1. Introduction
Strawberry is one of the most valuable cash crops for both domestic consumption and export. It is high in vitamins (B9 and C), minerals (P, K, and Ca), and fiber, and it is one of the best natural sources of antioxidants [
1]. The use of prohibited chemical fertilizers and pesticides has recently caused environmental pollution in the strawberry cultivation sector, and the untrustworthy results of biological monitoring have been thoroughly investigated [
2]. Although that chemical fertilizers increase crop productivity, they produce several hazardous residues that have massive negative impacts on the ecosystem and human health, as well as sustainability losses, and water contamination [
3,
4,
5].
Innovative products, such as nontoxic fertilizers, should be used to lessen agricultural chemicals’ risks and adverse effects. In this regard, nanotechnology has provided useful and distinctive applications in the agricultural industry [
6]. Nanofertilizers have been widely used to promote plant growth and to increase crop yields [
7]. Nano-engineered elements have been shown to be less toxic than macro- and micro-engineered elements [
8]. Nanofertilizers having a size that is less than the pore size of the cell wall, and therefore can easily move through the cell wall and migrate up to the plasma membrane. After gaining entry into the cell, nano-sized elements can move through both apoplastic and symplastic pathways, ultimately affecting plants’ various physiological and metabolic activities [
9].
Potassium (K) is an essential nutrient for plant growth which influences carbohydrate metabolism, fruit quality, and stress tolerance [
10,
11]. Potassium in soil is classified into four forms depending on the basis of its availability to plants, i.e., water-soluble K, exchangeable K, K that is difficult to exchange or fix in the lattice structure of clay minerals, and total K [
12]. Among these forms, only two forms are available for plant absorption, water-soluble K (that constitutes about 0.1–0.2% of the total amount) which can be absorbed by plants and microbes but is prone to leaching, and exchangeable K (1–2% of the total K in soil) [
13]. Furthermore, it is well known that sandy soils are low in nutrients, especially K. Therefore, the addition of K to soil is essential for sustaining plant growth and yields [
13]. Compared with traditional fertilizers, nano-K fertilizer for grapes has been shown to significantly increase plants’ growth, yields, and nutrient contents [
14]. In addition, [
9] found that K nanoparticles enhanced wheat’s morphological, biochemical, and yield-based parameters.
Nano-chitosan is a chitin-derived biopolymer that improves plant root growth, soil porosity, and water-use efficiency. Chitosan derivatives (e.g.,
N-succinyl and
N,
O-carboxymethyl chitosan) have been reported to improve chlorophyll fluorescence and photosynthesis [
15]. Chitosan has been shown to regulate gene expression in plant defense pathways, and this has helped to protect fruit for long periods, especially after fruit has been harvested and is in storage [
16]. Furthermore, nano-chitosan is a biocompatible and biodegradable material, and composites containing chitosan have performed well without harming naturally occurring beneficial soil microbes [
15,
17]. In addition, because it improves particle–particle cohesion, nano-chitosan has the potential to be a soil stabilizer [
17].
Nanofertilizers assist in minimizing fertilizer waste, reducing environmental contamination, and improving plant nutrient bioavailability [
18,
19]. According to [
20], nano-chitosan-NPK fertilizer increased the growth and productivity of wheat planted in sandy soil. It also acted in soil as a slow-release fertilizer that conserved nutrients from chemical processes such as denitrification, hydrolysis, and leaching that affect the presence of nutritients in soil [
20]. To the best of our knowledge, no research has been conducted on the effect of nano-chitosan-K fertilizer on strawberry growth, nutrition, and antioxidant properties. We hypothesized that the combination of K and chitosan would have a compatible and beneficial effect on the yield and quality of strawberry crops. The primary goal of this study was to determine the effect of nano-chitosan-K fertilizer on strawberry plants growth, yields, nutrient uptake, and juice quality. In addition, in this study, we examined K fractions in cultivated soil.
4. Discussion
Regarding the concentration of available K in the studied soil before cultivation, it reached 168 µg g
−1 (
Table 1), which is considered to be medium, as reported by [
33]. Many factors affect K dynamics in the soil such as the soil physio-chemical properties, soil microbial activities, and soil–plant interactions [
13]. Therefore, it is necessary to add K fertilizers to replenish the depletion of K in soil fertility to meet a plant’s requirements during its different growth physiological stages. In the combined two studied seasons of 2019/2020 and 2020/2021, it was found that increasing the soluble K, exchangeable K, and total K by applying 150.0 K
2SO
4 (100% of the recommended K
2SO
4) followed by 112.5 K
2SO
4 + 1000 mg L
−1 nano-chitosan-K (75% of the recommended K
2SO
4 combined with 1000 mg L
−1 nano-chitosan-K) increased soil K availability (
Figure 2). K
2SO
4, a good solubility, chlorine-free K fertilizer, is the most effective way to increase soil K content [
34]. However, applying 1000 mg L
−1 nano-chitosan-K without K
2SO
4 or adding 50% of the recommended dosage yielded the highest percentage of fixed K in sandy soil. Due to the decreased K fertilizer application in the soil, the concentration in this fraction was incorporated into the crystal lattice structure of clay minerals [
13]. In this regard, [
9] found that nano-K-sprayed wheat plants utilized the maximum amount of exchangeable K from sandy loam soil leading to the least amount of K loss by leaching. Nano-chitosan is a cationic biopolymer considered to be one of the most attractive highly reactive biopolymers mainly due to the presence of amino and hydroxyl functional groups on its backbone structure that embrace chemical linking with nutrients such as K [
20,
35]. The new nanocomposite works in the soil as a slow-release fertilizer, conserving K from leaching and increasing its bioavailability. Therefore, combining the two components of nano-chitosan and nano-K had a better effect on soil, and consequently on the cultivated plants [
20].
