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Introduction
Potassium (K+) is one of the most important macronutrients and is considered the quality element of crops (Lester et al. 2010), it plays various biochemical processes in plants (Akhtyamova et al. 2023), such as protein synthesis, sugar transport (Uthman and Garba 2023), stomatal conductance, photosynthesis, respiration, signal transduction and ion homeostasis (Shen et al. 2017). In addition, K plays a functional role in the activation of more than 60 carbohydrate synthesis enzymes and proteases in plants (Oosterhuis et al. 2014). K ions are involved in the opening and closing of stomata, helping to maintain optimal CO2 levels for photosynthesis, preventing water loss, and ensuring efficient sugar production in plants (Sheoran et al. 2021). The uptake and transport of K within the plant depends on the availability of K in the soil (Rogiers et al. 2017); however, despite a high availability of K in the soil especially in calcareous soils, many factors can limit its adequate uptake by plants impacting on poor fruit quality and yield (Jifon and Lester 2011). Among the soil factors, the ionic competition between cations (Ca, Mg) stands out (Weil and Brady 2002), and in the plant, the phenological stages of the crop (Ho 1988). In view of this situation, foliar fertilization with potassium is an alternative to improve the yield and quality of crops (Lo'ay et al. 2021). Nanomaterials are finding increasing applications in agriculture as they increase crop yields and reduce environmental pollution (Qibin et al. 2024).
Foliar application of nano fertilizers is becoming increasingly important compared to that of traditional fertilizers. This is mainly due to higher plant uptake efficiency and efficacy at low concentrations (Butt and Naseer 2020). K NPs provide sufficient K to the plant and improve photosynthetic processes, and yield (Lo'ay et al. 2021); also, K NPs improve crop productivity under salt stress conditions (Mahmoud et al. 2022). Salama et al. (2024) reported that the application of a K based nanofertilizer to onion (Allium cepa L) plants improved yield and the content of bioactive compounds. Likewise, Doaa et al. (2019) mention that the application of nano potassium + potassium sulfate increased N, P, and K concentrations in petioles, improved shoot length and diameter, leaf area, significantly increased yield, total soluble solids, total anthocyanins and total sugars in grapevine (Vitis vinifera) cv. Flame seedless. Despite the advantages of the use of nanoparticles, it is difficult to determine the optimal dose, since their effects vary depending on several factors such as the type of crop, the nanoform, the duration of exposure, and the interactions with the nanoelement (Guillén-Enríquez et al. 2022).
Melon (Cucumis melo L.) is known for its excellent nutritional profile, as it contains proteins, lipids, vitamin C, beta-carotene, antioxidants, bioactive polyphenols, and other phytochemical compounds crucial for disease prevention (Rivera-Gutiérrez et al. 2021). Given its economic importance and the continuous growth of production in Mexico, there is a production of 648 541.00 ton, with an average yield of 32.82 tons ha-1 (SIAP 2023),the objective of the present study was to determine the optimal dose of potassium nanoparticles (K NPs) in melon plants and evaluate their impact on yield, nutraceutical quality, and K content in the fruits.
Materials and methods
Growth conditions
The study was conducted in fields located in Concordia, Coahuila, Mexico (25° 48'31'' north latitude and 103°5'56.4'' west longitude, altitude 1 016 m). The climate in the region is semi-warm, with average annual temperatures ranging from 20 to 22 °C and annual rainfall between 125 and 400 mm. Melon var Cruiser was planted and grown according to standard commercial practices for melon production in this region, including fallow, cross-cropping, and leveling. Double-row borders were constructed forming beds at a distance of 4 m between borders and a plant spacing of 30 cm for a density of 16 665 plants per ha. The soil texture is silt loam (40% sand, 46% silt and 14 clay); bulk density 1.56 g cm3; pH 7.67; water holding capacity 23.4%; electrical conductivity 4.28 dS m-1; organic matter content 0.8%; total nitrogen 4.82 mg kg-1; available phosphorus 4.27 mg kg-1, potassium 603.93 mg kg-1; calcium 6 181.93 mg kg-1 and magnesium 202.74 mg kg-1.
The soil is calcareous. Fertilization was according to local recommendations, consisting of 120-60-00 (N-P2O5-K2O), applying all the phosphorus and half of the nitrogen at planting and the rest of the nitrogen at flowering. The fertilizers used were NH4H2PO4 and NH4SO4. Irrigation was provided by gravity. In pre-sowing, one irrigation was applied with a 0.30 m sheet; subsequently, in pre-sowing, one irrigation was applied with a 0.25 m sheet; subsequently, six auxiliary irrigations were applied with 0.09 m sheets each; a total of 0,74 m was applied during the crop cycle.
Treatments and experimental design
The commercial nanofertilizer PHC* NANO K was applied, which is a suspension, where K is available in ionic form, which allows it to be quickly absorbed by the plant (Marquez-Prieto et al. 2022). The treatments consisted of five doses of K NPs: 100, 200, 300, and 400 mg L-1 and a control treatment (distilled water). In all formulations, 0.1% urea was added as the carrier ion and a non-toxic commercial surfactant (INEX-A®, 0.02% v:v). A completely randomized block design was used, with six replicates, resulting in a total of 30 experimental units. The application of treatments was carried out directly on the plant using a manual sprayer. Four applications were made, the first one 30 days after planting, and the following three applications were made every 15 days.
