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Full Research Article

Increasing Fruit Yield and Bioactive Compounds of Strawberry Plants by Foliar Spraying with Some Environmentally Friendly Substances

A
Ashwaq Wadi Majeed1,*
https://orcid.org/0000-0002-4420-1272
Q
Qusay T. Salem1
M
Muhammad Qasim Abdulhamid1
N
Noor Yahya Muneer Shaker2
R
R.A.A. Al-Asadi1
N
Nisreen M. Hathal3
1Medicinal and Aromatic Plants Research Unit, College of Agriculture Engineering Sciences, University of Baghdad, Iraq.
2Department of Medical and Pharmaceutical Sciences, Ibn Sina University, Baghdad, Iraq.
3College of Agriculture, University of Diyala, Diyala, Iraq.

ABSTRACT

Background: The research study was conducted in the plastic houses belonging to the College of Agricultural Engineering Sciences/University of Baghdad / Al-Jadriya, for the autumn season of 2023-2024. The experiment was laid out in factorial experiment under a randomized complete block design (RCBD) with three replications and six plants per treatment.

Methods: The experiment was of two factors: the first factor represented foliage spraying by liquid potassium silicate fertilizer with three concentrations (0, 4 and 8) mL L-1 and the second factor was Roselle extract (RE) at three concentrations (0, 5, 10) g L-1.

Result: The results indicated a significant increase in most of the studied traits when treated with potassium silicate fertilizer and Roselle extract individually. Furthermore, the interaction treatment (Si2R2) showed a significant effect in increasing leaf area (46.23 cm2), leaf chlorophyll content (283.1 mg.100 g-1 fresh weight), leaf content of Si, N, P and K (12.64, 2.61, 0.485 and 1.725%, respectively), fruit content of vitamin C, anthocyanin and TSS percentage (12.000, 106.95 and 9.00%, respectively), number of fruits (47.00 fruits.plant-1) and fruit weight (23.35 g). The interaction treatment (Si2R1) excelled in leaf content of total phenols (0.915 mg g-1 dry weight).

INTRODUCTION

Strawberry (Fragaria × ananassa Duch.) is a crop of high economic importance, cultivated widely throughout the world (Xiong et al., 2021). It has high nutritional demands relative to its fruit yield, is susceptible to disease and pest damage and produces perishable fruit with a short storage life owing to its high respiration rate and water content (Barkaoui et al., 2021; Hocking et al., 2016). Strawberry fruits are rich in vitamin C, anthocyanins, minerals and other bioactive compounds (Sharma et al., 2024). Turkey is the world’s third-largest strawberry producer, with a production of 669,195 tons annually (FAO, 2018).
       
Silicon plays an important role in regulating sugar metabolism (Hosseini et al., 2019), nutrient transport across the cell membrane and cell wall properties, thereby benefiting plant growth and productivity. Potassium silicate further contributes through potassium’s regulatory functions in plant metabolism (Abou-Baker et al., 2011) and by improving water relations and photosynthetic activity under stress (Das et al., 2017; Goyal et al., 2025; Vennapusa et al., 2024). Foliar potassium silicate has been reported to increase plant height, leaf number, vitamin C and TSS content in strawberry (Nada, 2020; Mohammed and Majeed, 2024) and to improve growth, productivity and quality in strawberry and other crops such as Jerusalem artichoke and sweet potato (Abou El-Khair and Mohsen, 2016).
       
Researchers have shown growing interest in plant extracts as environmentally friendly alternatives to chemical substances. Roselle, in particular, contains a distinctive combination of bioactive components including sugars, organic acids and polyphenols; its anthocyanins are natural, non-toxic, water-soluble antioxidant pigments (Gadel-Kareem and Abdel-Rahman, 2013; Radwan et al., 2019), with documented antioxidant, anti-glycosylation and antibacterial properties (Lai et al., 2024). Foliar spraying with Roselle extract (0, 10, 20 and 30%) increased strawberry leaf number, leaf area, chlorophyll content and vegetative dry weight, with the greatest response at the 30% concentration (Salloom and AL-Hchami, 2020).
       
In pursuit of sustainable development goals, this study aimed to examine how strawberry plants respond to silicon and Roselle extract and their effect on the qualitative characteristics of the fruits and their content of total phenols and some antioxidants.

