2.6. Nanofertilizers and Phytotoxicity
There are research gaps that are related to nanofertilizer production, including with regard to the energy use and the technology expense, as well as with regard to their unknown interactions in the environment and their toxicities. There are several reports that are based on the phytotoxicities of nanoparticles in crops, which are common, and which result from the induction of the synthesis of reactive oxygen species (ROS) and the oxidative damage from the use of many metal-oxide nanoparticles, such as AgO, CeO
2, CuO, NiO, TiO
2, and ZnO (e.g.,
[35][36][38]).
2.7. Nanofertilizers and Climate Change
It is well known that nutrients can be lost from agricultural fields through the leaching process and/or through gaseous emissions, which may lead to environmental pollution and climate change. These environmental issues may be mitigated by applying nanofertilizers, even under stress
[39]. There are a few studies that have been published on the impact of applied nanofertilizers on reducing gas emissions to the atmosphere (such as
[40][41]). Nanoparticles are the most important adsorbents in soil, and they can affect the nutrient transportation, the organic matter fixation, and the precipitation of the new mineral phase. In the future, the presence of nanoparticles in intact soil formations will be crucial. Furthermore, nanoparticles have direct effects on plants, which include increased enzyme activity, better seed germination, increased plant tolerance to adverse conditions, increased carbon sequestration and nitrogen fixation, and increased photosynthetic and respiratory activities
[42]. Therefore, the role of nanofertilizers in mitigating climate change is an urgent global concern.
3. Application of Nanofertilizers
The intensive global production of foods requires the correct and proper amount of applied nutrients. However, the primary sources of nutrients were, for a long time, the chemical forms of the nutrients, which caused an environmental crisis. Nowadays, the organic, bio-, and nanoforms of these nutrients are applied at an ascent rate to replace the chemical forms of the nutrients
[43]. Nanofertilizers may achieve sustainability in agriculture through their high use efficiency and by minimizing the pollution of the environment
[44]. These nanofertilizers, as nanofertilizers, may also improve the crop productivity by increasing the bioavailability of these nutrients in the soil, as well as their uptake by cultivated plants. The advantage is the self-regulated, time-controlled, and spatially targeted delivery of the active ingredients
[45]. Nanofertilizers can be defined as a type of fertilizer that contains nutrients in the nanoscale form, and/or nutrients in encapsulation, which can systematically release different nutrients to targeted plants
[22]. Nanofertilizers represent any applied nanomaterials for which the nutrients are converted into the nanoscale, and that act as plant nutritional mediators that mainly provide nutrients to the plant in order to support its growth
[45][46]. These nanofertilizers can be encapsulated through nanocoating nanoporous polymeric materials, which are supplied in the nanoforms of emulsions, or as slow-release fertilizer particles
[45].
4. Crop Response to Applied Nanofertilizers under Individual Stress
In nature, plants may be exposed to one or more stresses (i.e., biotic and abiotic stresses), either individually or in combination, which ultimately cause losses in the crop yields. These stresses include abiotic stresses, such as drought, salinity, flooding, water deficits, and low and high temperatures (cold and heat stress), as well as biotic stresses, such as diseases or the pathogens of viruses, bacteria, and fungi
[47]. These plants can face individual or multiple stresses during their lifecycles, as is reported by several studies
[48]. Many antistressors, such as nanofertilizers, have been applied in order to support plant growth under stress, particularly nanoselenium for salinity stress
[49], nano-silicon for drought
[50], and nano-copper for salinity stress
[51], which can increase the plant tolerance against abiotic and biotic stresses (
Figure 2;
[17]). Plant nutrients have a prominent role in ameliorating several stresses, as is cited by many researchers, such as Ahmed et al.
[52]. Several reports confirm the significant role of plant nutrients under different stress conditions, such as salinity stress
[28], heat stress
[53], heat and drought
[54], and drought
[55].
5. Combined Stress and Applied Nanofertilizers
After several studies on the effects of individual stresses on cultivated plants, many studies have begun to investigate combined stress, which is more representative of the reality of nature
[56][57]. This combined stress may include abiotic stress, such as drought; biotic stress, such as bacterial infection (
Pseudomonas syringae)
[58]; or two different abiotic stresses, such as salinity and heat stress
[14]. The damage of this combined stress depends on the duration and the type of stress, the type of plant and its growing stages, the soil and its amendments, and the type of growth medium
[59]. The most common case studies of combined stress include soil salinity and heat stress [14;18], salinity and drought
[60][61][62][63][64][65]; drought and heat stress
[39][66][67]; and soil salinity and heavy metal stress
[68][69]. Nanofertilizers have already been applied to stressed plants, such as nano-Se to cucumber under salinity and heat stress
[14][70], and SiO
2 NPs to common beans under soil salinity and heavy metal stress
[69].
6. Applied Nanofertilizers under Multiple Stresses
Recently, there has been an urgent need for studies on environmentally stressed plants that closely imitate the field conditions (i.e., combinations of stresses instead of individual stresses), as there are serious challenges facing the global crop productivity
[71]. This requires the development of different strategies for making agriculture more resilient while reducing the negative impacts of the combination of drought and heat stress on crop productivity. These strategies may include many practices and superior crop varieties, and they may include the application of biofertilizers, which have the potential to improve the plant tolerance to the combined stresses
[66]. The effects of multiple stresses largely depend on the plant age; on the inherent stress-resistance of the plant, or the susceptibility of its nature; as well as on the severity of the stresses. The plant responses under combined stress comprise the morphophysiological, generic, and molecular features that result from these stresses
[71]. Under salinity, drought, and chilling stresses, osmo-protectants may accumulate, which is due to the induction of the osmotic effect on stressed plants. In general, ROS production may occur under almost all abiotic stress conditions. Moreover, it is well known that salt and heat stress commonly impact the transport and compartmentation of the ions in plants, whereas a physiological water deficit in plants is created because of drought and salinity stress, as well as a decrease in the CO
2 diffusion in the chloroplast because of the reduction in the stomatal opening, which leads to a reduction in the metabolism of the carbon
[71]. Under drought and heat stress, tea plants have resistance to these stresses through the accumulation of metabolites, such as caffeine, catechins, and theanine
[72].
7. Conclusions
Nanofertilizers are an essential source of fertilizers, which can improve the crop production, compared to traditional chemical fertilizers. Nanofertilizers have many advantages, which include their slow/controlled delivery of nutrients, and their abilities to reduce the loss of nutrients in soils by adsorption or fixation, increase the bioavailability of nutrients, extend the effective duration of the nutrient release in the soil, and increase the efficient use of nutrients. On the other hand, nanofertilizers have some problems because of their high reactivity and variability, and their phytotoxicity to plants. There are also human health risks, such as safety concerns for agricultural workers and consumers. Nanofertilizers should definitely be evaluated very carefully before marketing in order to examine not only their advantages for plant growth, but also their potential limitations in terms of the environment and human health. A genuine relationship between nanofertilizers and the plant stresses, in their individual or combined states, has been reported in the published literature; however, there are only a few studies that have been published on multiple stresses. The cited studies confirm the positive role of nanofertilizers on crop production under single and/or combined stress; however, their role under multiple stresses requires more effort and investigation.