Petrophytum cinerascens (Piper) Rydb. (Rosaceae) is a highly restricted endemic found on steep rocky outcrops and sandy soils along the Columbia River in eastern Washington. Our goal was to examine the plant's ability for photosynthetic acclimation to increased growth temperature. Plants collected from the field were subjected to three sets of day/night growth temperatures and to two different watering regimes. Leaf gas exchange was measured at step-intervals of leaf temperature. Data were fitted to individual temperature response curves, and the optimum temperature for assimilation (T_opt), the high temperature compensation point (T_high) and assimilation rate at the optimum temperature for assimilation (A_opt) were calculated from the regressions. We analyzed the data using a 3 × 2 split plot experimental design with growth temperature as the main effect and watering treatment as the split plot factor. A_opt for plants grown at 30/16 and 34/20 C was significantly greater than that for plants grown at 38/24 C, but the main effects of growth temperature on T_opt and T_high were not significant. The relative decrease in A_opt from the lowest growth temperature to the highest was almost 60% for well-watered plants and over 20% for plants with periodic drought treatments. The lack of change in T_opt and T_high, coupled with the decrease in A_opt with increased growth temperature, suggests that P. cinerascens is not able to acclimate to increased global temperature and therefore may serve as a sensitive indicator species of global warming.