Temperature response of photosynthesis of silver fir (<i>Abies alba</i> 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 o C. Response curves of net CO 2 assimilation to substomatal CO 2 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 o C were 37, 91 and 2.6 mol m -2 s -1 for V cmax , J max and R d , respectively. Optimal temperature was higher for V cmax (36.6 o C) than for J max (33.3 o C), 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 o C. It nevertheless fully recovered after 1 day at 25 o C even after 24 h heat stress at 40 o C. Irreversible injuries to PS II revealed by severe increases of ground fluorescence occurred at about 47 o C. This critical temperature for PS II increased with the air temperature imposed during the night preceding the measurements. maximal carboxylation rate / maximal light driven electron flow / dark respiration / optimal temperature / thermostability Rsum -Rponse thermique de la photosynthse de jeunes semis de sapin (Abies alba Mill.). La rponse la temprature de la photosynthse du sapin pectin (Abies alba, conifre particulirement tolrant l'ombre) a t caractrise en utilisant des mesures d'changes gazeux foliaires et de fluorescence de la chlorophylle a. Des semis de quatre ans levs dans des conteneurs et en serre dans le N-E de la France (INRA Nancy, Champenoux) ont t transports dans une chambre climatise et maintenus 6 tempratures (10, 18, 26, 32, 36 et 40 o C) pendant 24 h. Des courbes de rponse de l'assimilation nette de CO 2 la concentration intercellulaire de CO 2 ont t tablies sous clairement saturant sur des rameaux de l'anne portant une seule couche d'aiguilles. La vitesse maximale de carboxylation (V cmax ) et le flux maximal d'lectrons (J max ) ont t estims pour chacune de ces courbes en ajustant les rsultats exprimentaux au modle de photosynthse de Farquhar. La respiration des aiguilles (R d ) a t estime aprs chaque courbe de rponse en mesurant les changes gazeux aprs 5 min d'obscurit dans la chambre. Les valeurs moyennes des paramtres du modle 25 o C ont t estimes 37, 91 et 2,6 mol m -2 s -1 pour V cmax , J max et R d , respectivement. L'optimum thermique tait plus lev pour V cmax (36,6 o C) que pour J max (33,3 o C), et aucun optimum n'a pu tre estim pour R d . L'augmentation de la temprature conduisait une diminution du rapport J max /V cmax . Les nergies d'activation ont t estimes 56, 50 et 23 kJ mol -1 pour V cmax , J max et R d , respectivement. Le rendement