How does Lobaria pulmonaria regulate photosystem II during progressive desiccation and osmotic water stress? A chlorophyll fluorescence study at roomtemperature and at 77 K

The effects of decreasing mater potential (psi) on O-2 evolution and fluore scence yield at room temperature and at 77 It were investigated using the L ichen Lobaria pulmonaria. Changes in psi were created either by atmospheric desiccation or by osmotic dehydration, with either sucrose, sorbitol or Na Cl as osmoticum, Independent of the method used to establish psi, similar i nactivation patterns mere obtained and were reversible after reincubation i n pure water for 10 min. Our data indicate that exposure to increasing wate r stress acts at two levels. In the first phase, at 'mild' stress, i.e, at psi greater than -13, -16 and -20 MPa for drying, NaCl and sucrose treatmen ts, respectively, a progressive decline in O-2 production and the fluoresce nce yield (Delta F/F-m' and F-v/F-m) was correlated with increases in non-p hotochemical quenching (q(N)), At the same time the photochemical quenching (q(p)) changed only sligthly, indicating the absence of overreduction, The F-o level remained relatively constant in this first stage of water loss. A Delta pH mediated down regulation and a donor side Limitation of photosys tem (PS) II are discussed. When the mater stress was severe, a further decr ease in the fluorescence yield was observed and correlated with a considera ble decrease in F-o (second phase). Kinetic analysis of the 77 Ii emission showed that osmotic stress and atmospheric desiccation possibly lead to an increased spillover from PS II to PS I. In addition, a strong negative effe ct of NaF on the recovery from dehydration was found. This may indicate a s tate transition mediated by the displacement/recoupling of light harvesting complex (THC) LT from/to PS II. The photoprotective role of spatial rearra ngements of antenna complexes during desiccation is discussed.