AgNPs trigger a range of anatomical-morphological and biochemical-physiological reactions in plants, generating positive or negative effect. These effects depend on the size, concentration, duration of action, and surface coating type of nanoparticles, application site and method as well as species and plant vegetative phase29. Most often, NPs are applied in water/medium and applied to the substrate leading to the highest accumulation of nanoparticles in plant root and negatively affecting the rhizosphere microorganisms30. Meanwhile, nanoparticle application by spraying produces monodisperse particles and prevents their agglomeration31. In this form, NPs retain their dimensions, which determines their effective penetration through stomata32,33.
The physical and chemical properties of nanomaterials depend on particle size34. The size and shape of metal nanoparticles can be controlled by the choice of stabilizer34, in the presented study using SDS or citrate. The UV-Vis absorbance maxima obtained for AgNP_citr and AgNP_SDS corresponded to diameters of nanocrystalline silver core ranging between 10 and 50 nm11, respectively. Differences in the size of nanoparticles obtained by the two methods (with citrate and SDS) were also demonstrated by DLS and SEM image analysis. In addition, SEM analysis showed that the nanoparticles stored for 6 months agglomerated to a small extent, aligning well with known nanoparticle stabilization strategies35. Absorbed nanoparticles are basipetal transported, depending on their size, by apoplastic pathway, when the NPs size is about 200 nm, or by symplastic pathway, when NPs size is equal or smaller than 50 nm36. In the case of potato, nanoparticle size ranged from 15 to 126 nm depending on the synthesis method so they could be transported both apoplastic and symplastic ways.
Since silver ions in excess can pose a potential risk to humans, it is important to examine the distribution of Ag ions in the sprayed plants, especially in the edible parts, and thus assess the nutritional risk37. Most silver ions were accumulated in the potato shoots, especially in leaves and younger internodes, reflecting the application method. However, it was observed that SDS as a nanoparticle stabilizer reduced the mobility of silver in the plant, which was associated with the larger size of these nanoparticles. The study also revealed that despite the potential for apoplastic and symplastic transport, the concentration of Ag ions in potato tubers was low, especially in the case of spraying with AgNPs synthesized with SDS (Fig. 1c). Significantly, in the tubers, most silver ions were accumulated in the periderm, that is usually discarded before consumption38. Nevertheless, considering that the silver ingestion limit is 5 µg·kg−1·day−1 (about 350 µg·day−1 for an average adult weighing about 70 kg)37, the recorded Ag content in potato tubers does not pose a nutritional risk.
Potato tubers are an excellent source of minerals which are essential for the proper functioning of the human body38,39. Furthermore, they demonstrate high bioavailability of minerals due to a high content of substances that enhance the absorption of essential microelements39 and a low content of anti-nutritional factors40. Therefore, assessing the mineral content of tubers is an important part of determining their consumption values. In the collected potato tubers, the content of chosen macro- and microelements was examined in different parts depending on the spray applied. Among the macroelements present in potatoes, potassium (K+) content was examined as the element most abundant in the tubers and crucial for potato’s health-promoting properties, such as regulation of heart function and lowering blood pressure41. Whereas, sodium (Na+) content, although occurring at relatively low levels in potato tubers, was analysed because a high-sodium diet causes increased blood pressure and hypertension41. Moreover, in humans, sodium and potassium regulate and control total electrolyte management, participate in the body’s acid-alkaline balance, and serve a major role in the stimulus conduction in all nerve cells42. Recommended Dietary Allowance (RDA (US)) of potassium and sodium is 4700 mg and 1500 mg, respectively (for adults age 31 to 50)43, and 100 g of boiled potatoes can provide up to 16% of the Adequate Intake (AI) of potassium41. In the present study, potassium content was higher in pulp (total content in core and pith) than in peel (periderm) of potato tubers. In contrast, other studies observed about two times higher K+ content in peel than in pulp38. The sodium content of potato tubers in the studies described was very high, ranging from about 200 to 800 mg·kg−1, depending on the part of the tuber and the spray applied. In comparison in the studies of other authors, the values for Na+ content for ‘Tajfun’ variety varied between about 220 and 360 mg·kg−1 d.w., depending on the year and cultivation method44. Interestingly, the content of Na+ ions in the pulp (core and pith) of control tubers was almost 800 mg·kg−1 but in the pulp of AgNPs_SDS_1 and 10 sprayed tubers was lower, approximately 660 and 280 mg·kg−1, respectively.
Equally important as macroelements, in the human diet, are microelements, that, performing structural and functional roles, serving as an integral part of many enzymes and regulating metabolism, ensure the proper functioning of the body38. In the present study, zinc and iron were determined because their content in potato tubers is the highest among micronutrients. The contents of iron and zinc in raw potatoes range from 2.5 to 8.3 mg·kg−1 f.w. and 2.3 to 3.9 mg·kg−1 f.w., respectively41. Moreover, microelements deficiencies are common in both developing and developed countries. To give an example, it is estimated that 60% of the current world population suffers from iron deficiencies and 30% from zinc deficiencies38,39. RDA (US) for iron and zinc, amounts 18 and 11 mg, respectively43. According to Burgos et al.41, content of iron and zinc in 100 g of cooked potatoes ranges between 0.29 and 0.69 and 0.29–0.48 mg, respectively, covering 1.6–3.8 and 2.6–4.3% recommended dietary allowance, respectively. In presented study, content of Fe2+ and Zn2+ was the highest in periderm. Several times higher content of these elements in peels (periderm) compared to pulp was also observed by other authors38,40. However, the pulp (core and pith) from tubers of plants sprayed with AgNPs_SDS_10 contained more of these elements than peels.
