Effects of carbonaceous nanomaterials on soil-grown soybeans under combined heat and insect stresses

Because carbonaceous nanomaterials (CNMs) are expected to enter soils via agricultural CNM applications and disposal of biosolids or wastewater, the exposure implications to crop plants should be understood. Most prior investigations have been under ideal growth conditions, yet crops commonly experience abiotic and biotic stresses. Here, we investigated the effects of 1000 mg kg-1 multi-walled carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), and industrial carbon black (CB) on soybeans grown to the bean production stage in soil. Following seed sowing, plants experienced high temperatures and became infested with an insect (thrips). Consequently, all plants had similarly stunted stem and leaf growth, greater leaf damage, reduced final biomasses, and fewer root nodules, compared with healthy control soybeans previously grown without heat and thrips stresses. Thus, CNMs did not significantly influence the growth and yield of stressed soybeans, and the previously reported nodulation inhibition by CNMs was not specifically observed here. However, CNMs significantly altered two leaf health indicators: the leaf chlorophyll a/b ratio which was higher in the GNP treatment than in either the control (by 15%) or the CB treatment (by 14%), and leaf lipid peroxidation which was elevated in the CNT treatment compared with either the control (by 47%) or the GNP treatment (by 66%). Overall, these results show that, while severe environmental stresses may strongly impair plant production, CNMs (including CNTs and GNPs) in soil could additionally impact foliar health of an agriculturally important legume.