Effect of light‐induced changes in leaf anatomy on intercellular and cellular components of mesophyll resistance for CO₂ in Fagus sylvatica

Mesophyll resistance for CO 2 diffusion ( r m ) is one of the main limitations for photosynthesis and plant growth. Breeding new varieties with lower r m requires knowledge of its distinct components. We tested new method for estimating the relative drawdowns of CO 2 concentration ( c ) across hypostomatous leaves of Fagus sylvatica . This technique yields values of the ratio of the internal CO 2 concentrations at the adaxial and abaxial leaf side, c d / c b , the drawdown in the inter cellular air space (IAS), and intra cellular drawdown between IAS and chloroplast stroma, c c / c bd . The method is based on carbon isotope composition of leaf dry matter and epicuticular wax isolated from upper and lower leaf sides. We investigated leaves from tree‐canopy profile to analyse the effects of light and leaf anatomy on the drawdowns and partitioning of r m into its inter‐ ( r IAS ) and intracellular ( r liq ) components. Validity of the new method was tested by independent measurements of r m using conventional isotopic and gas exchange techniques. 73% of investigated leaves had adaxial epicuticular wax enriched in 13 C compared to abaxial wax (by 0.50‰ on average), yielding 0.98 and 0.70 for average of c d / c b and c c / c bd , respectively. The r IAS to r liq proportion were 5.5:94.5% in sun‐exposed and 14.8:85.2% in shaded leaves. c c dropped to less than half of the atmospheric value in the sunlit and to about two‐thirds of it in shaded leaves. This method shows that r IAS is minor but not negligible part of r m and reflects leaf anatomy traits, i.e. leaf mass per area and thickness.