Effects of frost hardening, dehardening and freezing stress onin vivochlorophyll fluorescence of seedlings of Scots pine (Pinus sylvestrisL.)

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Abstract
The kinetics ofin vivochlorophyll fluorescence of photosystem II (PS II) was measured at room temperature and 77 K during frost hardening of seedlings of Scots pine (Pinus sylvestrisL.), and after exposure of frost‐hardened shoots to sub‐freezing temperatures.A more pronounced decrease in variable fluorescence yield for the upper exposed than for the lower shaded surface of the needles suggested that some photoinhibition occurred during prolonged frost hardening at 50 μmol photons m−2s−1and 4°C.Reversible inhibition of photosynthesis after exposure to sub‐freezing temperatures was initially manifested as an increase of steady‐state energy‐dependent fluorescence quenching (qE) and a reduction in the rate of O2evolution. Further inhibition after treatment at still lower temperatures caused a progressive decline of steady‐state photochemical quenching (qQ) and the rate of O2evolution, whereas qEremained high. This implies an inactivation of enzymes in the photosynthetic carbon reduction cycle decreasing the consumption of ATP and NADPH, which is likely to cause an increase of membrane energization and a reduction of the primary electron acceptor (QA) of PS II. Alternatively, the changes in qQand qEmight be attributed to an inhibition of photophosphorylation. Severe, irreversible damage to photosynthesis resulted in a suppression of qEand of variable fluorescence yield, probably because the photochemical efficiency of PS II was impaired.Changes in the fast fluorescence kinetics at room temperature after severe freezing damage were interpreted as an inhibition of the electron flow from QAto the plastoquinone pool. It is suggested that irreversible freezing injury to needles of frost‐hardenedP. sylvestriscauses damage to the QB,‐protein.

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