PHOTOSYNTHETIC ACCLIMATION TO TEMPERATURE AND DROUGHT IN THE ENDEMIC CHELAN ROCKMAT, PETROPHYTUM-CINERASCENS (ROSACEAE)

Petrophylum cinerascens (Piper) Rydb. (Rosaceae) is a highly restricte d endemic found on steep rocky outcrops and sandy soils along the Colu mbia 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 compensati on point (T-high) and assimilation rate at the optimum temperature for assimilation (A(opt)) were calculated from the regressions. We analyz ed the data using a 3 x 2 split plot experimental design with growth t emperature 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 significantl y 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 re lative decrease in A(opt) from the lowest growth temperature to the hi ghest was almost 60% for well-watered plants and over 20% for plants w ith periodic drought treatments. The lack of change in T-opt and T-hig h, coupled with the decrease in A(opt) with increased growth temperatu re, suggests that P. cinerascens is not able to acclimate to increased global temperature and therefore may serve as a sensitive indicator s pecies of global warming.