(1) Seasonal nitrate reductase (NR) activity of the young, uppermost leaves of three genotypes (diploid, tetraploid and hexaploid) of Atriplex canescens growing in situ showed maximal activity during reproductive growth. Because rainfall coincided with reproductive growth, higher NR activity during this period may have been due to new leaf growth and increased soil-NO₃) availability rather than an increased need for nitrogen due to developing seeds. The wing-like bracts of the immature fruits possessed NR and may have been a source of reduced nitrogen for seeds. (2) Seasonal leaf water content was significantly correlated with NR activity in all genotypes, and water potentials in the stem xylem were correlated with NR activity only in the tetraploid plants. (3) In a glasshouse the hexaploid plants had significantly higher NR activity and reduced-nitrogen concentrations than did either the diploid or tetraploid plants. (4) Although thermal adaptation of NR would be advantageous for plants inhabiting a desert environment where seasonal temperatures fluctuate widely, there was no evidence to suggest that acclimation of NR occurs in A. canescens growing in situ.