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

Seedlings of seven temperate tree species (Acer pseudoplatanus L., Betula p endula Roth, Fagus sylvatica L., Fraxinus excelsior L., Juglans regia L., Q uercus petraea Matt. Liebl. and Quercus robur L.) were grown in a nursery u nder neutral filters transmitting 45% of incident global irradiance. During the second or third year of growth, leaf photosynthetic capacity (i.e., ma ximal carboxylation rate, V-cmax, maximal photosynthetic electron transport rate, J(max), and dark respiration, R-d) was estimated for five leaves fro m 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 equati ons of the Farquhar model on response curves of net CO2 assimilation (A) to sub-stomatal CO2 mole fraction (c(i)), at high irradiance. Primary paramet ers describing the kinetic properties of Rubisco (specificity factor, affin ity for CO2 and for O-2, and their temperature responses) were taken from p ublished data obtained with spinach and tobacco, and were used for all spec ies. 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 refere nce temperature of 25 degreesC were 77.3 and 139 mu mol m(-2) s(-1), respec tively. The activation energy was higher for V-cmax than for J(max) (mean v alues of 73.1 versus 57.9 kJ mol(-1)) resulting in a decrease in J(max)/V-c max ratio with increasing temperature. The mean optimal temperature was hig her for V-cmax than for J(max) (38.9 versus 35.9 degreesC). In addition, di fferences in these temperature responses were observed among species. Tempe rature optima ranged between 35.9 and above 45 degreesC for V-cmax and betw een 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 statisti cally significant differences among species. The optimal temperature for J( max) was highest in Q. robur, Q. petraea and J, regia, and lowest in A, pse udoplatanus and F. excelsior. Measurements of chlorophyll a fluorescence re vealed 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 b e of particular interest when adapted to heterogeneous forests comprising m ixtures of species with diverse ecological requirements.