MATERIALS AND METHODS

Work site
 
The research the Festival strawberry variety was conducted in plastic houses belonging to the College of Agricultural Engineering Sciences / Baghdad University/ Al-Jadriya, in autumn season 2023-2024, Strawberry plants (Fragaria × ananassa Duch.) cv. Festival were used in this study. Beds were made with a height of 25 cm and a width of 70 cm for the upper surface and then the beds were covered with black plastic (mulching) to maintain moisture, prevent weed growth and preserve the fruit. Then the young seedlings were planted in three rows with a distance of 50 cm between rows and a distance of 25 cm between seedlings.Greenhouse environmental conditions during the experiment were 18-34°C and 50-85% relative humidity, while the average outdoor temperature in the Al-Jadriya area of Baghdad ranged from a maximum of 42°C at the beginning of the autumn season to a minimum of 11°C by its end, with a gradual increase in relative humidity as winter approached.
 
Experimental design
 
A factorial experiment was conducted using a randomized complete block design (RCBD) with three replications and six plants per experimental unit, making 162 plants in all the experiment. The experiment performed with two factors: the first factor was spraying with liquid potassium silicate fertilizer at three rates (0, 4 and 8) ml L-1, designated (Si0, Si1 and Si2) respectively; the second factor was adding Roselle extract at three rates (0, 5 and 10) g L-1, designated (R0, R1 and R2) respectively. The fertilization and spraying treatments were repeated five times every 20 days, from October 1st to January 20th.
 
Potassium silicate fertilizer
 
The potassium silicate fertilizer used (LandCo®, Land Co Company, Iran) is a liquid organic fertilizer containing 12% potassium (K2O), 27% silica (SiO2), 1.35% free amino acids and 1.5% organic matter. It was applied as a foliar spray at concentrations of 4 and 8 mL L-1 (Si1 and Si2, respectively), with plants sprayed until full leaf wetting (spray-to-run-off), five times during the growing season at 20-day intervals, as described above.
 
Preparation of Roselle extract
 
Roselle (Hibiscus sabdariffa L.) leaves were washed and cut into small pieces, then dried in a hot-air oven at 45°C for 48 h and ground into a fine powder using an electric grinder. The powder was macerated in 70% ethanol, stirred until homogeneous and allowed to stand for 24 h at room temperature. The mixture was filtered through filter paper and the residue was re-extracted twice with fresh solvent under the same conditions (24 h each). The combined filtrates were concentrated using a rotary evaporator at 70°C under reduced pressure. The concentrated crude extract was subsequently diluted with distilled water to prepare the working concentrations of 5 and 10 g L-1 used in this study (R1 and R2, respectively), which were applied as a foliar spray five times during the growing season at 20-day intervals, together with the potassium silicate treatments described above.
 
Treatments
 
Si0R0 = control (sprayed with water only)
Si0R1 = 0 mL L-1 potassium silicate + 5 g L-1 Roselle extract
Si0R2 = 0 mL L-1 potassium silicate + 10 g L-1 Roselle extract
Si1R0 = 4 mL L-1 potassium silicate + 0 g L-1 Roselle extract
Si1R1 = 4 mL L-1 potassium silicate + 5 g L-1 Roselle extract
Si1R2 = 4 mL L-1 potassium silicate + 10 g L-1 Roselle extract
Si2R0 = 8 mL L-1 potassium silicate + 0 g L-1 Roselle extract
Si2R1 = 8 mL L-1 potassium silicate + 5 g L-1 Roselle extract
Si2R2 = 8 mL L-1 potassium silicate + 10 g L-1 Roselle extract
 
Measured traits
 
Leaf area (cm2)
 
Measured using the disc area method (Dvornic, 1965) and estimated according to the following formula:
 
Leaf Chlorophyll Content (mg 100 g-1 fresh weight)
 
Leaf chlorophyll content was assessed following the protocol outlined by Goodwin (1976), Six sheets were taken from each experimental unit and washed with distilled water. A weight of 0.2 g of the sample was taken and placed in an opaque glass bottle. 20 ml of acetone was added and the sample was left for 48 hours. The filtrate was then separated from the precipitate using filter paper and the results were read using a spectrophotometer at wavelengths of 645 and 663. The quantity was then calculated according to the following equation:
Chlorophyll a (μg/mL) = 12.7 D663 - 2.69 D645
Chlorophyll b (μg/mL) = 22.9 D645 - 4.68 D663
Total chlorophyll (μg/mL) = 20.2 D645 + 8.02 D663 (Note that D stands for wavelength reading).
 
