Effect of light-induced changes in leaf anatomy on intercellular and cellular components of mesophyll resistance for...

Mesophyll resistance for CO₂ 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 CO₂ concentration (c) across hypostomatous leaves of Fagus sylvatica. This technique yields values of the ratio of the internal CO₂ concentrations at the adaxial and abaxial leaf side, c_d/c_b, the drawdown in the intercellular air space (IAS), and intracellular 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 ¹³C 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.