DIFFERENCES IN UV-B SENSITIVITY BETWEEN PSII FROM GRANA LAMELLAE AND STROMA LAMELLAE

UV-B treatment of thylakoid membranes or fractions derived from them a ccelerates the first rise (F-o-F-i phase) of variable fluorescence and slows down the second (F-i-F-m) rise. This can be observed for all me mbrane fractions but to different extents. The grana fraction is affec ted more by the UV-B treatment than are the intact thylakoids and the stroma lamella fraction. An exogenous electron acceptor, 2,6-dichloro- p-benzoquinone, inhibits both the initial and second rises of the fluo rescence induction kinetics including the changes induced by UV-B. How ever, other electron accepters, such as 2,5-dimethyl-p-benzoquinone an d methyl viologen quench only the second rise of the curve. Oxygen evo lution from grana-derived membranes is more sensitive to UV-B treatmen t than oxygen evolution from stroma lamella-derived membranes.