Leaf silicon content (%)
 
Silicon was estimated according to the colorimetric method mentioned in Elliott and Snyder (1991).
 
Leaf nitrogen content (%)
 
Nitrogen was estimated according to the method mentioned in Chapman and Pratt (1961).
 
Leaf phosphorus content (%)
 
Phosphorus was determined according to the method reported in Page (1982).
 
Leaf potassium content (%)
 
Potassium was determined according to the method proposed by Haynes (1980).
 
Leaf Total phenolic compounds concentration (mg g-1 dry weight)
 
Leaf total phenolic content was determined according to the method reported by Mahadevan and Sridhar (1986).
 
Ascorbic acid (V.C mg 100 ml-1)
 
Fruit vitamin C content was determined using the direct colorimetric method described in Abbas and Abbas (1992).
 
Fruit anthocyanin (mg 100 g-1 fresh weight)
 
Fruit anthocyanin pigment content was measured as described in Ranganna (1977).
 
Number of fruits plant-1
 
The total fruits harvested were counted and then divided by the number of plants in both seasons.
 
Single-fruit weight (g)
 
Fruit weight was calculated by dividing the total fruit weight per plant by the total number of fruits per plant.
 
Total soluble solids percentage (T.S.S)
 
This percentage, which represents sugars, organic acids and other soluble solids in the fruit juice, was measured using a hand refractometer (Abbas and Abbas, 1992).
 
Statistical analysis
 
The ready-made Genstat program was used to analyze the results and the differences between the means were compared according to the Least Significant Difference (LSD) test under a probability level of 0.05 (Al-Rawi and Khalaf Allah, 2000).

RESULTS AND DISCUSSION

Vegetative growth and leaf nutrient content
 
Potassium silicate fertilizer and Roselle extract significantly increased leaf area, chlorophyll content and leaf silicon, nitrogen, phosphorus, potassium and total phenolic content (Table 1-7). For chlorophyll, silicon, phosphorus and potassium, Si2 gave the highest and Si0 the lowest values, Roselle extract followed a RE2 > RE1 > RE0 pattern and the Si2RE2 interaction consistently gave the highest values. Leaf area was highest under Si1 (37.94 cm2) and RE2 (40.56 cm2, not differing from RE1 at 30.83 cm2), with the Si2RE2 interaction (46.23 cm2) not differing from Si1RE2 (42.50 cm2). For leaf nitrogen, Si2 (not differing from Si1) and RE1 (not differing from RE2) gave the highest means. For total phenolic content, Si2 was highest, RE1 exceeded RE0 and the Si2RE1 combination (rather than Si2RE2) gave the overall highest value.

Table 1: Effect of spraying with potassium silicate and Roselle extract on the leaf area (cm2) of strawberry plant.



Table 2: Effect of spraying with potassium silicate and Roselle extract on the chlorophyll content of leaves (mg.100g-1 fresh weight) of strawberry plants.



Table 3: Effect of spraying with potassium silicate and Roselle xtract on the silicon content (%) of strawberry leaves.



Table 4: Effect of spraying with potassium silicate and Roselle extract on the nitrogen content (%) of strawberry leaves.



Table 5: Effect of spraying with potassium silicate and Roselle extract on the phosphorus content (%) of strawberry leaves.



Table 6: Effect of spraying with potassium silicate and Roselle extract on the potassium content (%) of strawberry leaves.



Table 7: Effect of spraying with potassium silicate and Roselle extract on total phenol content (mg g-1 dry weight) of strawberry leaves.


 
Fruit yield and quality traits
 
Potassium silicate and Roselle extract also significantly increased fruit vitamin C, anthocyanin, number of fruits, single-fruit weight and TSS content (Table 8-12), with Si2 consistently giving the highest values (not differing from Si1 for TSS) and Si0 the lowest. Roselle extract likewise increased all five traits, with RE2 generally highest-except vitamin C, where RE1 was highest-versus RE0. The Si2RE2 interaction gave the highest value for all five traits, not significantly different from Si2RE1 (vitamin C), Si2RE0 (anthocyanin), or Si2RE1, Si2RE0 and Si1RE2 (TSS).

