Temperature response of photosynthesis of silver fir (Abies alba Mill.) seedlings

Abstract

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°C. 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 (Vcmax) and maximal light driven electron flow (Jmax) were estimated by fitting Farquhar's model to the response curves at each temperature. "Dark" respiration (Rd) 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°C were 37, 91 and 2.6 μmol m-2 s-1 for Vcmax, Jmax and Rd, respectively. Optimal temperature was higher for Vcmax (36.6°C) than for Jmax (33.3°C), and no optimum was detected for Rd. Such values are very close to those of broadleaved tree species. The Jmax/Vcmax ratio decreased with temperature. Activation energies were estimated at 56, 50 and 23 kJ mol-1 for Vcmax, Jmax and Rd, respectively. The maximal quantum efficiency of PS II estimated from chlorophyll a fluorescence declined significantly above 36°C. It nevertheless fully recovered after 1 day at 25°C even after 24 h heat stress at 40°C. Irreversible injuries to PS II revealed by severe increases of ground fluorescence occurred at about 47°C. This critical temperature for PS II increased with the air temperature imposed during the night preceding the measurements.