Aims: Two genetically distinct clones of Phragmites australis were used to investigate the immediate response induced by osmotic stress. The study aimed at elucidating if the response time, the inhibition rate and the recovery from salinity stress vary between these two genotypes. The experimental work was conducted at the laboratory of the Institute of Bioscience, Aarhus University, Denmark. Methods: The light-saturated photosynthetic rate (Pmax), stomata conductance (gs) and transpiration rate (E) were measured over different periods of salt exposure (15, 70 and 240 minutes) and at different salt concentrations (20 and 40 parts per thousand salinity). Important findings: The osmotic stress induced stomata closure and reduction of Pmax and E for both clones. The clone-specific responses as measured through physiological parameters were negatively correlated with exposure time and salt concentration. During the 4-hour exposure at 20 ppt, the two clones were inhibited at different rates. The salt-sensitive Land-type showed an immediate reduction of Pmax, gs and E. No recovery was observed after removing the salt solution. At the same salt concentration, the reduction of Pmax gs and E of the Greeny-type was lower and immediate recovery was observed when the root zone was rinsed. Both clones were irreversibly inhibited after 4 hours of exposure to 40 ppt. Recovery was primarily related to exposure time, as Pmax, gs and E rates of both clones recovered completely after fresh-water rinsing in the 15-minute experiment. The Greeny-type also recovered after the 70-minute exposure, but not the Land-type. We conclude that the response to osmotic stress is genotype-dependent and that the salt-tolerant clone possesses very efficient signaling pathways to detect changes in the soil water potential and adjust accordingly.