Physiological Response to Salinity by Four Salt Marsh Plants

Water relations, rates of photosynthesis and respiration, and patterns of CO2 fixation were studied in four salt marsh species: two grasses, Spartina foliosa Trin. and Distichlis spicata (L.) Greene, and two succulents, Salicornia europea L. and Batis maritima L. Mean water potentials (ψw) of all four species ranged from -29.82 to -42.20 atm. The Δψw between the plant and external medium was of such a magnitude that water uptake did not appear to be a problem. Mean osmotic potentials (ψπ) ranged from -41.65 to -52.52 atm. The grasses showed moderately high relative phosphoenolpyruvate (PEP) carboxylase activity. This observation and the distinct Kranz anatomy of the grass leaves indicated the presence of C4 metabolism. Succulents lacked dark acidification, and day/night tests of PEP carboxylase gave the same enzyme activity as in C3 plants; therefore, these succulents are not CAM plants. The ψπ of the growth medium from 0 to -25.4 atm appeared to have little effect on respiration rates. The photosynthetic response to varying osmotic potentials of the growth medium was different in both groups of plants. Photosynthetic rates in the succulents decreased slightly or not at all with decreasing ψπ. Distichlis was very strongly affected even by ψπ of -12.5 atm, while photosynthetic rates in Spartina decreased significantly only at ψπ of -25.4 atm.