Arundo donax L. (Poaceae) — a C₃ Species with Unusually High Photosynthetic Capacity

Gas exchange characteristics, chlorophyll a fluorescence and leaf water potential were investigated in the giant reed, Arundo donax , under natural conditions in an estuarine mangrove swamp in Durban, South Africa. Maximum photosynthetic CO 2 uptake ranged between 19.8 and 36.7 μmol m −2 s −1 , depending on irradiance, and appeared to be regulated by leaf conductance. There was no saturation of CO 2 uptake or electron transport through PSII (ETR) with increasing irradiance up to 2500 μmol photons m −2 s −1 . A linear relationship between CO 2 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 CO 2 . 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 CO 2 uptake. The molar water use efficiency (WUE) ranged between 4.1 and 9.3 μmol mmol −1 . Percentage photorespiration was between 36 and 39%.