Chlorophyll fluorescence and water relations of salt-stressed plants

The yield of chlorophyll a variable fluorescence from leaves of grapevine, bean, barley, spinach, citrus (Citrus myrtifolia) and mangrove (Avicennia marina) remained unaffected by salt treatment declining only when excessive ion accumulation led to losses of cell turgor pressure. Salt treatment was found to accelerate the rate of fluorescence quenching after maximal fluorescence yield was attained in barley, spinach, citrus and mangrove, species which accumulate both sodium and chloride into their leaves, but not in grapevine or bean, species which accumulate chloride but exclude sodium from their leaves. Stimulation of fluorescence quenching in intact chloroplasts and thylakoids isolated from spinach was induced upon addition of sodium chloride, but not sorbitol. Energy-dependent fluorescence quenching associated with establishment of a pH gradient appears to be stimulated by sodium ion in isolated chloroplasts and thylakoids. In intact leaves stimulation of fluorescence quenching is more likely a consequence of accelerated photochemical reoxidation of Q, the stable electron acceptor of photosystem II.