The comparative responses of photosynthesis and growth to salinity were investigated for two C3 and one C4 species native to the tidal marshes of the San Francisco Bay-Sacramento River estuary of Northern California. At low salinities (0 or 150 meq l-1), where photosynthetic rates were maximal for all species, the C4 grass Spartina foliosa maintained the highest photosynthetic capacity and the C3 stem-succulent shrub Salicornia virginica the lowest; photosynthetic rates of the C3 sedge Scirpus robustus were intermediate. Differences in photosynthetic responses to intercellular CO2 pressure and temperature were consistent with those generally observed between C3 and C4 plants.
CO2 uptake was reduced at salinities above 150 meq l-1 in Scirpus and 300 meq l-1 in Spartina. In contrast, Salicornia exhibited no inhibition of CO2 uptake even at 450 meq l-1 salinity. Analysis of the responses to intercellular CO2 partial pressures showed that the inhibition of photosynthesis by high salinity in both Spartina and Scirpus is primarily accounted for by reduced photosynthetic capacity of the mesophyll, and secondarily, by reduced leaf conductances.
Species differences in relative growth rate (RGR) almost exactly opposed the differences in photosynthetic rates; the highest RGR was found in Salicornia and the lowest in Spartina. This reversal is accounted for by the greater allocation to photosynthetic shoots in Salicornia, which more than compensated for the lower photosynthetic capacity per unit surface area. RGR was more sensitive to salinity than photosynthetic rate in all three species, but the same relative sensitivities held. For Scirpus, reduced leaf elongation rates and changes in allocation patterns account for the greater limitation by salinity of RGR than of photosynthesis, and may be a primary factor restricting productivity of this species in saline habitats.