Fruit weight and yield
The fruits of all treatments were harvested at commercial maturity. All harvested fruits were weighed using a digital scale (Torrey®, México) with a maximum capacity of 5 kg. Yield was estimated per hectare considering the total weight of fruit in each experimental unit. The polar and equatorial diameter was measured using a digital vernier (Truper®, Mexico) and the result was reported in cm.
Soluble solids and firmness
The determination of total soluble solids (TSS) and firmness was carried out on one fruit per replicate. TSS (°Brix) was measured with a manual refractometer with a measuring range from 0 to 32% (Atago® Master 2311, Tokyo, Japan). Firmness was measured using a penetrometer model FH20000 (Extech®, USA) with an 8 mm measuring head. The method involved peeling the fruit, placing it on a sturdy, level surface, creating four punctures per fruit, averaging the readings, and presenting the findings as the maximum compression force in Newtons (N).
Preparation of extracts for non-enzymatic antioxidants
One melon was randomly chosen from each treatment and replication for the assessment of non-enzymatic antioxidants. Each fruit was sampled by extracting two g of fresh pulp and combining it with 10 mL of 80% ethanol in a plastic tube sealed with a screw cap. The tube was then placed in a rotary shaker (ATR Inc., USA) for 6 hours at 5 °C and 20 rpm. Subsequently, were centrifuged at 3000 rpm for 5 min and the supernatant was removed for analytical tests.
Flavonoids totals
Total flavonoids were determined by colorimetry (García-Nava 2009). Samples were quantified in a UV-Vis spectrophotometer at 510 nm (Metash, UV-6000, Shanghai, China). The standard was prepared with quercetin dissolved in absolute ethanol. Results were expressed as mg QE 100 g-1 FW.
Total phenolic content
Total phenolic content was determined by the Folin-Ciocalteau method (Garcia-Nava 2009). Samples were quantified in an ultraviolet (UV)-Vis spectrophotometer at 760 nm (Metash, UV-6000, Shanghai, China). The standard was prepared with gallic acid. The results were expressed in presented in milligrams of gallic acid (GA) equivalent per gram of fresh weight (mg equiv GA g-1 FW).
Antioxidant capacity
The assessment of antioxidant capacity was conducted through the in vitro DPPH+ method, with a modification based on the method of Brand-Williams et al. (1995). 50 μL of the sample was mixed with 950 μL of DPPH+ solution, and after a 3 min reaction period, the absorbance was measured at 515 nm. A standard curve was created using Trolox (Sigma-Aldrich), and the results were presented in micromoles (μM) of Trolox per gram of fresh weight (μM equiv Trolox g-1 FW).
K content
The K concentration in melon pulp was determined by AOAC (1990) guidelines using atomic absorption spectrophotometry with an air-acetylene flame (VARIAN-SPECTR AA 3110, Palo Alto, CA, USA), the results were expressed in mg kg-1 dry weight (DW).
Statistical data analyses
The data obtained underwent Bartlett's test to assess variance homogeneity, and normality was examined using the Bartlett and Kolmogorov-Smirnov tests. Following these tests, an analysis of variance (ANOVA) was conducted, identifying differences between treatments where applicable, Tukey's test was used (p ≤ 0.05). Using the statistical package SAS version 9.1 (Statical Analysis System Institute).
Results
Yield and fruit quality
The foliar application of K NPs did not show significant differences in the variables yield, weight, and equatorial diameter of melon fruits (Table 1).
Table 1 Yield, fruit weight, total soluble solids (TSS), and firmness of melon fruit subjected to different doses of potassium nanofertilizer.
| K NPs (mg L-1) | Yield (ton ha-1) | Fruit weight (kg) | Polar Diameter (cm) | Equatorial Diameter (cm) | Firmness (N) | TSS (°Brix) |
|---|---|---|---|---|---|---|
| Control | 11.58 | 1.43 | 14.92 b* | 13.90 | 12.33 c | 9.83 c |
| 100 | 13.501 | 1.44 | 15.12 ab | 14.06 | 13.26 bc | 10.83 bc |
| 200 | 13.857 | 1.53 | 15.15 ab | 14.05 | 15.83 a | 12.50 a |
| 300 | 14.388 | 1.53 | 15.40 a | 14.19 | 14.32 ab | 11.50 ab |
| 400 | 15.541 | 1.58 | 15.14 ab | 14.00 | 13.32 bc | 9.67 c |
| Significance | NS | NS | * | NS | * | * |
*Means with different letters in the same column are statistically different (Tukey p ≤ 0.05); NS = not significant, * = significant.
Significant differences were observed in the variable's firmness, total soluble solids, and polar diameter. Foliar spraying of K NPs at 200 mg L-1 increased firmness by 28.39% and TSS by 27.17% in melon fruits compared to the control; however, when the dose of K NPs was increased, a decrease in these quality parameters was observed. The highest values for polar diameter were found in the 300 mg L-1 treatment, exceeding the control by 3.11% (Table 1).
