Lionel G. Klikoff

Carbohydrate Cycle of Plantago insularis var. Fastigiata, a Winter Annual from the Sonoran Desert

Carbohydrate content during the life cycle of Plantago insularis was determined at seven different phenological stages. As a percentage of alcohol-insoluble dry weight, carbohydrate was maximum very early in the life cycle and continually declined to a minimum at fruit set. As an absolute amount, in milligrams per plant, carbohydrate increased gradually until the cotyledons plus first- and second-leaves stage and then increased sharply to a peak at the preflowering stage. From the preflowering to the flowering stage at floral bud opening, the amount of carbohydrate rapidly decreased to a minimum. From the flowering stage to fruit set, the amount of carbohydrate sharply increased to a maximum at fruit set. This carbohydrate balance may represent adaptations to a desert environment.

Physiological Response to Moisture Stress as a Factor in Halophyte Distribution

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.

Moisture Stress in a Vegetational Con- tinuum in the Sonoran Desert'

The desert vegetation north of Phoenix, Arizona, appar- ently forms a continuum from open, widely dispersed, small shrubs at the lower elevations to relatively dense, tall shrubs at the higher elevations. With several exceptions, this vegetation continuum is correlated with plant moisture stress. Phenological data strongly suggest that differences in plant moisture stress along the continuum markedly influence the com- position of the continuum.

Car6on Dioxide Exchange by Several Streamside and Scrub Oak Community Species of Red Butte Canyon, Utah

Species of a xeric scrub oak community (scrub oak, Quercus gam.belii; sagebrush, Artemisia tridentata; bitterbrush, Purshia tridentata) have lower seasonal water potentials than species of a mesic streamside habitat (box elder, Acer negundo; bigtooth maple, Acer grandidentatum; creeping barberry, Berberis repens). The restriction of box elder to mesic sites is, in part, a function of its physiological responses to moisture stress. Positive net photosynthesis of box elder ceases at water potentials 10 to 15 bars higher than net photo- synthesis of the other five species, and dark respiratory rates of box elder decline more rapidly. The relative photosynthetic and dark respiratory responses to water stress of scrub oak, sagebrush, bitterbrush, bigtooth maple and creeping barberry are apparently not significant factors influencing the distribution of these species.