Irwin R. Ungar, Ph.D.

     The main focus of my reserch effort is to determine the effect of environmental stress on plant growth and distribution using techniques of both physiological and population ecology.

     Field and laboratory investigations are being pursued to determine how seed dormancy is affected by environmental variables. The relationship between seed banks and salt marsh vegetation indicates that although species in plant communities occur in specific zones their seed banks contain taxa from all of the zonal halophyte communities. The dominant perennial salt marsh species generally have a transient seed bank, while salt tolerant annuals often produce a persistent seed bank.

     Laboratory investigations of germination indicate that seeds are inhibited by increases in salinity.  Induction of dormancy in halophytes by hypersaline conditions can often be alleviated by reducing salinity levels. Long term investigations indicate that seeds may remain viable after two years of exposure to se awater or higher NaCl concentrations. Seed dimorphism in species of Atriplex, Spergularia, and Salicornia provides multiple germination strategies.  Smaller seeds tend to be more dormant and less salt tolerant.  Buried large seeds of Atriplex all germinate in early spring, whereas small seeds form a persistent seed bank. Growth regulator substances, such as gibberellic acid, have been determined to alleviate seed dormancy and overcome the inhibitory effects of salt stress in several halophytes.

     Growth of obligate halophytes is stimulated by salinities in the range of 200 to 300 mmol NaCl. Osmotic adjustment in these species is achieved primarily by the accumulation of inorganic ions and they may have an ash content that is equal to from 40 to 50% of their dry mass.  Long-term flooding causes mortality in salt marshes. Laboratory investigations indicate that flooding may be a critical factor in determining growth and survival of species in zonal communities. It has been hypothesized that mature plants are more tolerant of salinity stress than are seeds. Under hypersaline conditions seeds often remain dormant and recover when salinity levels are reduced, while mature plants will either become dormant or die. Our data indicate that adaptations to overcome salinity stress by seeds may assure successful survival of populations of halophytes in salt marsh habitats.

     I am currently investigating the effects of salinity and other environmental variables on the seed germination and growth of halophytes under field and controlled laboratory conditions.  Experiments studying such variables as competition, salinity, flooding, and nutrition will help us better understand the zonal distribution and ecology of salt marsh vegetation.

Selected References:

• Gulzar, S. Khan, M.A. and I.A. Ungar. 2001. Effect of salinity and temperature on the germination of Urochondra setulosa (Trin.) C.E. Hubbard. Seed Science and Technology 29: 21-29.

• Ungar, I.A. and M.A. Khan. 2001. Effects of bracteoles on seed germination and dispersal of two species of Atriplex. Annals of Botany, 87: 233-239.

• Egan, T.P and I.A. Ungar. 2000. Similarity between seed bank and above-ground vegetation along a salinity gradient. Journal of Vegetation Science 11: 189-194.

• Khan, M.A., I.A. Ungar and A.M. Showalter. 2000. Effects of salinity on growth, water relations and ion accumulation of the subtropical perennial halophyte, Atriplex griffithii var. stocksii. Annals of Botany 85:225-232.

• Khan, M.A., I.A.. Ungar and A.M. Showalter. 2000. The effect of salinity on the growth, water status, and ion content of a leaf succulent perennial halophyte, Suaeda fruticosa (L.) Forssk. Journal of Arid Environments 45: 73-84.

• Egan, T.P. and I.A. Ungar. 1999. The effects of temperature and seasonal change on the germination of two salt marsh species, Atriplex prostrata and Salicornia europaea, along a salinity gradient. International Journal of Plant Sciences 160: 861-867.