Abstract
Production and distribution of carbon nanotubes (CNTs) is increasing. This is not only due to their physicochemical properties, but also their potential applications in commercial products. While CNTs could improve our quality of life, the potential for their accidental release into the environment is increasing. Despite more than a decade of extensive research into their phytotoxic effects on various organisms, our knowledge of the phytotoxicity of CNTs on crop plants is still poor. Moreover, it is clear that the potential impacts of CNTs on plants is complex, as CNTs are likely to interact with other environmental factors. Therefore, more investigations into the combined impacts of CNTs and other environmental stressors, for example salinity, are required. The aims of current study were 1) to understand the impacts of salinity and multi-walled carbon nanotubes (MWCNTs) on mung bean (Vigna radiata) and wheat (Triticum aestivum) seedlings, and 2) to compare the impacts on these two very different plants. To understand the combined impacts of salinity and MWCNTs, morphological and biochemical attributes were analysed by applying salinity-alone, MWCNTs-alone and salt + MWCNTs treatments. The growth attributes, germination percentage, root and shoot lengths, and shoot-root axis, cotyledon/grain and total seedling weights, and the number and length of lateral root were measured to see the gross effects of salinity-alone, MWCNTs-alone and salt + MWCNTs on mung bean and wheat seeds and seedlings. Markers of oxidative stress, including (Superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APOX), monodehydroascorbate reductase (MDHAR), e) dehydroascorbate reductase (DHAR), f) glutathione reductase (GR), g) glutathione peroxidase (GPOX)) activities, non-enzymatic antioxidant (total glutathione (Total GSH) and total ascorbate (Total Asc)), and oxidative damage markers (protein carbonyls (PC) and lipid peroxides (LOOH)) levels were then measured, in 10 days old mung bean and wheat seedlings. In addition to the above root surfaces and root tips were imaged using microscopy. Proline levels were also analysed as an estimation of osmotic stress caused by salinity. Finally, to compare and contrast, how mung bean, a legume, and wheat, a cereal, respond to NaCl and/or MWCNTs treatments, principal component analysis (PCA) and PCA loading was employed. The results of the current study showed: 1) growth reductions in 10 day old mung bean and wheat seedlings was caused by NaCl in a dose-dependent manner, 2) both mung bean and wheat seedlings treated with 20, 80 and 320 μg/mL MWCNTs alone had no significant differences in morphological and biochemical attributes, 3) the seedlings treated with 75 or 100 mM NaCl + 320 μg/mL MWCNTs showed significant reductions in growth and significant increase in oxidative stress marker levels compared to 75 or 100 mM NaCl alone and 320 μg/mL MWCNTs alone treatments, and 4) mung bean seedlings were more NaCl sensitive than wheat seedlings. The above suggests that the combination of high levels of salt and MWCNTs could possibly have unwanted effects on mung bean and wheat plants under saline soils.