Mineral nutrients are taken up by plants mainly from the soil solution through the roots. Their redistribution into the tuber, happens through the phloem from the aboveground parts of the plant40. The translocation of mineral nutrients and their accumulation in the underground organs of the plant are influenced by both environmental and genetic factors, including the availability of nutrients in the soil, the anatomy of the tuber, the mechanisms responsible for: transport and sequestration within the organ, loading and unloading of phloem and xylem, or transfer through the periderm39,40. Our research indicates that spraying with silver nanoparticles modifies the distribution and storage patterns of some macro- and microelements in the potato tuber.
L-ascorbic acid (LAA, vitamin C) plays a pivotal role in human nutrition and health, including prevention of scurvy. For humans, who are unable to synthesize vitamin C, the principal sources of this vitamin are vegetables and fruits45. The potato is one of the most widely consumed vegetables in the world today. Vitamin C represents the most abundant vitamin in potatoes, with an average content range of 0.8–3 g·kg−1 fresh weight45,46. Estimated Average Requirement of vitamin C for adults ranges between 60 and 75 mg per day47. Assuming a loss of approximately 50% of vitamin C during the preparation of potatoes41, a portion of 100 g of cooked potatoes, containing on average 30 mg of vitamin C, can provide 20–25% of the Estimated Average Requirement (EAR). As potato consumption tends to be much higher than that of other vegetables, it can make a significant contribution to total dietary intake of vitamin C41. Furthermore, L-ascorbic acid is of significant importance as it enhances the bioavailability of iron, due to its properties that mitigate the chelating effect of phytic acid46. Prior research indicates that the vitamin C content of potatoes is subjected to influence from both genotype and growing conditions, including soil type and climate45,46. The vitamin C content in the tubers of the tested variety ranged from 210 to 271 mg/kg, about 70% of which was in the pulp (core plus pith). In general, the application of silver nanoparticles at higher concentrations resulted in either no alteration in the vitamin C content (AgNPs_SDS) or a reduction in it (AgNPs_citr) when compared with the control. Nevertheless, a modification in the accumulation pattern was identified in specific parts of the tubers. The concentration of vitamin C increased in the pith and decreased in the core. Such alterations may be associated with the plant’s reaction to the stres factor – silver ions. However, due to the minimal rate of nanoparticle penetration into the tubers (i.e., modified underground shoots), LAA accumulates around the vascular bundles, as the rapid translocation to other plant parts is unnecessary48,49.
The concentration of sugars in potato tubers is a significant determinant of the quality of potatoes. High content of soluble sugars is not desirable in potato tubers. It is due to their properties, which lead to non-enzymatic browning, resulting in a reduction of tuber quality50. The level of sugars in potato tubers is affected by a number of factors, including genotype, environmental conditions and cultivation methods during growth, as well as post-harvest handling and storage51. It was recorded between approximately 100 and 150 g·kg−1 f.w. of soluble sugars in the tubers of the tested potato variety in control conditions. While, Pszczółkowski et al.52, for the same cultivar, recorded a sugar content between 40 and 83 g·kg−1 f.w. The elevated sugar content observed in the control tubers may be attributed to the processing methodology employed, which involved fragmentation of the tubers. Kumar et al.51 observed that mechanical treatment of tubers, particularly the removal of the periderm, resulted in an increase in sugar content. However, spraying with silver nanoparticles, especially those synthesised with citrate, altered the sugar content of the tubers, increasing their levels and altering their accumulation pattern in different parts of the tuber. Sugars are involved in plant growth and development as: structural components, energy sources for various metabolic activities, reserves and as signalling molecules in signal transduction pathways53. Given their diverse roles, an increase in the content and translocation of sugars within tubers may be linked to growth processes and also contribute to the immune response54 to an emerging stress factor such as Ag ions.
A comparative analysis of the antioxidant activity of various vegetables demonstrated that the potato exhibits relatively low antioxidant activity55. It should be noted, however, that potatoes have a relatively high consumption rate when compared with other vegetables. Consequently, a minor increase of the antioxidant activity and phenolic compounds content of potato tubers results in a proportional increase in the intake of bioactive compounds in the diet55. Phenolic content and antioxidant activity are influenced primarily by genotype55, but also by environment and growing conditions56,57. Furthermore, studies conducted on in vitro potato explants treated with silver nanoparticles demonstrated an increase in the level of lipid peroxidation, indicating an increase of oxidative stress58. Increased oxidative stress will lead to the activation of antioxidant mechanisms in the plant and increase, among others, the content of phenolic compounds. It is noteworthy that elevated antioxidant activity, in addition to strengthening the health-promoting properties of potato tubers, can enhance protection against pathogenic microorganisms during cultivation and tuber storage59. Consequently, it was reasonable to test whether spraying with silver nanoparticles would affect the degree of lipid peroxidation, content of phenolic compounds and antioxidant activity in the tubers of tested potato cultivar.
