Physiological Response to Moisture Stress as a Factor in Halophyte Distribution

Abstract

Research was conducted to determine seasonal water stress patterns in important species from four adjacent halophyte communities, and to evaluate their photosynthetic and respiratory response to moisture stress. Suaeda depressa and S. fruticosa, which grew in the most saline areas of the study site, consistently had lower (more negative) water potentials than Sarcobatus vermiculatus or Distichlis spicata. This suggested that the distribution of these species may, in part, be controlled by their relative tolerances to severe moisture stress. Net photosynthetic rates of S. fruticosa were at a maximum at lower water potentials than S. vermiculatus or D. spicata. The more gradual decline in net photosynthesis at decreasing water potentials in these three halophytes, compared with more mesic glycophytes, suggests a further physiological adaptation to their saline desert environment. Rates of dark respiration appeared to decline very gradually in response to decreasing water potential in these species, indicating that many metabolic processes could continue with minimal disruption during periods of severe water stress. INTRODUCTION Relatively few studies have sought to explain the natural distribution of the halophytic vegetation of western Utah, and the results of these studies are variable (Kearney et al., 1914; Shantz and Piemeisel, 1940; Flowers, 1934; Flowers and Evans, 1966; Fireman and Hayward, 1952; and Gates et al., 1956). With the exception of salt grass, Distichlis spicata (L.) Greene, the major salt desert communities are dominated by species of the family Chenopodiaceae. These communities are often separated by a very narrow ecotone, suggesting sharp changes in edaphic factors, although such sharp changes are not always found (Gates et al., 1956). Earlier studies indicated that soil salinity and soil moisture relations are primary factors governing halophyte distribution, though most species are able to tolerate a wide range of soil conditions. Since soil moisture and salinity are known to affect plant water potential (Kozlowski, 1964), it would be expected that plants of different halophyte communities would experience different water potentials. The physiological responses of these plants to various water potentials might then be expected to play an important role in their distribution. The purpose of the present research was to determine the variation in water potential in the field of important species from four halophyte communities and to evaluate their photosynthetic and respiratory response to moisture stress. The species included Sarcobatus vermic1 Present address: Urbana College, Urbana, Ohio 43078.