Effects of salinity stress on PSII in Ulva lactuca as probed by chlorophyll fluorescence measurements

Effects of salinity stress on photosystem II (PSII) performance in Ulva lactuca were investigated. Salinity stress (48-128parts per thousand) resulted in a significant decrease in photosynthetic O-2 evolution by 12.5%, 21.2%, 37.2% and 75.4% compared with the control treatment, whereas the contents of chlorophyll a, b and carotenoids during short-time (12 h) exposure were unaffected. The chl a fluorescence transients were recorded and analyzed according to the JIP-test which can quantify the PSII performance. The fluorescence yield at phase J, I and P markedly declined with increasing salinity and the transient almost reached a plateau at salinities higher than 64parts per thousand. Addition of 3-(3, 4-dichlorophenyl)-1,1-dimethylurea (DCMU) led to a transformation of the O-J-I-P rise into an O-J rise. Salinity stress (at salinities higher than 64parts per thousand) induced a decrease by 1.4%, 35.9% and 50.1% in the maximum quantum yield of primary photochemistry (phi(Po)), by 34.3%, 56.1% and 49.8% in the quantum yield of electron transport (phi(Eo)), and by 53.9%, 40.8% and 73.2% in the efficiency with which a trapped exciton can move an electron into the electron transport chain further than Q(A)(-) (psi(o)) compared to the control. The total number of active reaction centers per absorption (RC/ABS) and the amount of active PSII reaction centers per excited cross section (RC/CS) also decreased by 45.7% and 70.8% when exposed to the highest salinity stress (128parts per thousand), respectively. These results indicated that the main targets in PSII that were affected by elevated salinity were inactivation of reaction centers and inhibition of the electron transport at the acceptor side of PSII. (C) 2004 Elsevier B.V. All rights reserved.