Stimulation of photosynthetic electron transport in a salt-tolerant plant by high chloride concentrations

Abstract

Halophytes are plants which tolerate high concentrations of electrolytes of which NaCl is normally the dominating salt. Compartmentation of the electrolytes within the cytoplasm has not been unequivocally resolved¹. Several soluble enzymes from higher plant halophytes showed similar sensitivity to salinity when tested in vitro as did those from glycophytes². Also, isolated mitochondria from halophytes and glycophytes showed similar inhibition of oxidative phosphorylation by NaCl in vitro³. On the other hand, chloride (plus the accompanying cations) has been shown to be accumulated in the chloroplasts of Limonium vulgare^4–6, and it is also an essential cofactor in photosynthetic O₂ evolution^7–10. In the only report available on the effects of salt on chloroplasts from a halophyte¹¹, it was demonstrated that differences exist in the response to salts of thylakoids from halophytic or mesophytic plants. I show here that thylakoids from mature leaves of the grey mangrove, Avicennia marina, require high chloride levels for photosynthetic electron transport around photosystem II and discuss the possibility of chloride accumulation into the chloroplast as an adaptational mechanism for salt tolerance.