Developmental progression of photosystem II electron transport and CO2 uptake in Spartina alterniflora, a facultative halophyte, in a northern salt marsh

Leaves of Spartina alterniflora Loisel, a halophytic salt marsh grass, show rapid changes in photosynthetic performance and allocation of carbon and nitrogen resources to key macromolecules during the growing season. Photo system II (PSII) electron transport correlated primarily with the developmental state of the plants and less with the ambient temperature. The ratio of gross CO2 uptake to PSII electron transport showed high interleaf variation but rose steeply to about 0.22 mol CO2·mol PSII electrons–1 through June to late July, with subsequent decline. Seasonal variation in CO2 per PSII electrons correlated with the pattern of maximum capacity for CO2 uptake. RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase) and chlorophyll (Chl) (µmol·m–2) peaked in early June, before the period of maximum leaf elongation. RuBisCO then declined through net dilution as the leaves elongated to maximal length in mid-July. RuBisCO and Chl levels stabilized once leaf elongation stopped in mid-July, but from mid-August onward the leaves showed net loss of RuBisCO and Chl. The plants thus show a developmental program of early remobili zation of nitrogenous macromolecules from leaves even though the plants thereby forego maximal photo synthetic performance during 75 d in late summer and early autumn, when temperature and light are near optimal. Key words: electron transport, photosynthesis, PSII, RuBisCO, Spartina.