Temperature responses of photosynthesis were assessed in a shade tolerant tree species (silver fir, Abies alba Mill.) using leaf gas exchange and chlorophyll a fluorescence measurements. Four-year-old seedlings grown in a greenhouse in N-E France were transferred into a climate chamber and kept during 24 hours at six temperature levels: 10, 18, 26, 32, 36 and 40 degreesC. Response curves of net CO2 assimilation to substomatal CO2 partial pressure were obtained on small twigs bearing a single row of needles under saturating irradiance. Maximal carboxylation rate (V-cmax) and maximal light driven electron flow (J(max)) were estimated by fitting Farquhar's model to the response curves at each temperature. "Dark" respiration (R-d) was estimated at the end of each response curve by measuring gas exchange after 5 min darkness in the chamber. The temperature responses of the three parameters were fitted to a thermodynamic model. Mean values at a reference temperature of 25 degreesC were 37, 91 and 2.6 mumol m(-2) s(-1) for V-cmax, J(max) and R-d, respectively. Optimal temperature was higher for V-cmax (36.6 degreesC) than for J(max) (33.3 degreesC), and no optimum was detected for R-d. Such values are very close to those of broadleaved tree species. The J(max) /V-cmax ratio decreased with temperature. Activation energies were estimated at 56, 50 and 23 kJ mol(-1) for V-cmax, J(max) and R-d, respectively. The maximal quantum efficiency of PS II estimated from chlorophyll a fluorescence declined significantly above 36 degreesC. It nevertheless fully recovered after 1 day at 25 degreesC even after 24 h heat stress at 40 degreesC. Irreversible injuries to PS II revealed by severe increases of ground fluorescence occurred at about 47 degreesC. This critical temperature for PS II increased with the air temperature imposed during the night preceding the measurements.