Photosynthesis of co-existing Phragmites haplotypes in their non-native range: are characteristics determined by adaptations derived from their native origin?

The Gulf Coast of North America (GC) is a ‘hot spot’ of Phragmites diversity as several lineages (defined according to the haplotypes of their chloroplast DNA) differing in origin, genetic traits and phenotype co-exist and interbreed in this area. We analysed differences in photosynthetic characteristics among and within four haplotypes to understand if differences in gas exchange can be attributed to adaptations acquired in their native ranges. We collected rhizomes of four GC haplotypes (I2, M1, M and AI; including the phenotypes ‘Land-type’, ‘Delta-type’, ‘EU-type’ and ‘Greeny-type’) and propagated them in a common controlled environment to compare photosynthesis–irradiance responses, CO2 responses, chlorophyll fluorescence, the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), specific leaf area (SLA), pigment contents, stomatal density and guard cell length. The maximum light-saturated photosynthetic rate, Amax, in the haplotype I2 (Land-type) and haplotype M1 (Delta-type) (34.3–36.1 mmol CO2 m 22 s) was higher than that in the invasive Eurasian haplotype M (22.4+2.3 mmol CO2 m 22 s). The Amax of haplotype AI (Greeny3-type) was 29.1+4.0 mmol CO2 m 22 s and did not differ from the Amax of the other haplotypes. The carboxylation rate (Vcmax) and electron transport rate (Jmax) followed the same pattern as Amax. The haplotypes also differed in SLA (17.0–24.3 m 2 kg dry mass) and pigment content, whereas stomatal density and guard cell length, Rubisco activity and chlorophyll fluorescence did not differ significantly among haplotypes. The high photosynthetic activity and gas-exchange capacity of the two haplotypes originating in tropical Africa and the Mediterranean area (haplotypes I2 and M1) are apparently adaptations derived from their native ranges. Hence, the haplotypes can be regarded as ecotypes. However, it remains unclear how these differences relate to plant competitiveness and fitness in the GC of North America environment.