Table 8: Effect of spraying with potassium silicate and Roselle extract on the fruit content of vitamin C (mg 100 ml-1).



Table 9: Effect of spraying with potassium silicate and Roselle extract on the anthocyanin content (mg 100 g-1 fresh weight) of strawberry fruit.



Table 10: Effect of spraying with potassium silicate and Roselle extract on the number of fruits (fruit. plant-1) of strawberry plants.



Table 11: Effect of spraying with potassium silicate and Roselle extract on the weight of a single fruit (g) of strawberry plants.



Table 12: Effect of spraying with potassium silicate and roselle extract on the TSS (%) ratio of strawberry plants.


     
The significantly positive response recorded in the current findings can be attributed to the chemically-rich composition of Roselle extract, which is rich in anthocyanins, organic acids and phenolic compounds (Zydlik et al., 2024; Pons, 2003). These bioactive compounds act not only as exogenous antioxidants but also as biostimulant signaling molecules, an effect recently confirmed in Roselle itself, where foliar plant-extract biostimulants enhanced chlorophyll retention, antioxidant enzyme activity and secondary metabolite content under stress conditions (Bahgat et al., 2023), consistent with the improved growth, yield and fruit quality observed here.
     
These findings are in agreement with previous studies on foliar application of plant extracts in seedless grape (Gadel-Kareem and Abdel-Rahman, 2013), Calendula (Saleh et al., 2019), lemon (Majeed and Altaai, 2023) and pepper (Salloom et al., 2023), all reporting improved vegetative growth, yield and fruit quality. The higher TSS and vitamin C content observed in Roselle-treated fruits can be attributed to increased photosynthetic efficiency and greater photosynthate translocation to the fruits, together with a cytokinin-mediated reduction in respiration rate (Al-Asady et al., 2019); slower respiration lowers the consumption of soluble solids and protects vitamin C from oxidative loss. These results are consistent with Ismail and Ganzour (2021), who reported increased vegetative growth of strawberry using natural extracts.
     
The significance of potassium silicate in sustainable fruit production under stress conditions is well documented (Ladon et al., 2021) and was also evident here in flowering and yield parameters, reflecting the dual role of its two building blocks (Khan, 2025). Silicon controls nutrient absorption and translocation across the cell membrane and strengthens the cell wall (Xiao, 2022; Xu, 2023), favouring vegetative growth and yield. Recent strawberry-specific studies confirm that foliar/root-applied silicon enhances marketable yield, fruit weight and fruit number (Jarosz et al., 2025) and increases photosynthetic pigment concentration while reducing oxidative stress (Kashani et al., 2025), directly supporting the leaf area and chlorophyll improvements recorded here. Potassium itself is involved in numerous metabolic processes (Nithin, 2020; Cuin, 2018), directly affecting photosynthetic efficiency (Tränkner et al., 2018), promoting cell expansion (Prado, 2021) and increasing assimilate translocation to plant organs (Sustr et al., 2019).
       
The enhanced responses under the two-way interaction reflect additive and synergistic effects between the factors. Because potassium is involved in transporting assimilates and nutrients from leaves into the fruits, it also raised fruit TSS and vitamin C concentration, alongside its role in regulating sugar metabolism and hormonal balance (Hosseini et al., 2019). The enhanced leaf mineral content following silicon application may reflect stimulation of the H+-ATPase enzyme (Saaseea, 2018; Nada, 2020), improved transpiration regulation and cellular water status (Koly, 2024) and mechanical reinforcement of the cell wall (Tränkner et al., 2018), together with enhanced antioxidant enzyme activity (Zheng et al., 2019), consistent with similar studies (Nada, 2020; Liu et al., 2017).
       
The synergistic response observed under the combined Si + Roselle extract treatments (particularly Si2RE2) suggests that the two inputs act through complementary rather than redundant pathways: silicon primarily reinforces structural and water-regulatory functions, while the Roselle extract contributes an exogenous antioxidant and biostimulant load. This complementarity is consistent with reports of enhanced performance when silicon is co-applied with plant-derived biostimulant extracts in related species, where the combined foliar application of potassium silicate and a plant extract produced greater improvements in growth and physiological parameters in Roselle (Hibiscus sabdariffa L.) than either input alone (Abou-Sreea et al., 2022). To our knowledge, the present study is among the first to evaluate this specific combination in strawberry, which represents a key element of its novelty and practical relevance for sustainable, eco-friendly fruit production systems.