Bioactive compounds
K is one of the nutrients with the greatest influence on the quality parameters of crops. The K NPs foliar application at 100 mg L-1 influenced the highest phytochemical biosynthesis of flavonoids, total phenols and antioxidant capacity with increases of 31.25, 23.48 and 8.50%, respectively, in relation to the corresponding controls. However, by increasing the dose from 200 to 400 mg L-1 there was a decrease in phytochemical compounds (Figure 1).
Potassium concentration
K concentration presented a significant difference in melon pulp according to the applied treatments. The treatment 400 mg L-1 registered the highest K concentration in the fruits, exceeding the control by 34.93% (Figure 2).
Discussion
In the present study, foliar application of K NPs did not significantly modify yield relative to the control. The nule or low response of yield, weight, and fruit size to the foliar application of K NPs is probably due to the high K content in the soil, the level of K in the soil of the study site of the present work was 603.93 mg kg-1. Similar results have been reported by Hartz et al. (2005); and Jifon and Lester (2011), who found that at high potassium content in the soil there is no yield response to foliar application with this element, but there is an improvement in fruit quality, due to its role in the transport of sugars, metabolite biosynthesis and enzyme activation (Javaira et al. 2012).
Foliar applications of K NPs resulted in increases in TSS and fruit firmness. Adequate K nutrition has been shown to increase soluble sugars and pulp firmness (Abdullah and Alabdaly 2023). Melon fruit consumption is related to TSS, which is responsible for the sweet taste. It has been determined that a melon fruit is of lower quality if it has <9 °Brix. However, if it presents between 9 to 12 °Brix it is of excellent quality, and if it is >12 °Brix it is considered of extra quality (García-Mendoza et al. 2019). The higher values in firmness and TSS reflect the crucial role of K in enhancing photosynthesis and transporting sugar to demand sites (Marschner 2012), sugars act as osmoregulatory agents to maintain cell pressure, cell structure, stomatal conductance, and cell potential (Saddhe et al. 2021), the above explains the positive relationship between TSS and fruit firmness (Demiral and Köseoglu 2005) as TSS increases the pressure potential (ψp) of fruits (Lester et al. 2010). However, at higher potassium doses the TSS and firmness decrease because its excess alters sugar metabolism (Zhang et al. 2018, Wu et al. 2023), ionic homeostasis (Shen et al. 2017), altering normal calcium absorption affecting the integrity of cell walls; moreover, excess K can generate osmotic stress thus decreasing water absorption by the fruit which affects pressure potential (Lester et al. 2006). A similar relationship between high doses of potassium and decreased TSS and firmness have been reported by Akhtar et al. (2010), Javaira et al. (2012) and (Molina et al. 1992).
Regarding bioactive compounds, the results indicate that low doses (100 mg L-1) of K NPs increase the contents of total flavonoids, total phenols and antioxidant capacity, contrary to the K content in melon fruit, which increased with high doses (300 and 400 mg L-1). Previous studies have shown that foliar application of K at appropriate doses stimulates the biosynthesis of bioactive compounds (Gaaliche et al. 2024, Salama et al. 2024). However, high concentrations of K+ in the cytosol of cells inhibit the synthesis of primary compounds derived from nitrogen metabolism, reducing the amount of energy, amino acids, and consequently, proteins and enzymes, necessary for the biosynthesis of compounds of secondary metabolism (Xie et al. 2021). In this context, plant exposure to high levels of nanoparticles, regardless of their nature, exhibits an overproduction of ROS (Hatami and Ghorbanpour 2024) that cannot be eliminated at the expense of the consumption of antioxidant-derived compounds produced by plants (Huchzermeyer et al. 2022); since ROS play a dual role, at low concentrations, they act as signalers, triggering a moderate stress response in plants and activating the biosynthesis of bioactive compounds, and with overproduction, cellular homeostasis is disturbed, damaging cellular structures, proteins, DNA and lipids (Singh et al. 2024).
Foliar fertilization with K NPs can contribute to supplementing the average daily potassium intake ranging from 2000 to 3900 mg in people without kidney problems (Choi and Ha 2013); however, it is necessary to complete the requirements with sources (Asaduzzaman et al. 2018), since K plays an important role in the human body and maintains the normal functioning of muscles, heart, and nerves through acid-base balance, enzyme activation, and kidney function (Crawford and Harris 2011), its concentration in harvested agricultural products is a quality parameter in itself.
Conclusions
The nutraceutical quality of melon fruits is strongly influenced by the dose of K NPs used. At low concentrations of K NPs, there is a marked enhancement of non-enzymatic antioxidant activity. However, at higher doses, there is an inhibitory effect on the activity of these antioxidants, together with an increase in potassium concentration in melon flesh. It is not recommended to apply high doses of K NPs, as this leads to an overproduction of ROS, causing cellular stress that negatively affects the yield and nutraceutical quality of the crop. Foliar fertilization with K NPs is a viable strategy to enhance the biosynthesis of bioactive compounds in melon fruits.
