Seasonal changes in net photosynthesis of Atriplex hymenelytra shrubs growing in Death Valley, California

Field measurements of CO₂ and water vapor exchange were made on Atriplex hymenelytra (Torr.) Wats. shrubs growing in Death Valley, California during March 1971 and July 1972. Rates of CO₂ uptake and leaf conductances were substantially higher in March as compared to July. In spite of the large differences in ambient temperatures in March and July, no adaptive acclimation response was apparent in the temperature dependence of photosynthesis. Photosynthesis transpiration ratios at typical midday leaf temperatures were much higher in March than in July. High water use efficiency results from a combination of the C₄ photosynthetic pathway, relatively low leaf conductances, and maximum growth and photosynthetic activity during the cooler months of the year.