The process of lipid peroxidation, which results in the formation of malondialdehyde, is induced by the activity of reactive oxygen species (ROS) generated by stress factors. The transport of AgNPs resulted in the release of silver ions, which are known to disrupt cellular functions and induce phytotoxicity by binding to cellular components and modulating their activity, and thus generating ROS10. During lipid peroxidation, components of cellular membranes and consequently the overall structure of the membrane are damaged. This results in increased membrane permeability, which subsequently hinders the proper functioning of cells60. If this process occurs in the edible parts of plants, it would result in a reduction in their nutritional value61. An increase in lipid peroxidation results in a reduction in the consumption values of potato tubers, due to a more pronounced browning of these tubers62. The presence or absence of lipid peroxidation, and related changes in MDA content, may serve as an indirect indicator of the overall functionality of the antioxidant system. The results of the present study showed that the degree of lipid peroxidation varied according to the type of nanoparticles used, their concentration and the part of the tuber. The greatest increase in malondialdehyde (MDA) content, and thus the highest level of oxidative stress, was observed following the application of AgNPs_citr. The distribution of MDA content in the different tuber sections indicated that oxidative stress was highest in the pith, suggesting that this part of the tuber was the most susceptible to the effects of Ag ions. This was also indirectly reflected in the increase in soluble sugar content.
Polyphenols represent a significant group of antioxidants and are regarded as the most prevalent antioxidants in human diet57. A review of literature revealed no information concerning the impact of silver nanoparticles on the phenolic compounds content and antioxidant activity of potato tubers. In contrast, in vitro studies demonstrated that the addition of silver nanoparticles to the culture medium altered the phenolic compounds content in 4-week-old potato plantlets58. Nevertheless, the content of phenolic compounds and antioxidant activity of potato tubers was the subject of analysis in field experiments conducted under different cultivation conditions56,57. The average phenolic content in potato tubers of different cultivars varied between 62.6–1157. mg∙kg−1 f. w, while an average of 155.0 mg∙kg−1 f. w57. , 169.1 mg∙kg−1 f. w56. and 712.7 mg∙kg−1 f. w63. was recorded in tubers of the cultivar ‘Tajfun’ in earlier studies. The present study revealed that the phenolic content of the tubers exhibited considerable variation, ranging from 330 to 650 mg∙kg−1 f.w. This variability was observed to be dependent on both the specific part of the tuber and the spray applied. As observed by Kim et al.64, the highest phenolic contents were found in the periderm of the tubers (400–650 mg∙kg−1 f.w.). Overall, the application of silver nanoparticles, with the exception of AgNPs_SDS_0.1, resulted in a reduction in the phenolic compound content across all tested tuber parts. This suggests the existence of a disparate mechanism of response to the stress factor.
Reactive oxygen species, generated as a by-product of metabolic reactions, are tightly linked to processes such as growth, environmental stress regulation, development and defense mechanisms64. Given the toxicity of these reactive molecules, research into the mitigating effects of radical scavengers has become a focus for the prevention of disease. One of the most frequently employed radicals for the assessment of antioxidant activity is the DPPH radical64, which was utilised in this study to quantify the radical scavenging activity of extracts derived from various parts of the tuber. Kim et al.64 demonstrated that the antioxidant capacity of potato tuber periderm and pulp extracts, prepared in 80% methanol, exhibited an average antioxidant activity of approximately 40% in both cases. In contrast, Im and Suh65 reported antioxidant activity of different potato cultivars at 6.9–14.6% in the tuber pulp and 9.2–18.3% in the periderm. The presented studies revealed that the antiradical activity exhibited the lowest levels in the pith (0.5–5.6%) and the highest levels in the periderm (3.6–14.6%) of the tested tubers. The application of Ag nanoparticles, similar to phenolic compounds, with the exception of AgNPs_SDS_0.1, resulted in a reduction of antiradical activity.
The results presented in current study indicate that foliar spraying of silver nanoparticles affected the nutritional properties of potato tubers. The synthetic method for nanoparticles determined the final distribution and accumulation of silver ions in the plant. Lower amounts of silver ions were transported to the underground parts of the potato (tubers) when synthesized with incorporation with SDS as capping agent, rather than with citrate. This method of synthesis was also more favourable in terms of nutritional properties of potato tubers. Spraying with the highest tested concentration of AgNPs_SDS had a favourable effect on the nutritional parameters of potato tubers including a variety of macro- and micronutrients, ascorbic acid and soluble sugars. On the other hand, lower concentrations of AgNPs_SDS improved the antioxidant properties of tubers, increasing the content of phenolic compounds and free radical scavenging efficiency. Based on these results further research is needed to verify if and how spraying with silver nanoparticles will affect the resistance of potato plants to pathogens and pests during cultivation, as well as affect tubers upon prolonged storage conditions.
