NONPHOTOCHEMICAL REDUCTION OF THE PLASTOQUINONE POOL IN SUNFLOWER LEAVES ORIGINATES FROM CHLORORESPIRATION

We investigated the relationship between nonphotochemical plastoquinon e reduction and chlororespiration in leaves of growth-chamber-grown su nflower (Helianthus annuus L.). Following a short induction period, le aves of previously illuminated sunflower showed a substantially increa sed level of minimal fluorescence following a light-to-dark transition . This increase in minimal fluorescence was reversed by far-red illumi nation, inhibited by rotenone or photooxidative methyl viologen treatm ent, and stimulated by fumigation with CO. Using flash-induced electro chromic absorption-change measurements, we observed that the capacity of sunflower to reduce plastoquinone in the dark influenced the activa tion state of the chloroplast ATP synthase, although chlororespiratory transmembrane electrochemical potential formation alone does not full y explain our observations. We have added several important new observ ations to the work of others, forming, to our knowledge, the first str ong experimental evidence that chlororespiratory, nonphotochemical pla stoquinone reduction and plastoquinol oxidation occur in the chloropla sts of higher plants. We have introduced procedures for monitoring and manipulating chlororespiratory activity in leaves that will be import ant in subsequent work aimed at defining the pathway and function of t his dark electron flux in higher plant chloroplasts.