Bioenergy crops are an attractive option for use in energy production. A good plant candidate for bioenergy applications should produce a high amount of biomass and resist harsh environmental conditions. Carbon-based nanomaterials (CBNs) have been described as promising seed germination and plant growth regulators. In this paper, we tested the impact of two CBNs: graphene and multi-walled carbon nanotubes (CNTs) on germination and biomass production of two major bioenergy crops (sorghum and switchgrass). The application of graphene and CNTs increased the germination rate of switchgrass seeds and led to an early germination of sorghum seeds. The exposure of switchgrass to graphene (200 mg/l) resulted in a 28% increase of total biomass produced compared to untreated plants. We tested the impact of CBNs on bioenergy crops under salt stress conditions and discovered that CBNs can significantly reduce symptoms of salt stress imposed by the addition of NaCl into the growth medium. Using an ion selective electrode, we demonstrated that the concentration of Na+ ions in NaCl solution can be significantly decreased by the addition of CNTs to the salt solution. Our data confirmed the potential of CBNs as plant growth regulators for non-food crops and demonstrated the role of CBNs in the protection of plants against salt stress by desalination of saline growth medium.
The use of fossil fuels has accelerated since the dawn of the industrial revolution and demand will increase dramatically in response to an ever-increasing population and by a higher need for energy by mechanization . It is reported that the energy demand will be increased by more than 50% due to rapid progress in all sectors including infrastructure development by the year 2025 . However, the predominant fossil fuel power source is restricted . Limited resources, controversy, environmental concerns and frequently increasing prices associated with the fossil fuels underscore the need to develop an alternative source of energy . For this reason, scientists are looking to bioenergy as a complement to fossil fuels. The concept of bioenergy refers to new alternate renewable energy from biological materials that can generate heat, electricity and transportation fuels . Any plant materials that are used to produce energy are referenced as bioenergy crops. Bioenergy crops are mainly cultivated for power generation including electricity, heat, and liquid fuels for transportation of motor vehicle . Moreover, growing practice of bioenergy crops helps to reduce our dependence on existing fossil energy, reduce global warming by lowering the greenhouse gas, as well as creates job opportunities for thousands of people globally . Bioenergy crops can be cultivated in marginal soils as an energy source due to their potential for a higher amount of biomass production  with a low requirement for fertilizers and irrigation. Currently, many countries (mainly Europe, USA, Brazil and Australia) have implemented policies to encourage energy production from plants. It is estimated that about 273–1381 EJ/energy is provided by bioenergy . Early seed germination with higher germination rate, fast growth, and development, larger biomass yield, and tolerance to stresses are pivotal features of potential bioenergy crops .
The productivity of plants, including bioenergy crops, is limited by several critical factors such as genetic potential, biotic, abiotic, and nutritional stress. Thus, the search for new technologies that can lead to the enhancement of plant productivity is a constant task. It was demonstrated recently that certain nanomaterials may regulate productivity by the enhancement of plant growth . Our laboratory discovered that a wide range of CBNs in low doses can activate seed germination , plant growth, and development of model plants as well as crop species such as barley, corn, and soybean [11–17]. The uptake and accumulation of CBNs in exposed plant tissues was confirmed using microscopy (TEM) and spectroscopy (Raman Spectroscopy) [12, 15, 17]. Recently, the exact concentration of CBNs absorbed by exposed plants including carbon nanotubes and carbon nanohorns was measured by the microwave induced heating (MIH) technique in different plant organs [16, 18]. The documented presence of nanomaterials used as plant growth regulators can be taken as an alarming sign of a possible transfer of CBNs in the food chain by consumption of crops contaminated with nanomaterials. Thus, the potential toxicity of CBN-contaminated food derived from agricultural crops exposed to CBNs has to be investigated experimentally. However, concern about the safety of use of CBNs for plant growth regulation can be less significant if nanomaterials will be applied to non-food plant species such as bioenergy crops which are not subject to food consumption. Here, we describe the efficiency of two types of CBNs, graphene and multi-walled CNTs, for regulation of seed germination and activation of biomass production of two different bioenergy crops Sorghum bicolor L. Moench and Panicum virgatum L. We also made an attempt to understand how the application of CBNs will affect the abiotic stress response of exposed bioenergy species. It was previously reported that carbon nanotube membranes are efficient for desalination of salty water [19, 20]. It is well known that salt stress is one of the major abiotic factors that limits sustainable crop production throughout the world . A higher level of soil salinity limits seed germination as well as growth and development of plants . Salt stress is responsible for the reduction of the tremendous amount of biomass accumulation of energy crops (Miscanthus × giganteus) . Salinity not only adversely affects plant productivity but also quality. More than 20% of agricultural land is already damaged by salinization due to poor drainage system and irrigation of salty water . Here, we demonstrated that CBNs (CNTs) added to growth medium can significantly reduce symptoms of salt stress in bioenergy crops exposed to salt stress by removal of toxic Na+ ions from salt solution. Fig 1 illustrates the experimental design for a study focused on the effects of CBNs on germination, growth and stress response of sorghum and switchgrass.
The effects of CBNs in seed germination, growth, and development of bioenergy crops. The seeds of sorghum and switchgrass were exposed to graphene or multi-walled CNTs by addition to the growth medium. Germination and plant growth between CBN-treated and control bioenergy crops were calculated. The quantification of multi-walled CNTs inside the shoots of matured bioenergy crops was performed using the microwave induced heating (MIH) technique. For salt stress experiments, seeds were exposed to growth medium supplimented with NaCl and different concentration of CNTs or graphene (50, 100, 200, 500, 1000 μg/ml) and germination and seedling growth was monitered. The physical interaction between multi-walled CNTs and ions (Na+ or Cl¯) presented in salty solutions supplemented with CNTs was confirmed by measuring the electrode potential using ion-selective electrodes.