This study is designed to investigate the effects of carbon nanomaterials (multi-walled carbon nanotubes, MWCNTs) under controlled conditions on three different plant species. The study covers the effects of MWCNT dosage, treatment duration, and the plant-developmental stage, including imbibition, germination and seedling development. Germination experiments are conducted under standardized laboratory conditions based on the protocols of the International Seed Testing Association with aqueous MWCNT suspensions at a dosage of 0, 100 and 1000 mg L-1 applied as seed treatments during 36 h after sowing prior to radicle emergence, using soybean (Glycine max (L.) Merr. cv. BR-16 Conquista), common bean (Phaseolus vulgaris L. cv. Bohnen maxi) and maize (Zea mays L. cv. Surprise) as test plants. The seed treatment with MWCNTs reduced the speed of water uptake particularly by soybean seeds. This is associated with an increased germination percentage and reduced development of abnormal seedlings, while mean germination time is unchanged. However, during later seedling development, negative effects on root growth, particularly affecting fine root development are recorded for all investigated plant species. In soybean, this effect is first detected at 8 days after sowing and requires a minimum MWCNT seed exposure of 36 h. Inhibition of root growth is associated with reduced metabolic activity of the root tissue as indicated by tetrazolium vitality staining. The nitrate uptake was lower in MWCNT-treated plants, which is mainly attributed to the smaller root system. The results demonstrate that even under standardized experimental conditions, excluding environmental factors and effects induced by carbon nanomaterials, plant responses to MWCNT exposure exhibit differences, depending on plant species but also on the physiological status and the developmental stage of individual plants.