Abstract: Seasonal changes of net photosynthesis (A), stomatal conductance (gs) and transpiration (E) were researched in Platanus acerifolia Willd. and Acer pseudoplatanus L. Measurements were carried out four times during vegetation period (July 16, August 20, September 5 and September 22) 2011 at seedling age 2+1, cultivated in the nursery. Soil in the nursery belongs to the type of undeveloped alluvial soil (fluvisol), sandy loam form, with the dominant fraction of fine sand (63.5%) and silt+clay fraction in a lower concentration (34.7%). A sharp reduction in net photosynthesis was observed in the second part of August and at the beginning of September, when both species exhibited their lowest net CO2 assimilation. Minimum A was 7.27 and 7.0 μmol m-2 s-1 in P. acerifolia and A. pseudoplatanus, respectively. Stomatal conductance followed rise-and-fall pattern of net photosynthesis during the entire period of observation. The seasonal course of transpiration was similar in P. acerifolia and A. pseudoplatanus, with a peak at the beginning of September. Maximum transpiration rate was 2.87 and 3.62 mmol m-2 s-1 in P. acerifolia and A. pseudoplatanus, respectively. Statistically signiœcant correlation between net photosynthesis and stomatal conductance, in both species, suggests that stomatal closure was the main factor controlling CO2 assimilation. Incomplete recovery of stomatal conductance and net photosynthesis in A. pseudoplatanus, after the rainfall at the end of September, suggests remaining stomatal limitations. Inferiority of A. pseudoplatanus was probably related to the sensitivity of this species to summer heat and drought owing to the fact that this species is native for sites determined by large share of silt in the soil texture and high content of moisture and nutrients in the soil.
        Keywords: Acer pseudoplatanus L., Platanus acerifolia Willd., net photosynthesis, transpiration, stomatal conductance.