Changes of the photosystem 2 activity and thylakoid proteins in spruce seedlings during water stress

We subjected spruce [Picea glauca (Moench) Voss x P. engelmanni Parry hybrid complex] seedlings to a severe water stress (shoot water potential less than or equal to-3.5 MPa) to permit assessment of stress effects on photosystem 2 (PS2) in isolated photosynthetic membranes. The thylakoids and Triton-treated membranes isolated from stressed seedlings showed declines in O-2-evolving capacity (H2O --> 2,6-dichloro-p-benzoquinone, DCBQ) and electron transport activity (H2O --> 2,6-dichlorophenol indophenol, DCIP). A partial restoration of O-2-evolution by adding CaCl2 suggested an effect of water stress on the oxygen-enhancing extrinsic (OEE) polypeptides. Water stress had an additional impact on the reaction centre, shown by the inability of 1,5-diphenylcarbazide (DPC) to restore the electron transport (DPC --> DCIP) to the levels seen in control membranes. Quantification of specific photosynthetic membrane proteins by immunoblots strengthened the above suggestions: after drought stress, concentrations of OEE1 and OEE2 declined by 40 %, and amount of the reaction centre protein D1, ATP synthetase, and cytochrome f also declined. The specific effect of stress on these proteins was confirmed by the fact that the amount of chlorophyll-protein complex CP2 was unchanged in membranes isolated from drought-stressed seedlings.