Net photosynthesis and dark respiration studies were conducted on Atriplex confertifolia (Torr. and Frem.) S. Wats and Ceratoides lanata (Pursh) J.T. Howell under field and laboratory conditions. These woody species are commonly found in salt desert shrub communities of the Intermountain West. During these investigations, the effects of air temperature, plant moisture stress, soil water potential, irradiation, and plant phenological status were examined with respect to their influence on carbon dioxide (CO2 ) exchange. Intensive field studies were carried out between April and October. This interval corresponded to the major period of physiological activity in both species. The factors of moisture stress and phenological status appeared to regulate photosynthesis and respiration on a seasonal basis. They set the limits within which daily CO2 exchange could take place. Diel patterns of CO2 exchange were primarily controlled by prevailing temperature and irradiation. Irradiation was more critical during the spring, and temperature became more limiting in the summer. Two alternate photosynthetic strategies of dealing with existing harsh environmental conditions appeared to have evolved in Atriplex confertifolia and Ceratoides lanata. Atriplex confertifolia exhibited an endurance strategy whereby it continued moderate rates of photosynthesis throughout the season. Ceratoides lanata, in contrast, completed the majority of its net assimilation in the spring; then it was relatively inactive when moisture stress became great. These differences seemed to be correlated with water use efficiencies of both species. Rates of net photosynthesis were greatest during the spring in both species. At that time CO2 fixation in Ceratoides lanata exceeded that of Atriplex confertifolia. Later in the year, photosynthetic rates were reduced; and the assimilation rate of Atriplex confertifolia was greater than that observed in Ceratoides lanata. These seasonal patterns of CO2 exchange offered an insight into differences between species using different assimilation pathways. Atriplex confertifolia utilizes the dicarboxylic acid pathway (C4) for carbon fixation, while Ceratoides lanata uses the pentose pathway (C3). Since both species can coexist in the same reasonably stable community, it appeared that both carboxylation pathways were efficient with respect to prevailing environmental conditions. Atriplex confertifolia had lower net assimilation rates than C4 species from warmer climates. It carried on moderate rates of photosynthesis at low temperature (5 to 10 C), and it had relatively low thermal optima (15 to 27 C) for net photosynthesis. An acclimative shift in temperature optima was also noted. This photosynthetic pattern seemed to be related to the climatic conditions under which Atriplex confertifolia evolved. Ceratoides lanata exhibited assimilation rates which were comparable to other C3 species in arid environments. As with Atriplex confertifolia, Ceratoides lanata carried on photosynthesis at relatively low temperatures, but it did not undergo an acclimative shift in the temperature optimum (15 C). Both species were physiologically adapted to severe moisture stress. They carried out active photosynthesis and respiration at soil water potentials between -15 and -50 bars. As soil water potential decreased below -50 bars, CO2 exchange in Ceratoides lanata was curtailed. Photosynthesis and respiration continued at a moderate level in Atriplex confertifolia until soil water potential was reduced below -70 bars.

Coleogyne ramosissima Torr. (blackbrush) and Atriplex confertifolia [Torr. & Frem.] Wats. (shadscale) are cold desert shrubs from different families that often grow together in the Great Basin and the Colorado Plateau in the life zone between 800 and 2000 m elevation. Tissue and seeds from the two species were collected from several localities. Metabolic heat and carbon dioxide production rates were measured with calorespirometry on tissue from field-grown plants and on seedlings at temperatures from 5 to 35°C at 5°C intervals. Blackbrush adapts to higher temperatures earlier than does shadscale, and once it has adjusted to higher temperatures, anabolic rates keep rising with increasing temperature while for shadscale rates flatten out or drop off between 25 and 35°C. Both species are metabolically adapted to temperatures during the growth season determined by the historical rainfall pattern.

Root respiratory response of mature Atriplex confertifolia plants growing in established communities was measured during two growing seasons using freshly excised root segments and gas chromatography techniques. Respiratory response at fixed test temperatures changed significantly during the growing season, and this pattern of respiratory adjustment varied for root segments located at different depths in the soil profile. Respiration measured at a constant test temperature was highest during early sumner, and declined to minimum values in late summer and fall. Root segments taken from the top 30 cm of the soil profile displayed peak activity several weeks before root segments from greater depths reached maximum activity. The significance of these patterns of respiration is discussed in relation to the carbon balance of Atriplex plants and the uses of respiratory energy within the root system.