title: Light response characteristics of net CO2exchange in brackish wetland plant communities.
abstract: Photosynthetic responses to incident photon flux density (400-700 nm; PPFD) was studied in a grass community consisting of Spartina patens and Distichlis spicata and a mixed community having the two grasses and a sedge, Scirpus Olneyi. Net community CO2exchange and incident PPFD were monitored from dawn to dusk in a large open gas exchange system, and a hyperbolic light response model was fit to the data for each day. Light response curves from five growing seasons were evaluated for seasonal trends in the compensation value, initial slope, and maximum net CO2exchange rate calculated from the model at PPFD=1670 μmol m-2s-1.All response curves were curvilinear. Data from approximately 30% of the 113 days studied fit saturation curves which occurred primarily in spring and fall. Approximately 5% of all curves constructed required a different response curve for the morning and afternoon. These occurred during mid-summer and were interpreted to be evidence of water stress.The compensation flux density was very high early in the growing season, but rapidly decreased and during the months June, July and August, it averaged near 100 and 120 μmol m-2s-1in the mixed and grass communities. The initial slope and maximum net CO2exchange rate increased from early May to maxima in July and declined thereafter. Mid-summer mean values for the mixed and grass communities respectively were 34.3±10.3 mmol mol-1and 39.1±9.1 mmol mol-1for the initial slope and 20.3±4.2 μmol m-2s-1and 23.0±3.8 μmol m-2s-1for maximum net CO2exchange. Daytime respiration accounted for approximately 20% of maximum gross photosynthesis in both communities.Photosynthetic efficiency, CO2assimilated per unit total incident solar radiation, was approximately 4.1% and 4.7% at dawn or dusk and 2.3% and 2.6% at midday for the mixed and grass community. Gross photosynthesis, maximum photosynthesis plus midday respiration, accounted for 2.7% and 3.0% of total incident solar radiation in the mixed and grass communities.
authors.affiliation: Smithsonian Environmental Research Center, 12441 Parklawn Drive, 20852, Rockville, MD, USA.
authors.forename: Bert G
authors.initials: BG
authors.lastname: Drake