RECOMMENDATION

Foliar application of Roselle extract five times per year is an effective strategy to enhance the growth, yield and quality of strawberry fruits naturally and safely. This approach also highlights its high value in terms of sustainability, nutritional benefits and environmental friendliness, making it an ideal option for farmers seeking eco-friendly and high-quality production.

CONCLUSION

The results showed that the Roselle extract foliar treatment increased growth and productivity as well as physical and chemical properties. So, it is clear that five times foliar spraying of Roselle extract per year is needed for high yield and quality improvement of strawberry fruit. This strategy is also suggested because of its high sustainability and nutritional value as well as for being green process of treatment and a natural biostimulant.

ACKNOWLEDGEMENT

The authors would like to thank the Department of Horticulture, College of Agriculture, University of Baghdad, for providing the facilities necessary to conduct this research.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions.
 
Informed consent
 
Not applicable. This study involved plant materials only and did not involve humans or animals.

CONFLICT OF INTEREST

The authors declare no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

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    Full Research Article

    Increasing Fruit Yield and Bioactive Compounds of Strawberry Plants by Foliar Spraying with Some Environmentally Friendly Substances

    A
    Ashwaq Wadi Majeed1,*
    https://orcid.org/0000-0002-4420-1272
    Q
    Qusay T. Salem1
    M
    Muhammad Qasim Abdulhamid1
    N
    Noor Yahya Muneer Shaker2
    R
    R.A.A. Al-Asadi1
    N
    Nisreen M. Hathal3
    1Medicinal and Aromatic Plants Research Unit, College of Agriculture Engineering Sciences, University of Baghdad, Iraq.
    2Department of Medical and Pharmaceutical Sciences, Ibn Sina University, Baghdad, Iraq.
    3College of Agriculture, University of Diyala, Diyala, Iraq.

    ABSTRACT

    Background: The research study was conducted in the plastic houses belonging to the College of Agricultural Engineering Sciences/University of Baghdad / Al-Jadriya, for the autumn season of 2023-2024. The experiment was laid out in factorial experiment under a randomized complete block design (RCBD) with three replications and six plants per treatment.

    Methods: The experiment was of two factors: the first factor represented foliage spraying by liquid potassium silicate fertilizer with three concentrations (0, 4 and 8) mL L-1 and the second factor was Roselle extract (RE) at three concentrations (0, 5, 10) g L-1.

    Result: The results indicated a significant increase in most of the studied traits when treated with potassium silicate fertilizer and Roselle extract individually. Furthermore, the interaction treatment (Si2R2) showed a significant effect in increasing leaf area (46.23 cm2), leaf chlorophyll content (283.1 mg.100 g-1 fresh weight), leaf content of Si, N, P and K (12.64, 2.61, 0.485 and 1.725%, respectively), fruit content of vitamin C, anthocyanin and TSS percentage (12.000, 106.95 and 9.00%, respectively), number of fruits (47.00 fruits.plant-1) and fruit weight (23.35 g). The interaction treatment (Si2R1) excelled in leaf content of total phenols (0.915 mg g-1 dry weight).

    INTRODUCTION

    Strawberry (Fragaria × ananassa Duch.) is a crop of high economic importance, cultivated widely throughout the world (Xiong et al., 2021). It has high nutritional demands relative to its fruit yield, is susceptible to disease and pest damage and produces perishable fruit with a short storage life owing to its high respiration rate and water content (Barkaoui et al., 2021; Hocking et al., 2016). Strawberry fruits are rich in vitamin C, anthocyanins, minerals and other bioactive compounds (Sharma et al., 2024). Turkey is the world’s third-largest strawberry producer, with a production of 669,195 tons annually (FAO, 2018).
           
