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Three poplar clones of section Populus (Brauna 11 [Populus tremula], L447 [Populus canescens] and Esch 5 [Populus tremula × Populus tremuloides]) were used to analyse the effects of ploidy levels on primary productivity and water use efficiency. The clones were established in tissue culture (2N) and lines with different ploidy levels (2N/4N and 4N) were generated via colchicine treatment. Light response curves were modelled based on gas exchange measurements carried out three times during the growing season on the 1st fully developed leaf under controlled conditions. The plants were harvested in September to analyse biometric parameters. The photosynthetic capacity was greatest in May, decreased throughout the season and increased slightly again in September. The decrease in Brauna 11 and Esch 5 varied from 20-50% compared with values in May and it was not as pronounced in L 447. Photosynthesis and intrinsic water use efficiency differed between clones, but not among the single isogenic lines within each clone. The seasons in which the experiments were conducted influenced the rankings of the 9 variants in physiological measurements, but these rankings were not uniform during the growing period. Differences in biometric parameters were detected between the clones and variants, e.g.; leaf masses were greater in all of the polyploid variants and lowest in the diploid variants. All of the L 447 lines showed the highest wood densities, which were higher in the mixoploid and lower in the tetraploid variants within each clone. A compact wood structure could prevent damage to the water-conducting system under drought stress.