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 potent ial less than or equal to-3.5 MPa) to permit assessment of stress effe cts on photosystem 2 (PS2) in isolated photosynthetic membranes. The t hylakoids and Triton-treated membranes isolated from stressed seedling s showed declines in O-2-evolving capacity (H2O --> 2,6-dichloro-p-ben zoquinone, DCBQ) and electron transport activity (H2O --> 2,6-dichloro phenol indophenol, DCIP). A partial restoration of O-2-evolution by ad ding CaCl2 suggested an effect of water stress on the oxygen-enhancing extrinsic (OEE) polypeptides. Water stress had an additional impact o n 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 m embrane proteins by immunoblots strengthened the above suggestions: af ter drought stress, concentrations of OEE1 and OEE2 declined by 40 %, and amount of the reaction centre protein D1, ATP synthetase, and cyto chrome f also declined. The specific effect of stress on these protein s was confirmed by the fact that the amount of chlorophyll-protein com plex CP2 was unchanged in membranes isolated from drought-stressed see dlings.