Leaf photosynthetic characteristics of beech (Fagus sylvatica) saplings during three years of exposure to elevated CO2 concentration

Beech(Fagus sylvatica L.) seedlings were cultivated from seeds sown in pots or directly in the ground in outdoor chambers that were transparent to sol ar radiation, and provided either ambient air or CO2-enriched air (ambient + 350 mu mol mol(-1)). The rooting Volume was high in all experiments. In t he short-term experiment, potted plants were assigned to a factorial CO2 x nutrient treatment (optimal nutrient supply and severe nutrient shortage) f or 1 year. In the long-term experiment, plants were grown directly in the g round and received an optimal supply of water and nutrients in both CO2 tre atments for 3 years. Nutrient stress caused carboxylation capacity (V-m) to decrease in the pott ed seedlings exposed to CO2-enriched air during their first growing season. In the long-term experiment with optimal nutrient supply, CO2-enriched air did not affect V-m, but caused an upward acclimation of maximum electron t ransport rate (J(m)). Consequently, there was a 14% increase in the J(m)/V- m ratio, indicating nitrogen reallocation to maintain an equilibrium betwee n RuBP consumption and RuBP regeneration. Both V-m and J(m) decreased durin g the growing season in both CO2 treatments. Although upward acclimation of J(m) was no longer apparent at the end of the third growing season, plants in CO2-enriched air maintained a higher J(m)/V-m ratio than plants in ambi ent air, indicating that photosynthetic acclimation always occurred. Second flush leaves appeared during each growing season. When expressed on the ba sis of foliar nitrogen concentration, their photosynthetic characteristics (V-m and J(m)) were enhanced compared with other leaves. Because the number of second flush leaves was also increased in the elevated CO2 treatment, t his response should be taken into account when modeling the effects of elev ated CO2 concentration on canopy photosynthesis. Stomatal conductance decre ased in response to atmospheric CO2 enrichment; however, the stomatal respo nse to irradiance followed a single relationship based on two stomatal cond uctance models.