A concern for any agrochemical is the safety of plants, environment, farmers, and consumers. In recent reviews, NP is a biosafety solution. However, nanotoxicology still remains to be noticed
[23][24]. When applying NPs to plants, they will enter the tissues and cause positive and negative impacts depending on their size, shape, and concentration. NPs usually enhance shoot elongation, root elongation, seed germination at low concentration, and in contrast at high concentration
[25]. The effective concentration varies between NPs and crops. In the study by the authors of
[21], the CS-NP-loaded SA and silver were tested for phytotoxicity with the cassava by leaf disk assay method before being applied to cassava plants at net house condition. Results showed that these formulations did not cause damage in leaf disk up to 800 ppm. Then, researchers varied concentrations of 25–800 ppm for stalk-soaking and foliar spraying to enhance cassava growth and reduce leaf spot disease. This is an easy way to know what “safe” concentrations are for the plant. When applied to soil, NPs can cause negative impacts on soil microflora but will be less damaging than agrochemical applications
[26]. On the other hand, the amount of agrochemical and fertilizer applied to agriculture is reduced if they are replaced by NPs, which leads to a reduction in their toxicity. Usually, the safety-by-design principle is applied to screen potential risks from materials and methods synthesis to NP formulation
[27]. As mentioned above, ionic gelation method and CS—a natural polymer—are friendly, safe, and biodegradable solutions.
In addition to its ability to directly inhibit pathogens or induce plant defense system against diseases, CS-NPs or CS-NPs-loaded active ingredients have the ability to stimulate plant growth. At this time, they act as fertilizers or nutrients, affecting plant physiological processes, including nutrient uptake, cell division, cell elongation, enzymatic activation, and the synthesis of protein that leads to increase yield
[28]. Efficiency depends on both the CS-NPs and the active ingredient, even when it releases all the active ingredients because the main component of CS is nitrogen, which takes 9–10%
[29]. Furthermore, the rich positive charge of CS leads to increased affinity toward the plant cell membrane, which enhances reactivity in the plant system
[30].
In the seeding stage, CS-NPs increased the seeding vigor index (57.1%), the number of lateral root (133.3%), and dry weight (200%) of chickpeas
[5]. Additionally, the chickpea seeds were soaked with CS-NPs-loaded thiamine at 0.1% overnight, leading to the seeding vigor index increasing by 64.2%, with Indole-3-acetic acid content increased 10-fold
[17]. Treatment with CS-NPs-loaded Cu at 0.08, 0.1, and 0.12% increased seedling vigor index (33.9, 33.7, and 24.3%), fresh weight (18.9, 21.6, and 16.2%), and dry weight (20.0, 26.7, and 13.3%) in tomato, respectively
[12]. Additionally, CS-NPs-loaded Cu at 0.01–0.16% increased seeding vigor index (15.6–48.6%), fresh weight (7.1–11.4%), and dry weight (21.4–57.1%) in maize seedings, which were related to increasing α-amylase and proteases at days 5 and 7, respectively
[31].
Under greenhouse conditions, the dry weight and yield of finger millet increased by 148.8% and 93.2% when treated with CS-NPs, respectively
[2]. The plant height, stem height, and root length of maize increased by 30.3–60.3, 66.3–237.5, and 2.7–61.1% when treated with CS-NPs-loaded Zn at 0.01–0.16%, respectively
[14]. In chickpea, the shoot length, number of leaves per plant, fresh weight, dry weight, and number of secondary roots per plant increased by 15.3, 14.4, 37.7, 20.0, and 52.8% when sprayed with CS-NPs-loaded thiamine at 0.1%, respectively
[17]. In present year, pre-treated CS-NP-loaded SA at 400 ppm and CS-NP-loaded Ag at 200, 400, 800 ppm by stalk-soaking for 5 min and foliar spraying at 28, 42 DAP could increase the number of leaves (45.1–82.4%), the number of shoots (38.5–46.2%), the largest leaf area (29.6–41.9%), root length (11.6–29.9%), and root weight (27.6–82.8%) of cassava, at 75 DAP in net house condition
[21]. In addition, CS-NPs-loaded Cu at 0.06% could increase plant height (56.8%), root length (40.3%), and pod number (7.2%). NPs treatment at 0.02% could increase root weight (46.8%), nodule number (44.2%), nodule weight (125.8%) under greenhouse conditions and also increase root length (60.9%), root weight (46.8%), and pod number (29.7%), in soybean under field conditions
[22]. Furthermore, NPs at 0.01–0.08% increased plant height (15.9–47.0%), stem diameter (82.9–102.9%), root length (9.5–15.8%), root number (20.9–46.3%), and chlorophyll content (67.3–182.6%) under greenhouse conditions and increased grain yield (25.4–29.3%), 100 grain weight (14.4–16.9%) in maize under field conditions
[12]. However, the treatment at 0.16% reduced root length (9.8%) and chlorophyll content (4.6–9.7%), although the difference was not significance. The CS-NPs-loaded SA at 0.01–0.16% increased leaf area (160.6–224.7%), shoot length (38.5–76.9%), root length (66.9–111.5%), root number (59.6–91.8%), stem diameter (22.8–53.9%), and total chlorophyll (54.2–141.4%) in maize under greenhouse conditions. Moreover, the treatment of these NPs at 0.08%, days to 50% tasselling was early by 4 days under field conditions. Moreover, the plant height (25.5%), ear height (12.1%), cob length (44.8%), test weight (71.1%), and grain yield (48.3%) also increased
[16]. The authors of
[32] synthesized CS-NPs-loaded NPK (ionic gelation) with slow-release N (66.7%), P (3.1%), and K (57.7%) for 240 h. The leaf number, leaf area, plant height, and stem diameter of coffee increased 22.8, 46.9, 12.7, and 28.3% when these NPs were treated at 30 ppm, respectively. This synthesized CS-NPs-loaded NPK improved N (17.04%), P (13.1%), K (67.5%), chlorophyll (30.68%), carotenoid (21.4%) content, and photosynthesis rate (71.7%) in the coffee leaves. Another nano fertilizer, CS-NPs-loaded silicon at 0.04–0.12%, increased the seeding vigor index in maize seeding by 167.5–285.2%. Furthermore, foliar spraying induced antioxidant defense enzyme activity; equilibrated cellular redox; and balanced O
2− and H
2O
2 in leaf, leading to homeostasis. In the field trial, the yield and test weight of maize was increased by 186.6 and 77.1% by treated CS-NPs-loaded silicon at 0.08 and 0.04%, respectively
[33] (Figure 1).