The effect of ionic stress on photosynthesis in Dunaliella tertiolecta : Chlorophyll fluorescence kinetics and spectral characteristics.

A comparison of the effects of ionic stress and an uncoupler on long-term fluorescence transients (the 'Kautsky effect') in the green alga Dunaliella tertiolecta indicated that the large quenching induced by ionic stress was caused by a pH gradient across the thylakoid membrane. This possiblity was given support by the increase in the slow phase of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in algae subjected to ionic stress. Low-temperature fluorescence emission spectra indicated that salt stress enhanced photosystem-I emission in the dark, and a comparison of simultaneous emissions at 695 and 720 nm at room temperature indicated a further increase in photosystem-I emission during the fluorescence transients. Taken together with the decrease in the fast phase of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in stressed algae, our results indicate that ionic stress stimulates cyclic electron flow, and that non-cyclic flow is inhibited. The effect of sucrose-induced osmotic stress was similar to, but less marked than, the effects of NaCl and KCl; the effect of decreasing the external salinity was small.

The effects of coumarin on the growth and viability of sporelings of red algae.

The growth of sporelings of the red algae Plumaria elegans, Antithamnion plumula and Polysiphonia brodiaei was markedly influenced by coumarin. Growth of Plumaria and Antithamnion was totally inhibited by immersion for 7 days in media containing 200 mg coumarin/l, and showed 46% and 41% growth inhibition respectively in 100 mg coumarin/l; a significant reduction in growth was obtained in 50 mg/l of the phytostatic agent (e.g. 15% growth inhibition with Plumaria; and 10% with Antithamnion). A noticeable stimulation of growth was observed in 10 mg coumarin/l. The viabilities of the sporelings remained high after immersion in the toxic agent. The inhibitory effects were of a similar order both with the young plants treated immediately after commencement of growth, and with sporelings grown normally for 14 days before contact with coumarin. With Plumaria sporelings the maximum inhibitory effects were observed after 3 days immersion in 200 mg coumarin/l, and after 5 days in 100 mg coumarin/l. Immersion for 7 days in 200 mg/l of the reagent induced irreversible changes in the sporelings; such effects were less marked at 100 mg/l; and at 50 mg/l there was a complete recovery from the effects of the compound when speorelings were transferred to normal culture medium. The significance of the results is discussed in terms of the possible factors involved which may influence sporeling growth.