Essential nutrients are most commonly applied to soil and plant leaves, and soil fertilization is more desirable and effective for larger nutrient needs[19]。In some cases, however, foliar fertilization has become a widespread and common method of crop management due to its more cost-effective and efficient characteristics[19].

The leaves of the plant protect the plant from water loss, pests and pathogens, while allowing gas exchange for the photosynthetic reaction[20].The surface of a leaf is usually composed of features: trichosomes, stomata, and phloem pores. NPs is absorbed by leaves in two ways, namely the cuticle pathway and the stomatal pathway[21]（图2）。NPs with a diameter of less than 4.8 nm can enter the leaf directly through the cuticle channel, while NPs with a larger diameter can enter through the stomata. Due to the high density of the stomata itself, the stomatal pathway is considered to be a more efficient way to absorb NP[23].NF is absorbed by the stomata and transferred to the rest of the plant through the phloem. Nanoparticles can enter phloem in two ways, either directly from phloem cells or through the interstitiums of phloem cells [24]. Nano-fertilizer can promote the rapid acquisition of nutrients in plant growth parts, thereby increasing chlorophyll production, photosynthetic rate, and ultimately increase plant growth and development[25].

Nano-fertilizers are becoming more and more important in improving nutrient utilization efficiency due to their unique properties[28]。Rice fertilizer helps to release nutrients in a slow, controlled manner in order to deliver nutrients to the target location, thereby minimizing losses[29]。Nano-fertilizers have greater absorption and retention capacity than traditional fertilizers due to their small size [30]. Nano-fertilizer can improve the physiological and biochemical indexes of plants, such as photosynthetic rate and nutrient absorption efficiency, and enhance the defense system of plants[7]。Some studies have reported that zinc NFs has better physical and chemical properties and plays a positive role in promoting seed germination and plant growth [31\u201232]. Ahmed et al. have shown that sulfur nanofertilizers can not only reduce arsenic toxicity, but also improve the yield and quality of rice[33]。Hu et al. proved that low levels of TiO2NPs improved plant nutritional quality without causing significant oxidative stress [34]. In general, nano-fertilizer is more effective than traditional fertilizer and has good prospects for development.

Foliar fertilization is more effective than soil fertilization and is a useful method to meet the needs of higher tree species (Figure 3). Compared with foliar application, soil application is more harmful to the environment, and nano-fertilizer has lower bioavailability in the soil, while foliar application has lower environmental risk, so plants generally prefer foliar application of nano-fertilizer. Foliate fertilization has a low exposure dose, can be applied repeatedly, and can be applied regularly according to the weather to avoid nutrient loss. In addition, foliar application provides faster nutrient uptake than soil application[37].Figure 3 illustrates the advantages of foliar fertilization and its positive effects on plants.

Foliar application of nano-fertilizer can improve fertilizer utilization efficiency and crop yield and quality, and reduce adverse effects to a certain extent.

Zinc (Zn), as an essential micronutrient, has a major impact on plants, including protein, DNA, and RNA synthesis, and is also an essential cofactor for many antioxidant enzymes. There are many studies on the beneficial effects of zinc-based nano-fertilizer spraying on crops. Lorenzo et al found that ZnO NPs improved the growth and physiology of coffee due to the increased ability of ZnO NPs to penetrate leaves, and had a more positive effect on fruit and quality than ZnSO4 [Garcia-Lopez et al demonstrated, ZnO NPs (1000 mg/L and 2000 mg/L) sprayed on the leaf surface increased the antioxidant capacity of Habanos pepper fruit and significantly improved the fruit quality[38].Davarpanah et al. concluded that spraying a lower concentration of B or Zn nano-fertilizer on the leaf surface could promote the yield of pomegranates without affecting the characteristics of the fruit[40].Other studies have shown that leaf surface application of zinc nano-fertilizer not only increases the number of leaves and essential oil content, but also significantly improves plant growth, yield and nutrient content [41,42,43,44,45,46]. It can be seen that leaf surface application of zinc nano-fertilizer has a positive effect on the improvement of crop yield, quality, nutrient content and physiological parameters, and there may be no potential toxicity, but it should be noted that the optimal concentration of different plants may vary greatly.

Environmental stress effects will change ecosystem processes [53], destroy ecosystem balance, and then destroy the environmental balance related to food production, which may lead to crop yield reduction [54]. Heavy metals, salinization, drought, and high temperatures are all key environmental stressors that have serious impacts on global crop productivity and quality [55]. Various strategies have been sought to enhance the ability of plants to withstand these numerous environmental stresses [56]. Nano-fertilizer has high efficiency and slow release, and has become a suitable choice to reduce environmental stress effects [57\u201258] and promote crop cultivation in harsh environments [59\u201260]. A number of studies have confirmed the positive effects of NPs on plants under temperature stress, including improving photosynthetic capacity [61] and promoting growth and development [62]. However, heat stress has not been studied enough; Therefore, this paper introduces the application of nano-fertilizer in alleviating heavy metal stress, salt stress and drought stress, which are relatively concerned aspects in current research.

