Abstract.Seedlings ofBetula pendulawere grown in a controlled environment chamber at quantum flux densities of 50, 250 and 600 μmol m−2s−1. The relationship between the flux densities of absorbed CC2and quanta was determined for shoots of whole seedlings. Rates of both light‐saturated andin situ(measured under the growing conditions) net photosynthesis were determined and the pholosynthetic quantum yields under light‐limiting conditions were calculated. Anatomical leaf characteristics, chlorophyll contents and sizes and densities of the photosynthetic units (chlorophyll/P700) were determined. Chloroplasts were isolated and their rates of 2,6‐dichlorophenol indophenol photoreduction were measured together with their pool sizes of the electron transport carriers plastoquinone and cylochrome ƒ.Although acclimated to different quantum flux densities, the three birch populations showed the same quantum yield of net photosynthesis. This was approximately 0.028 in normal air (21.2 kPa oxygen) and about 0.040 when photorespiration was largely inhibited in 2.0 kPa oxygen. In addition, thein situnet photosynthesis rates were limited by the absorbed quantum flux density for low, intermediate and high light grown seedlings. It was concluded that birch acclimated to the three light regimes at different levels of organization (metabolic and anatomical). Thus, the quanta which were absorbedin situcould be transferred into chemical equivalents at an optimal and constant efficiency. The use of different reference bases for expressing rates of net photosynthesis are als