3.1. The effect of nutritional treatments on Morphological characteristics and yield
The highest bunch fresh and dry weight (73.97 g, and 69.24 g, respectively) were recorded for S.K2×H2 treatment, and the lowest (38 g, and 33.79 g respectively) for the control (Fig. 1). Mean comparison also showed that the highest fresh and dry weight of grape berries (6.99 g and, 1.26 g respectively) were observed in S.K2×H2 treatment. The mean comparison revealed that the S.K2×H2 treatment produced the highest yield (10.56 kg/tree).
Silica has been reported to improve the growth and yield of cucumber [22], tomato [23, 24], and strawberry [25].
3.2. The effect of nutritional treatments on a, b, and total Chlorophyll
2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid treatment led to the highest total chlorophyll content (2.67 mg FW), and the lowest total chlorophyll content was in the control treatment (1.19 mg g− 1 FW) (Fig. 2a). Mean comparison also showed that the highest concentrations of chlorophyll a (1.633 mg g− 1 FW) was in 2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid treatment (Fig. 2b), whereas the highest concentration of chlorophyll b (1.04 mg g− 1 FW) was found in 2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid and 2000 mg. L− 1 K2SO3 × 1000 mg. L− 1 humic acid treatments (Fig. 2c).
Park et al. [26] found that the use of different sources of silicon affected the amount of photosynthesis-related proteins. Soundararajan et al. [27], studying Rosa hybrida ‘Rock Fire’ under salt stress, reported that Si increased photosynthesis-related proteins by 20%. Application of potassium silicate increased the chlorophyll content (a, b) on orange trees [28]. Silicon can affect dry weight through two mechanisms: (1) maintaining the integrity of the chloroplast and chlorophyll structures, hence photosynthesis, which leads to increased light absorption [2, 29, 30]; and (2) increasing water storage capacity by raising the level of aquaporin gene expression especially SbPIP genes in the root [32] and increasing the hydraulic conductivity of the root [31]. Mohamadineia et al. [33] stated that the highest level of total chlorophyll in grapes was obtained with foliar application of humic acid at 5 g.L− 1. Khazaie and Kafi [34] stated that the increase in chlorophyll concentration could be due to the effect of humic acid in absorbing elements such as N.
3.3. The effect of treatments on the contents of flavonoids, total phenolics, anthocyanins, and soluble sugars
The highest levels of phenolics and flavonoids were recorded for 2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid (77.7, and 0.17 mg. g− 1 FW, respectively). The highest anthocyanin level was observed in the 2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid treatment (1.22 mg. g− 1), and the lowest in the control (0.25 mg. g− 1)., There was no significant difference between S.K2×H2, S.K1×H2, and S.K2×H1 treatments regarding soluble sugar content, being 1.076, 0.848, and 0.870 mg.g− 1 FW, respectively.
Phenolic compounds, a group of secondary metabolites, are widely distributed throughout plants and have several biological effects, including antioxidant and antibacterial activities, which play an important role in neutralizing free radicals and suppressing oxygen molecules through the displacement or decomposition of peroxides [35]. In addition, anthocyanins are among the secondary metabolites that are produced in the process of fruit ripening depending on the variety and environmental situations and nutritional status of the plant. They also provide protection from oxidative stress in plants; the aromatic ring is scavenging ROS to reduce oxidative damage with free OH groups attached and chelating metals [36]. The use of K2SiO3 in ‘Paros’ strawberry cultivar increased the content of flavonoids and phenolics in the fruit [25], which is consistent with our findings. Abd-Elkader et al. [37] stated that the foliar spraying of K2SiO3 increased the total phenolics in Zucchini fruit (Cucurbita pepo L.), while it had no effect on its flavonoid content. In some studies, the foliar application of potassium and iron, while improving the carbohydrate status of the plant, led to an increase in the concentration of anthocyanin and increased the quality of the fruit [38]. Zahedi et al [36] showed that the use of silica nanoparticles increased the antioxidant activity and content of phenolics in strawberry. The increase in total phenolics following the treatments of K2SiO3 could be due to the effect of Si in the induction of several changes in the phenolic compounds especially under abiotic and biotic stresses [39].
In the study of Ferrara et al. [40], foliar application of humic acid on Vitis vinifera L. increased soluble carbohydrates, which is in agreement with our findings. Moreover, with the use of foliar K, the nutrients are readily available to the plant. By facilitating the transfer of photosynthetic products, elements and secondary metabolites are better available to the plant, thus increasing the concentration of carbohydrates [41]. By improving the absorption of nutrients, humic acid enhances photosynthesis and improves the transport of nutrients to different parts of the plant, thus increasing soluble carbohydrates [42].