    Silicon plays an important role in regulating sugar metabolism (Hosseini et al., 2019), nutrient transport across the cell membrane and cell wall properties, thereby benefiting plant growth and productivity. Potassium silicate further contributes through potassium’s regulatory functions in plant metabolism (Abou-Baker et al., 2011) and by improving water relations and photosynthetic activity under stress (Das et al., 2017; Goyal et al., 2025; Vennapusa et al., 2024). Foliar potassium silicate has been reported to increase plant height, leaf number, vitamin C and TSS content in strawberry (Nada, 2020; Mohammed and Majeed, 2024) and to improve growth, productivity and quality in strawberry and other crops such as Jerusalem artichoke and sweet potato (Abou El-Khair and Mohsen, 2016).
           
    Researchers have shown growing interest in plant extracts as environmentally friendly alternatives to chemical substances. Roselle, in particular, contains a distinctive combination of bioactive components including sugars, organic acids and polyphenols; its anthocyanins are natural, non-toxic, water-soluble antioxidant pigments (Gadel-Kareem and Abdel-Rahman, 2013; Radwan et al., 2019), with documented antioxidant, anti-glycosylation and antibacterial properties (Lai et al., 2024). Foliar spraying with Roselle extract (0, 10, 20 and 30%) increased strawberry leaf number, leaf area, chlorophyll content and vegetative dry weight, with the greatest response at the 30% concentration (Salloom and AL-Hchami, 2020).
           
    In pursuit of sustainable development goals, this study aimed to examine how strawberry plants respond to silicon and Roselle extract and their effect on the qualitative characteristics of the fruits and their content of total phenols and some antioxidants.

    MATERIALS AND METHODS

    Work site
     
    The research the Festival strawberry variety was conducted in plastic houses belonging to the College of Agricultural Engineering Sciences / Baghdad University/ Al-Jadriya, in autumn season 2023-2024, Strawberry plants (Fragaria × ananassa Duch.) cv. Festival were used in this study. Beds were made with a height of 25 cm and a width of 70 cm for the upper surface and then the beds were covered with black plastic (mulching) to maintain moisture, prevent weed growth and preserve the fruit. Then the young seedlings were planted in three rows with a distance of 50 cm between rows and a distance of 25 cm between seedlings.Greenhouse environmental conditions during the experiment were 18-34°C and 50-85% relative humidity, while the average outdoor temperature in the Al-Jadriya area of Baghdad ranged from a maximum of 42°C at the beginning of the autumn season to a minimum of 11°C by its end, with a gradual increase in relative humidity as winter approached.
     
    Experimental design
     
    A factorial experiment was conducted using a randomized complete block design (RCBD) with three replications and six plants per experimental unit, making 162 plants in all the experiment. The experiment performed with two factors: the first factor was spraying with liquid potassium silicate fertilizer at three rates (0, 4 and 8) ml L-1, designated (Si0, Si1 and Si2) respectively; the second factor was adding Roselle extract at three rates (0, 5 and 10) g L-1, designated (R0, R1 and R2) respectively. The fertilization and spraying treatments were repeated five times every 20 days, from October 1st to January 20th.
     
    Potassium silicate fertilizer
     
    The potassium silicate fertilizer used (LandCo®, Land Co Company, Iran) is a liquid organic fertilizer containing 12% potassium (K2O), 27% silica (SiO2), 1.35% free amino acids and 1.5% organic matter. It was applied as a foliar spray at concentrations of 4 and 8 mL L-1 (Si1 and Si2, respectively), with plants sprayed until full leaf wetting (spray-to-run-off), five times during the growing season at 20-day intervals, as described above.
     
    Preparation of Roselle extract
     
    Roselle (Hibiscus sabdariffa L.) leaves were washed and cut into small pieces, then dried in a hot-air oven at 45°C for 48 h and ground into a fine powder using an electric grinder. The powder was macerated in 70% ethanol, stirred until homogeneous and allowed to stand for 24 h at room temperature. The mixture was filtered through filter paper and the residue was re-extracted twice with fresh solvent under the same conditions (24 h each). The combined filtrates were concentrated using a rotary evaporator at 70°C under reduced pressure. The concentrated crude extract was subsequently diluted with distilled water to prepare the working concentrations of 5 and 10 g L-1 used in this study (R1 and R2, respectively), which were applied as a foliar spray five times during the growing season at 20-day intervals, together with the potassium silicate treatments described above.
     
