The loss in DPC supported DCPIP photoreduction coincides, intime, with the ageing-induced blue shift of the red absorptionband and thus suggests structure and function relations of theplastid membrane. (Received October 30, 1978; )