DIFFERENTIAL IMPACT OF MULTI-WALLED CARBON NANOTUBES ON GERMINATION AND SEEDLING DEVELOPMENT OF GLYCINE MAX, PHASEOLUS VULGARIS AND ZEA MAYS
In face of highly variable and so far largely unexplained plant responses after exposure to carbon nanomaterials, this study was designed to dissect the effects of a defined type of carbon nanomaterials (multi-walled carbon nanotubes, MWCNTs) under controlled conditions on three different plant species, depending on MWCNT dosage, treatment duration, and the plant-developmental stage, including imbibition, germination and seedling development. Germination experiments were conducted under standardized laboratory conditions according to 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 was associated with an increased germination percentage and reduced development of abnormal seedlings, while mean germination time was unchanged. However, during later seedling development, negative effects on root growth, particularly affecting fine root development were recorded for all investigated plant species. In soybean, this effect was first detectable at 8 days after sowing and required a minimum MWCNT seed exposure of 36 h. Inhibition of root growth was associated with reduced metabolic activity of the root tissue as indicated by tetrazolium vitality staining. Also nitrate uptake was lower in MWCNT-treated plants, which could be mainly attributed to the smaller root system. The results demonstrated that even under standardized experimental conditions, largely excluding environmental factors and effects induced by the variable properties of 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.
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