Diurnal profiles of gas exchange, leaf temperature, and stem xylem pressure potentials were measured at three field sites where plants representative of diploid (2n = 18), tetraploid (2n = 36) or hexaploid (2n = 54) cytotypes of fourwing saltbush (Atriplex canescens) occurred together in various combinations. Results indicated that differences among cytotypes in leaf morphology were related to differences in leaf temperatures, net photosynthesis, leaf conductances, and internal C0₂ concentrations. Mean area per leaf for diploids was 0·228 cm², for tetraploids 0·427 cm², and for hexaploids 0·613 cm². Leaf temperatures of diploids were consistently 1-5°C higher than the other cytotypes. In contrast, leaf temperatures of hexaploids were at or below ambient levels while leaf temperatures of tetraploids were intermediate to the other cytotypes. Photosynthetic rates of diploids decreased rapidly when leaf temperatures approached 37°C, and 63% of the variation of photosynthesis was accounted for by leaf temperatures. Increasing internal C0₂ levels in diploids indicated internal limitations to C0₂ uptake in the mesophyll were responsible for depressed photosynthesis. In contrast, photosynthesis of tetraploids and hexaploids was largely unaffected by leaf temperatures. Higher leaf temperatures in diploids were also translated into higher conductances and lower water-use efficiency (WUE) than the other cytotypes. Tetraploids and hexaploids had higher WUE that were related to either lower conductances, increased photosynthesis, or a combination of reduced conductances and higher photosynthesis. The intermediate leaf form and physiology of tetraploids may represent a compromise to diploids and hexaploids.