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Gas exchange characteristics, chlorophyll a fluorescence and leaf water potential were investigated in the giant reed, Arundo donor, under natural conditions in an estuarine mangrove swamp in Durban, South Africa. Maximum photosynthetic CO2 uptake ranged between 19.8 and 36.7 mu mol m(-2) s(-1) depending on irradiance, and appeared to be regulated by leaf conductance. There was no saturation of CO2 uptake or electron transport through PSII (ETR) with increasing irradiance up to 2500 mu mol photons m(-2) s(-1). A linear relationship between CO2 uptake, corrected for respiration (A), and ETR has only been reported for C-4 species and C-3 species when photorespiration is eliminated. From this relationship, it was calculated that 8.5 electrons were transported through PSII for the fixation of one mole of CO2. Predawn leaf water potential was about - 0.5 MPa and decreased to - 1.5 MPa on a cloudy day and to - 2.1 MPa on a clear day. Diurnal change in leaf water potential had little influence on leaf conductance and hence CO2 uptake. The molar water use efficiency (WUE) ranged between 4.1 and 9.3 mu mol mmol(-1). Percentage photorespiration was between 36 and 39%.