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A greenhouse experiment, which imitated a short (4-day-long) and progressive (3-week-long) soil drought, was employed to assess, with an IR gas analyzer, leaf CO2 exchange rate (CER) in intact one-year-old seedlings of Betula platyphylla as related to the flux of photosynthetically active radiation ranging from 0 to 1400 μE/(m2 s). The registered indices comprised leaf temperature, leaf transpiration conductivity, and the average daily increment of the leaf area. Within a week period following the transition from the short severe soil drought (20% H2O per soil weight) to the conditions of sufficient water content (35–40%), the plants completely regained the initial leaf CER. Under the progressive soil drought, leaf CER was reduced by 30–35%, as compared to the conditions of sufficient water content, evidently due to a 3.7-fold drop in the transpiration conductivity as compared to the control plants. The apparent constant of Rubisco carboxylation and leaf respiration in the light were not affected by the drought period. The rate of leaf growth under the progressive drought was reduced by 64% as compared to the sufficient moisture conditions. Thus, under the progressive drought, the diminished stomatal conductivity reduced CO2 concentration inside the leaf and lowered carbon photosynthetic assimilation. Meanwhile, the leaf source activity considerably increased in spite of diminished photosynthesis.