Photoinhibition and Recovery of Photosynthesis in Canker-susceptible and Resistant Needles of Cypress (Cupressus sempervirens L.)

The aim of the present investigation was to test the hypothesis that the cypress canker caused by a fungus (Seiridium cardinale) infection induced effects on photosynthesis which could be related to photoinhibition and the process of recovery in susceptible and resistant needles. Photoinhibition of photosynthesis and recovery was studied in canker-infected susceptible and resistant needles of cypress (Cupressus sempervirens L.) under controlled conditions (irradiation of detached needles to approximately 1900 lmol/m2/s). The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence (Fv/Fm) and electron transport measurements. The potential efficiency of photosystem (PS) II, Fv/Fm declined, and Fo increased significantly in canker-susceptible needles, while Fo did not change in resistant needles. In isolated thylakoids, high light (HL) decreased the rate of whole chain and PS II activity markedly more in susceptible than in resistant needles. A smaller reduction of PS I activity was noticed only in susceptible needles. Upon subsequent dark incubation, fast recovery was noticed in both needle types and reached maximum rates of PS II efficiency similar to those noticed in non-photoinhibited needles. The artificial exogenous electron donors such as diphenyl carbazide (DPC), NH2OH and Mn2+ failed to restore the HL induced loss of PS II activity in susceptible needles, while DPC and NH2OH significantly restored it in resistant needles. The results suggest that HL inactivates the donor side of PS II in resistant and the acceptor side of PS II in susceptible needles. The results on the quantification of the PS II reaction centre protein D1 and 33 kDa protein of water-splitting complex following HL exposure showed pronounced differences between susceptible and resistant needles. The marked loss of PS II activity in HL-irradiated needles was due to the marked loss of D1 protein in susceptible and 33 kDa protein in resistant needles, respectively