ABSTRACT
The occurrence of genetically defined races for species of halophytes in saline habitats would indicate that natural selection is taking place in these highly stressful environments. A number of environmental factors have been hypothesized to induce the two extreme height forms in Spartina alterniflora, including soil salinity, nitrogen content of soil, sulfide levels, water-logging, and reduced aeration. Microspecies can develop in salt marsh habitats because of geographic or temporal isolation of populations. Another factor influencing reproductive isolation is that in some genera such as Salicornia there is apparently selection for a breeding system that favors selling. The costs of salt resistance and the evolutionary consequences need to be considered. Although some data are available concerning the mechanisms that increase salt tolerance in some crop species, little research has been carried out regarding the costs and precise mechanisms that favor ecotypic differentiation of highly salt-tolerant halophytes.
