Temperature response of leaf photosynthetic capacity in seedlings from seven temperate tree species

Seedlings of seven temperate tree species (Acer pseudoplatanus L., Betula pendula Roth, Fagus sylvatica L., Fraxinus excelsior L., Juglans regia L., Quercus petraea Matt. Liebl. and Quercus robur L.) were grown in a nursery under neutral filters transmitting 45% of incident global irradiance. During the second or third year of growth, leaf photosynthetic capacity (i.e., maximal carboxylation rate, V-cmax, maximal photosynthetic electron transport rate, J(max), and dark respiration, R-d) was estimated for five leaves from each species at five or six leaf temperatures (10, 1 8, 25, 32, 36 and 40 degreesC). Values of V-cmax and J(max) were obtained by fitting the equations of the Farquhar model on response curves of net CO2 assimilation (A) to sub-stomatal CO2 mole fraction (c(i)), at high irradiance. Primary parameters describing the kinetic properties of Rubisco (specificity factor, affinity for CO2 and for O-2, and their temperature responses) were taken from published data obtained with spinach and tobacco, and were used for all species. The temperature responses of V-cmax and J(max) which were fitted to a thermodynamic model, differed. Mean values of V-cmax and J(max) at a reference temperature of 25 degreesC were 77.3 and 139 mu mol m(-2) s(-1), respectively. The activation energy was higher for V-cmax than for J(max) (mean values of 73.1 versus 57.9 kJ mol(-1)) resulting in a decrease in J(max)/V-cmax ratio with increasing temperature. The mean optimal temperature was higher for V-cmax than for J(max) (38.9 versus 35.9 degreesC). In addition, differences in these temperature responses were observed among species. Temperature optima ranged between 35.9 and above 45 degreesC for V-cmax and between 31.7 and 43.3 degreesC for J(max), but because of data scatter and the limited range of temperatures tested (10 to 40 OC), there were few statistically significant differences among species. The optimal temperature for J(max) was highest in Q. robur, Q. petraea and J, regia, and lowest in A, pseudoplatanus and F. excelsior. Measurements of chlorophyll a fluorescence revealed that the critical temperature at which basal fluorescence begins to increase was close to 47 degreesC, with no difference among species. These results should improve the parameterization of photosynthesis models, and be of particular interest when adapted to heterogeneous forests comprising mixtures of species with diverse ecological requirements.