Long-term photosynthetic response in single leaves of A C3 and C4 salt marsh species grown at elevated atmospheric CO2 in situ.

Mono-specific communities of the C3 sedge, Scirpus olneyi and the C4 grass, Spartina patens, were exposed to normal ambient or elevated CO2, (ca. 680 μl l-1) throughout the 1987 and 1988 growing seasons in open-top field chambers located on a tidal marsh. Single stems of C3 plants grown in ambient or elevated CO2 showed an increased photosynthetic rate when tested at elevated CO2 for both seasons. This increase in photosynthetic response in the C3 species was maintained throughout the 1987 and 1988 growing season. The stimulation of photosynthesis with elevated CO2 appeared to increase as temperature increased and decreased as photosynthetic photon flux (PPF) increased. Analysis of the photosynthetic response of the C3 species during the 1988 season indicated that significant differences in light-saturated photosynthetic rate between ambient and elevated CO2 conditions continued until October. In contrast to the C3 sedge, the C4 grass showed no significant photosynthetic increase to elevated CO2 except at the beginning of the 1988 season.