Heavy metals are absorbed by plants and accumulate in food crops for human and animal consumption, seriously endangering crop growth and human health [63\u201264]. Many studies have shown that nanoparticles can reduce plant stress to heavy metals [65\u201266]. Leaf application of SE and SI, NPK can alleviate metal stress of rice and improve the yield and quality of brown rice [67]. Spraying ZnO NPs on the foliage reduced Cd pollution and increased the plant height, biomass and chlorophyll concentration of maize plants [68]. Foliar application of TiO2NPs significantly reduced She content and CD-induced toxicity in stems. However, the application of TiO 2NPs to soil increased the absorption of Cd by corn in CD-contaminated soil [69]. In summary, the application of nano-fertilizer on the surface of leaves has a alleviating effect on soil heavy metal pollution, and may be more useful than soil application to a certain extent. However, it should be noted that the presence of heavy metals may promote the absorption and enrichment of nanoparticles in plants and produce co-toxicity, leading to food safety issues [70].
2.2.2. Salt stress
Salinity is considered to be one of the major abiotic stresses that limit crop yields globally. Salt stress limits growth, reduces biomass, causes chlorophyll degradation and alters the state of water [73]. Abdelaal et al proved that leaf surface application of silicon alleviated the adverse effects of salt stress on sweet pepper by improving water state, increasing photosynthetic rate, regulating certain osmotic pressure and plant hormones, and increasing antioxidant enzyme activity [75].Perez-Labrada et al. Foliar application of Cu nanoparticles enhanced salt tolerance by increasing the Na+/K+ ratio and stimulating the antioxidant mechanism of plants [77]. Sheikhalipour et al. proved that foliar application of Cs-Se NPs could increase photosynthetic pigment content in leaves, promote plant production, and reduce oxidative damage under salt stress by increasing SOD, POD and CAT enzyme activities [78]. Mustafa et al. found that leaf spraying with low-dose TiO2 nanoparticles could improve the germination characteristics, water and permeability potential of wheat, and help improve plant tolerance to salt stress [79]. Nano-silicon fertilizer plays a positive role in alleviating salt stress [55,80,81]. For example, Alsaeedi et al. showed that amorphous silica nanoparticles (Si NPs) contribute to the normal growth of cucumber plants under salt stress without any significant symptoms of dehydration throughout the growing season [82]. However, foliar application of silicon fertilizer has not been fully studied and is a new research direction. Silicon nanoparticles are also relatively new as a coating for spraying other nanoparticles. A study showed that ZnO NPs and Zno-Si NPs had different leaf application effects on pea plants under salt stress [83]. Higher concentrations of ZnO NPs can produce certain phytotoxic effects, while Zno-Si NPs is non-toxic to plants under physiological conditions, and even has a slight irritating effect at higher concentrations. The application of nano-fertilizer on leaf surface is one of the trends to alleviate salt stress.
2.2.3. Drought stress
Drought condition is also a key factor restricting crop yield, causing many adverse stresses on plant morphology, physiology and molecular level [84], affecting plant growth, physiology and yield [85]. In addition, as drought worsens, soil salinization and calcification increase, which in turn leads to a significant decline in productivity [91]. In semi-arid tropical regions where water is scarce, foliar application of nanomaterials may be the best option for increasing yield, as it requires a large amount of water to dissolve and be absorbed by the root system [92]. Foliar application of nano-fertilizer as a growth regulator can promote crop development and productivity under drought conditions [92]. For example, leaf spraying ZnO NPs can improve yield and crop quality [41,93] and seed nutritional quality [94], as well as stomatal conductance and crop drought stress index [94]. In addition, Moitazedi et al found that zinc fertilizer applied to foliage could significantly improve the influence of drought stress on membrane stability index (MSI) [95]. Studies have also shown that under water shortage conditions, spraying Fe and Zn nano-fertilizer on leaf surface can improve the physiological characteristics and seed yield of legumes under normal irrigation [85\u201296]. Leaf surface application of K nanochelate can improve growth, physiological and biochemical characteristics, increase quantitative and qualitative traits, and reduce the negative effects of water stress [97]. The application of magnesium nano-fertilizer and chitosan fertilizer on the surface of leaves can improve the total chlorophyll yield, seed yield and oil content, and alleviate drought stress [98]. In addition to several metallic nano-fertilizers, non-metallic nano-fertilizers have also been used in many applications.

In conclusion, the application of nano-fertilizer on leaf surface under abiotic stress can improve plant enzyme activity and enhance plant antioxidant capacity. These improvements can increase crop resistance to adversity, resulting in higher yields and quality.