Effects of elevated CO 2 concentration on stomatal conductance and respiration of beech leaves in darkness.

Beech saplings (Fagus sylvatica L.) were grown in 10-litre pots with medium-fertile soil at ambient-air CO2 concentration (control) and at ∼700 µmol/mol in temperature- and humidity-controlled cabinets standing in the field. Respiration in darkness and transpiration of leaves in response to CO2 concentration were measured with a mini-cuvette system. Stomatal conductance was calculated. Despite clearly decreasing stomatal conductance (∼70%) and transpiration rate (∼49%) with elevated CO2 concentration from subambient (200 µmol/mol) to 800 µmol/mol, dark respiration rates did not decrease significantly (∼3% on average). Thus suggests that stomata opened in response to a low CO2 supply without increased CO2 losses via respiration. This contradicts earlier suggestions in the literature assuming direct effects of ambient-air CO2 concentration on respiration during darkness. Stomatal conductance and transpiration rate of leaves grown under elevated CO2 were greater than in the control and responded less to increased CO2 concentrations. This indicates acclimatization of stomatal aperture to the permanently given CO2 level.