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PSII photochemistry in vegetative buds and needles of Norway spruce (Picea abies L. Karst.) probed by OJIP chlorophyll a fluorescence measurement

Sequence of events during chloroplast maturation is still under study. Vegetative buds represent developmental stage of Norway spruce (Picea abies L. Karst.) needles where chloroplast biogenesis and photosynthetic activity begin. In this work the analyses of polyphasic chlorophyll a fluorescence rise (OJIP) was used to compare photosystem II (PSII) functioning in vegetative buds and fully photosynthetically active mature current-year needles. Photosynthetic efficiency of investigated developmental state was estimated based on performance index (PIABS). Considerably decreased values of PIABS in vegetative buds pointed to their low photosynthetic efficiency. Values of maximum quantum yield of PSII (Fv/Fm) in buds were slightly decreased but above limiting value for functionality indicating that primary photochemistry of PSII is not holdback of photosynthetic activity of vegetative buds. The most significant difference observed between vegetative buds and mature current-year needles was accumulation of reduced form of primary quinine acceptor of PSII (QA-) in vegetative buds, as a result of limitation of its re-oxidation by passing electron to secondary quinone acceptor, QB. This was evident as major increase in relative variable fluorescence at J step leading to significant decrease in probability that a trapped exciton (ET0/TR0) or absorbed photon (ET0/ABS) can move an electron into electron transport chain beyond QA-. Electron transport beyond QA- expressed per reaction center (ET0/RC) was also significantly decreased in buds. Contrary to that, buds had increased absorption of light energy (ABS/RC) and trapping of exciton (TR0/RC) per reaction centre, what resulted in reduction of QA. Based on these results we suggest that absence of functional electron transport beyond QA- which may be contributed to a presence of QB-non-reducing PSII reaction centers that are unable to transfer electron from QA- to QB is the major limiting factor of photosynthesis in vegetative buds.

chlorophyll a fluorescence; OJIP transient; photosystem II; P. abies; vegetative buds

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