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1. Carbon isotope discrimination (DELTA) has been shown to be a valuable too for long-term estimates of water-use efficiency, because this parameter is influenced by the ratio of intercellular to ambient CO2 levels, which is in turn related to water-use efficiency. The objective of this study was to examine variation in DELTA-values of field materials and the extent to which changes in DELTA might be interpreted as changes in water-use efficiency.

2. On-line discrimination observations with four desert species confirmed theoretically expected relationships between DELTA and the ratio of intercellular to ambient CO2 concentrations, a prerequisite to applying DELTA-values to water-use efficiency. Seasonal changes in DELTA-values were measured in 15 desert species over the course of two growing seasons in western Arizona.

3. Two major patterns appeared: in leaves of some species there were 2-3 parts per thousand changes during the year, whereas in others the changes in isotopic composition were less than 1 parts per thousand.

4. When carbon isotope analyses are applied to water-use efficiency extrapolations, it has been assumed that leaf temperatures are equivalent among the species being compared. An error analysis of this assumption indicated a near-linear relationship between leaf temperature differential and the difference in isotopic composition necessary to distinguish differences in water-use efficiency. When leaf temperature differentials between plants were less-than-or-equal-to 2.5-degrees-C, a 1 parts per thousand difference in DELTA was sufficient to rank plants correctly and unambiguously with respect to water-use efficiency, indicating the extent to which changes in intercellular CO2 can offset possible differences in the evaporative gradient among plants when calculating water-use efficiencies.

5. As leaf DELTA-values among different species often range 4 parts per thousand on a given sampling date, carbon isotope discrimination appears to be a feasible approach for ranking relative water-use efficiency differences among arid-land plants.