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Photoinhibition of photosynthesis and photosynthetic recovery were studied in detached needles of cypress ( Cupressus sempervirens  L.) Clones 52 and 30 under controlled conditions of high irradiation (about 1900 μmol m −2  s −1  for 60 min; HL treatment), followed by 60 min in darkness. The degree of photoinhibition was determined based on the ratio of variable to maximum chlorophyll fluorescence ( F  v / F  m ), which is a measure of the potential efficiency of photosystem II (PSII), and on electron transport measurements. The  F  v / F  m  ratio declined in needles of both clones in response to the HL treatment. Minimal fluorescence ( F  o ) increased in HL-treated needles of both clones. The HL treatment decreased rates of whole-chain and PSII activity of isolated thylakoids more in Clone 52 than in Clone 30. In needles of both clones, PSI activity was less sensitive to photoinhibition than PSII activity. In the subsequent 60-min dark incubation, fast recovery was observed in needles of both clones, with PSII efficiencies reaching similar values to those in non-photoinhibited needles. The artificial exogenous electron donors diphenyl carbazide (DPC), hydroxylamine (NH 2 OH) and manganese chloride (MnCl 2 ) failed to restore the HL-induced loss of PSII activity in needles of Clone 30, whereas DPC and NH 2 OH significantly restored PSII activity in photoinhibited needles of Clone 52. Quantification of the PSII reaction center protein D1 and the 33-kDa protein of the water-splitting complex following HL treatment of needles revealed pronounced differences between Clone 52 and Clone 30. The large decrease in PSII activity in HL-treated needles was caused by the marked loss of D1 protein and 33-kDa protein in Clone 30 and Clone 52, respectively.

Photoinhibition of photosynthesis in needles of two cypress ( Cupressus sempervirens ) clones