    Treatments
     
    Si0R0 = control (sprayed with water only)
    Si0R1 = 0 mL L-1 potassium silicate + 5 g L-1 Roselle extract
    Si0R2 = 0 mL L-1 potassium silicate + 10 g L-1 Roselle extract
    Si1R0 = 4 mL L-1 potassium silicate + 0 g L-1 Roselle extract
    Si1R1 = 4 mL L-1 potassium silicate + 5 g L-1 Roselle extract
    Si1R2 = 4 mL L-1 potassium silicate + 10 g L-1 Roselle extract
    Si2R0 = 8 mL L-1 potassium silicate + 0 g L-1 Roselle extract
    Si2R1 = 8 mL L-1 potassium silicate + 5 g L-1 Roselle extract
    Si2R2 = 8 mL L-1 potassium silicate + 10 g L-1 Roselle extract
     
    Measured traits
     
    Leaf area (cm2)
     
    Measured using the disc area method (Dvornic, 1965) and estimated according to the following formula:
     
    Leaf Chlorophyll Content (mg 100 g-1 fresh weight)
     
    Leaf chlorophyll content was assessed following the protocol outlined by Goodwin (1976), Six sheets were taken from each experimental unit and washed with distilled water. A weight of 0.2 g of the sample was taken and placed in an opaque glass bottle. 20 ml of acetone was added and the sample was left for 48 hours. The filtrate was then separated from the precipitate using filter paper and the results were read using a spectrophotometer at wavelengths of 645 and 663. The quantity was then calculated according to the following equation:
    Chlorophyll a (μg/mL) = 12.7 D663 - 2.69 D645
    Chlorophyll b (μg/mL) = 22.9 D645 - 4.68 D663
    Total chlorophyll (μg/mL) = 20.2 D645 + 8.02 D663 (Note that D stands for wavelength reading).
     
    Leaf silicon content (%)
     
    Silicon was estimated according to the colorimetric method mentioned in Elliott and Snyder (1991).
     
    Leaf nitrogen content (%)
     
    Nitrogen was estimated according to the method mentioned in Chapman and Pratt (1961).
     
    Leaf phosphorus content (%)
     
    Phosphorus was determined according to the method reported in Page (1982).
     
    Leaf potassium content (%)
     
    Potassium was determined according to the method proposed by Haynes (1980).
     
    Leaf Total phenolic compounds concentration (mg g-1 dry weight)
     
    Leaf total phenolic content was determined according to the method reported by Mahadevan and Sridhar (1986).
     
    Ascorbic acid (V.C mg 100 ml-1)
     
    Fruit vitamin C content was determined using the direct colorimetric method described in Abbas and Abbas (1992).
     
    Fruit anthocyanin (mg 100 g-1 fresh weight)
     
    Fruit anthocyanin pigment content was measured as described in Ranganna (1977).
     
    Number of fruits plant-1
     
    The total fruits harvested were counted and then divided by the number of plants in both seasons.
     
    Single-fruit weight (g)
     
    Fruit weight was calculated by dividing the total fruit weight per plant by the total number of fruits per plant.
     
    Total soluble solids percentage (T.S.S)
     
    This percentage, which represents sugars, organic acids and other soluble solids in the fruit juice, was measured using a hand refractometer (Abbas and Abbas, 1992).
     
    Statistical analysis
     
    The ready-made Genstat program was used to analyze the results and the differences between the means were compared according to the Least Significant Difference (LSD) test under a probability level of 0.05 (Al-Rawi and Khalaf Allah, 2000).

    RESULTS AND DISCUSSION

    Vegetative growth and leaf nutrient content
     
    Potassium silicate fertilizer and Roselle extract significantly increased leaf area, chlorophyll content and leaf silicon, nitrogen, phosphorus, potassium and total phenolic content (Table 1-7). For chlorophyll, silicon, phosphorus and potassium, Si2 gave the highest and Si0 the lowest values, Roselle extract followed a RE2 > RE1 > RE0 pattern and the Si2RE2 interaction consistently gave the highest values. Leaf area was highest under Si1 (37.94 cm2) and RE2 (40.56 cm2, not differing from RE1 at 30.83 cm2), with the Si2RE2 interaction (46.23 cm2) not differing from Si1RE2 (42.50 cm2). For leaf nitrogen, Si2 (not differing from Si1) and RE1 (not differing from RE2) gave the highest means. For total phenolic content, Si2 was highest, RE1 exceeded RE0 and the Si2RE1 combination (rather than Si2RE2) gave the overall highest value.

