The Gulf Coast of America is a ?hot spot? of Phragmites australis diversity as several lineages including hybrids co-exist in this area. A recent study on the P. australis populations of the Gulf Coast shows that there are at least four types ofphylogeographic lineages, including (i) the native ?Land type? with woody, branched and red-stemmed shoots, (ii) the native ?Delta type?, which dominate in the Mississippi River delta, (ii) the introduced ?EU type? which is distinguished from the Land and Delta types by its purple panicles, and (iv) the introduced ?Greeny type?, characterized by its blue-green stand color, and exclusively found in the Mississippi River delta. Although the distribution of the lineages to some extent is determined by the hydro regime and salinity, the lineages grow side by side, and hence compete. We collected rhizomes of the four lineages and propagated them in a common controlled environment to study differences in ecophysiological characteristics of importance for growth and competition. We focused on the process of photosynthesis and
assessed the photosynthetic light-response, the CO2-response, the chlorophyll fluorescence characteristics, the content and activity of the primary carbon fixing enzyme Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo), specific leaf area, pigment contents and stomata density and length. The lightsaturated rate of photosynthesis, A max, differed significantly between the lineages and were higher in the Land and Delta type (34.3-36.1 ?mol CO 2 m -2 s-1 ) compared to the EU type (22.4 ±2.3 ?mol CO2 m -2 s-1 ).A max of the Greeny type averaged 19.1 ±4.0 ?mol CO2 m -2 s-1 , but did not differ statistically significantly from the other types. The pattern for electron transport rate (Jmax) mirrored A max with rates of 174- 178 ?mol m -2 s-1 for the Land and Delta type, 116±10 ?mol m -2 s-1 for the EU type and 139 ±17 ?mol m -2 s-1 for the Greeny type. The lineages also differed in specific leaf area (range 17 to 24 m2 kg-1 dry weight). However, total chlorophyll content, stomata density and length, and RuBisCO did not differ significantly between the lineages. A principal component analysis based on the measured ecophysiological characteristics divided the lineages into groups which are consistent with the phylogeopgraphic relationships between the lineages.