Mesophyll conductance to CO₂ in Arabidopsis thaliana

The close rosette growth form, short petioles and small leaves of Arabidopsis thaliana make measurements with commercial gas exchange cuvettes difficult. This difficulty can be overcome by growing A. thaliana plants in 'ice-cream cone-like' soil pots. This design permitted simultaneous gas exchange and chlorophyll fluorescence measurements from which the first estimates of mesophyll conductance to CO₂ (gm) in Arabidopsis were obtained and used to determine photosynthetic limitations during plant ageing from c. 30-45 d. Estimations of gm showed maximum values of 0.2 mol CO₂ m⁻² s⁻¹ bar⁻¹, lower than expected for a thin-leaved annual species. The parameterization of the response of net photosynthesis (AN) to chloroplast CO₂ concentrations (Cc) yielded estimations of the maximum velocity of carboxylation (Vc,max_Cc) which were also lower than those reported for other annual species. As A. thaliana plants aged from 30 to 45 d, there was a 40% decline of AN that was entirely the result of increased diffusional limitations to CO₂ transfer, with gm being the largest. The results suggest that in A. thaliana AN is limited by low gm and low capacity for carboxylation. Decreased gm is the main factor involved in early age-induced photosynthetic decline.