Comparison of the changes in net photosynthetic CO2 uptake and water vapour efflux during leaf ontogenesis with the differences between the leaves according to their descending insertion level

The differences between leaves of different age according to their descending insertion level (starting from the youngest, 18th leaf) were compared with the changes occurring during the corresponding period of ontogenesis of the 18th unshaded leaf using the gas exchange [net photosynthetic CO2 uptake (P N ), water vapour efflux (E)] of the adaxial and abaxial surfaces of tobacco leaves as an example. Experimental elimination of the influence of shading during the involved period of ontogenesis of the 18th leaf manifested itself by a relatively slower decrease inP N and by fluctuation of theE values at approximately the same level. Thus the differences between leaves of different insertion levels cannot be exclusively ascribed to the effect of their ontogenetic age.