3–4. The effect of treatments on enzymatic activity
The highest CAT, POX, and SOD activities (0.736, 0.162, and 0.321 nmol min mg protein) were found in 2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid treatment (Fig. 4). The activity of antioxidant enzymes depends on the plant species, organ/tissue, and stress extent (43). Zangeneh and Rasouli [44] stated that in white quince grapes, the SOD enzyme activity increased linearly with the increase of humic acid concentration from zero to 2000 mg/liter. Furthermore, the use of humic acid in Malus robusta increased the activity of the superoxide dismutase enzyme [45]. In another study, the application of K2SiO3 increased the activity of the superoxide dismutase in grapes [46], which is in agreement with the findings of this study. On the contrary, Hafez et al. [47] reported that the use of K2SiO3 was effective in reducing the activity of antioxidant enzymes (CAT, POD, and SOD) in Vicia faba L. plants. Our results show that the use of K2SiO3 and humic acid causes a synergistic effect in increasing the activity of antioxidant enzymes.
3.5. The effect of nutritional treatments on the uptake of micro elements
The treatment with K2SO3 and humic acid had a significant effect on the Zn, Mn, Fe, and Cu concentrations in grape leaves (Fig. 5). The highest concentration of leaf Cu was found in S.K2×H2, S.K2×H1, and S.K1×H1 treatments, being 0.118, 0.114, 0.113, and 0.110 µg g− 1 DW, respectively. The lowest value was in the control (0.016 µg g− 1 DW) (Fig. 5a). Moreover, the highest fruit Zn concentration was in S.K2×H2, S.K1×H2 and S.K2×H1 treatments (1.45, 1.38, and 1.12 µg g− 1 DW, respectively), and the lowest in the control (0.66 µg g− 1 DW) (Fig. 5b). The highest Mn concentration was in 2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid (3.36 µg g− 1 DW) and the lowest in control (without treatment) (Fig. 5c). The concentration of Fe was higher in 2000 mg.L− 1 K2SO3 × 2000 mg.L− 1 humic acid (26.24 µg g− 1 DW) and the lowest in control (13.85 µg g− 1 DW) (Fig. 5d). Our findings showed that the application of K2SO3 and humic acid can change Cu, Mn, Zn, and Fe levels in grapes. In all treatments, they had a synergistic effect on the measured micro-elements.
After being absorbed by the root, Si is transferred to the aerial parts of the plant and is deposited on the cell wall in the form of hydrated polymer, amorphous silica, silica-cuticle double layer, and silica-cellulose double layer on the surface of the leaf and stem and reduces transpiration [48, 49]. In addition, the presence of Si in the root endoderm may reduce the apoplastic uptake of water and some ions [50]. These results show that Si, probably by increasing the activity of ATPases and thus improving the function of the membrane, was able to increase the concentration of iron in the plant and reduce the severity of iron deficiency stress [51]. Miao et al. [52] confirmed that Si alleviates some of the consequences of potassium deficiency in soybeans with the aid of growing the interest of antioxidant enzymes and reducing the quantity of hydrogen peroxide. Cao et al. [53] showed that Si probably reduces the intensity of oxidative stress and chlorophyll deficiency in Oriza sativa L. by improving iron absorption and enhancing growth in Fe deficiency conditions. The research indicated that the use of Si increases the absorption and accumulation of Mg and Mn [54, 55]. Moreover, potassium accelerates the transfer of photosynthic materials and increases the concentration of Mn by increasing the photosynthetic capacity [56].
Davarpanah et al. [57] stated that the use of humic acid on pomegranate (Punica granatum cv. Ardestani) increased the leaf concentration of Zn, whereas it had no significant effect on the leaf Fe concentration. In the study of Poozeshi et al. [58] on Vitis vinefera cv. Peykani, the application of humic acid increased the concentration of leaf Mn. It seems that humic acid increases the concentration of Mn due to its property of chelating elements [59].
Accordingly, humic acid can improve the availability of essential nutrients such as nitrogen, phosphorus, and potassium to trees. Potassium silicate can also enhance the uptake of potassium, which is important for fruit tree growth and development [60, 61]. Moreover, water-holding humic acid can help improve the the soil by increasing its water holding capacity, aeration, and nutrient retention [62]. This can create an extra favorable surroundings for tree roots to grow and access nutrients. Potassium silicate can enhance the structural integrity of tree cells, making them stronger and better able to withstand external pressures. Overall, the use of humic acid and potassium silicate can contribute to healthier, more vigorous trees with improved growth and fruit quality [63].