The improvement of vegetative growth characteristics such as plant height, fresh and dry weights, numbers of leaves per plant, and leaf areas of strawberry plants with an increasing K fertilization rate either in soil or by spraying nano-chitosan-K on foliage was of interest (
Table 3). The best results were with the combination of the two applications. This could be attributed to the increased uptake of K and its associated role in enzyme activation, protein synthesis, photosynthesis, osmoregulation, stomatal movement, energy transfer, phloem transport, cation–anion balance, and stress resistance [
36,
37,
38]. Similarly, [
9] proved that K had a stimulatory effect on the weight of wheat plants and better results were obtained when nanoformulation was applied as compared with conventional fertilizers.
Because the application of the T2 and T3 treatments caused an increase in the availability of nutrients, which, in turn, caused an improvement in the ability of plant roots and leaves to absorb nutrients, a high level of NPK and chlorophyll was found in the strawberry plant foliage (
Table 4). Due to the fact that the nanoparticles had a high specific surface area and, consequently, a high reactive potential, higher absorption of nano-chitosan-K was achieved, and this resulted in a beneficial influence on plant growth [
18]. Increases in both the NPK and chlorophyll content in plant foliage have been linked to the use of chitosan-K, which aided plant absorption of soil-water and nutrients, increased chlorophyll synthesis in plant leaves, and ultimately improved photosynthesis [
39], which was reflected in the robust growth of plants.
Crop yields increased with increased plant growth and nutrient uptake. T2 and T3 improved the early fruit yield, marketable fruit yield, total fruit yield, and fruit firmness (
Table 5). Crop productivity increased within the range from 6% to 17% with nanofertilizers. The ease with which nanofertilizers penetrated the stomata of leaves caused the increase (via a topical application). Since nutrient use efficacy is increased by at least 20% for most applied nutrients, nano-based fertilizers are much more effective than conventional fertilizers [
17]. Chitosan nanoparticles are easily absorbed by leaves and translocated to stems to boost plant growth and the yields of different crops [
40].
Since both chitosan and K in the form of nanoparticles (
Figure 1) are tiny in size, have high absorption capacity, and are dispersed and rapidly and optimally absorbed and taken up by plants [
41], treating plants with nano-chitosan-K could effectively meet their nutrient needs [
42]. The physical and chemical properties of nanoparticles that are small (1 to 200 nm) seem to be more effective [
43,
44].
Chitosan-K inhibits cell wall disintegration, slows the aging process, and decreases the formation of H
2C=CH
2 [
36,
45]. Thus, it is likely that the effectiveness of the T2 treatment was demonstrated by maintaining fruit firmness due to a reduction in ethylene formation. In addition, nano-chitosan has been shown by [
46] to improve plant metabolic activity and to facilitate the transport of active chemicals across cell membranes. It also had positive effects on the productivity and quality of the plants. Because of its natural origin, non-toxicity, safety, and biodegradability, nano-chitosan has the potential to replace agrochemicals in the reduction of abiotic stresses [
45]. Furthermore, K is a macronutrient that serves the same purpose in protecting plants from abiotic stresses [
11]. Foliar application of nano-chitosan-K resulted in the increased accumulation of TSS, total sugar, and vitamin C in plant cells, which were osmotic adjustments and important indicators for fruit quality, in addition to fruit acidity and anthocyanin content which were responsible for fruit color [
36,
46] (
Table 6). The potential role of chitosan in enhancing the availability and uptake of water and nutrients by regulating cell osmotic pressure and enzyme activities could explain its augmentative effect on strawberry growth and yield while improving fruit quality [
47,
48]. Additionally, the foliar application of chitosan has been shown to maintain the membrane stability of the leaf and to increase the levels of antioxidant enzymes in apple [
49]. It has been demonstrated that chitosan improved sugar metabolism [
50], thereby improving the quality of strawberry fruit juice. Similarly, a K supply induced stress tolerance with improvements in growth, chlorophyll synthesis, antioxidant enzyme activation, gas exchange traits, and sugar content [
51]. In addition, K can play a crucial role in plant–water relations by encouraging water uptake by plants. This helps plants to achieve optimal turgor and membrane stability [
52]. K is essential for photosynthesis because it facilitates translocation [
53], modifies the osmotic charge [
54,
55], and improves crop growth, productivity, and quality [
56].
The principal component analysis (PCA) biplots were useful for visualizing the relationship between treatments and the fruit production and quality characteristics of the strawberry plants that were tested (
Figure 3). The first two principal components, as shown in the biplots, reflected more than 60% (90.62%) of the total variance. The utilized biplot model, therefore, provided a good fit. According to [
57], the first two principal components should reflect more than 60% of the total variation in order to obtain a good fit for the biplot model. In addition, the ideal test trait should have the largest vector of all traits because it should be able to effectively differentiate between treatments [
58]. The results that were obtained were consistent with those that were reported by [
59,
60] concerning the effect of nanofertilizers, including chitosan nanoparticles, on the quality and productivity of strawberries. As a result, the use of exogenously applied beneficial compounds in a nano form, such as chitosan and K, achieved favorable progress in plant nutrition, resulting in high quantity and a high quality of crop products.