Intraspecific Variation in Salt Tolerance in Panicum Hemitomon, Spartina Patens, and Spartina Alterniflora: Population Differentiation and Investigations of Underlying Factors.

Although it is known that wetland plant species exhibit considerable interspecific variation in salt tolerance across coastal plant communities, very little is known concerning the amount of intraspecific variation in salt tolerance within plant species. Panicum hemitomon, Spartina patens and Spartina alterniflora are dominant emergent macrophytes of fresh, brackish and salt marshes, respectively. To investigate intraspecific variation in salt tolerance, plant material was collected from Gulf Coast populations of each of these species and subjected to a salinity screening protocol. All three of the plant species displayed significant intraspecific variation in lethal salinity level and plant morphology. Lethal salinity levels ranged from 7.67$\perthous$ to 12.0$\perthous$ in Panicum hemitomon, from 63$\perthous$ to 93$\perthous$ in Spartina patens, and from 83$\perthous$ to 115$\perthous$ in Spartina alterniflora. Population morphological differences were most correlated with salt tolerance in Panicum hemitomon, the fresh marsh dominant and least correlated with Spartina alterniflora, the salt marsh dominant. Investigations conducted at sublethal salinity levels on subsets of populations showed that plant photosynthetic response was able to differentiate highly salt-tolerant and poorly salt-tolerant populations within each species to varying degrees. These differences were greatest in Panicum populations, with the highly salt-tolerant populations having higher photosynthetic rates and greater water use efficiencies. Highly salt-tolerant populations of Panicum hemitomon and Spartina patens were able to limit the total cation concentrations in their leaves, maintain greater leaf xylem pressures, and accumulate less proline than poorly salt tolerant populations, but apparently had only limited control over the ionic composition. Conversely, Spartina alterniflora populations showed no differences in leaf total cation concentrations, but the highly salt-tolerant populations were able to selectively decrease their Na:K ratio and accumulate more glycinebetaine than poorly salt-tolerant populations. It is concluded that in Panicum hemitomon plant size factors and photosynthetic rates are important in explaining population differences in salt tolerance by providing more mature tissue for the translocation of salts away from actively growing regions. The importance of plant morphology decreases as physiological/biochemical responses become progressively more important in explaining intraspecific variation in salt tolerance in Spartina patens and Spartina alterniflora