    Table 1: Effect of spraying with potassium silicate and Roselle extract on the leaf area (cm2) of strawberry plant.



    Table 2: Effect of spraying with potassium silicate and Roselle extract on the chlorophyll content of leaves (mg.100g-1 fresh weight) of strawberry plants.



    Table 3: Effect of spraying with potassium silicate and Roselle xtract on the silicon content (%) of strawberry leaves.



    Table 4: Effect of spraying with potassium silicate and Roselle extract on the nitrogen content (%) of strawberry leaves.



    Table 5: Effect of spraying with potassium silicate and Roselle extract on the phosphorus content (%) of strawberry leaves.



    Table 6: Effect of spraying with potassium silicate and Roselle extract on the potassium content (%) of strawberry leaves.



    Table 7: Effect of spraying with potassium silicate and Roselle extract on total phenol content (mg g-1 dry weight) of strawberry leaves.


     
    Fruit yield and quality traits
     
    Potassium silicate and Roselle extract also significantly increased fruit vitamin C, anthocyanin, number of fruits, single-fruit weight and TSS content (Table 8-12), with Si2 consistently giving the highest values (not differing from Si1 for TSS) and Si0 the lowest. Roselle extract likewise increased all five traits, with RE2 generally highest-except vitamin C, where RE1 was highest-versus RE0. The Si2RE2 interaction gave the highest value for all five traits, not significantly different from Si2RE1 (vitamin C), Si2RE0 (anthocyanin), or Si2RE1, Si2RE0 and Si1RE2 (TSS).

    Table 8: Effect of spraying with potassium silicate and Roselle extract on the fruit content of vitamin C (mg 100 ml-1).



    Table 9: Effect of spraying with potassium silicate and Roselle extract on the anthocyanin content (mg 100 g-1 fresh weight) of strawberry fruit.



    Table 10: Effect of spraying with potassium silicate and Roselle extract on the number of fruits (fruit. plant-1) of strawberry plants.



    Table 11: Effect of spraying with potassium silicate and Roselle extract on the weight of a single fruit (g) of strawberry plants.



    Table 12: Effect of spraying with potassium silicate and roselle extract on the TSS (%) ratio of strawberry plants.


         
    The significantly positive response recorded in the current findings can be attributed to the chemically-rich composition of Roselle extract, which is rich in anthocyanins, organic acids and phenolic compounds (Zydlik et al., 2024; Pons, 2003). These bioactive compounds act not only as exogenous antioxidants but also as biostimulant signaling molecules, an effect recently confirmed in Roselle itself, where foliar plant-extract biostimulants enhanced chlorophyll retention, antioxidant enzyme activity and secondary metabolite content under stress conditions (Bahgat et al., 2023), consistent with the improved growth, yield and fruit quality observed here.
         
    These findings are in agreement with previous studies on foliar application of plant extracts in seedless grape (Gadel-Kareem and Abdel-Rahman, 2013), Calendula (Saleh et al., 2019), lemon (Majeed and Altaai, 2023) and pepper (Salloom et al., 2023), all reporting improved vegetative growth, yield and fruit quality. The higher TSS and vitamin C content observed in Roselle-treated fruits can be attributed to increased photosynthetic efficiency and greater photosynthate translocation to the fruits, together with a cytokinin-mediated reduction in respiration rate (Al-Asady et al., 2019); slower respiration lowers the consumption of soluble solids and protects vitamin C from oxidative loss. These results are consistent with Ismail and Ganzour (2021), who reported increased vegetative growth of strawberry using natural extracts.
         
    The significance of potassium silicate in sustainable fruit production under stress conditions is well documented (Ladon et al., 2021) and was also evident here in flowering and yield parameters, reflecting the dual role of its two building blocks (Khan, 2025). Silicon controls nutrient absorption and translocation across the cell membrane and strengthens the cell wall (Xiao, 2022; Xu, 2023), favouring vegetative growth and yield. Recent strawberry-specific studies confirm that foliar/root-applied silicon enhances marketable yield, fruit weight and fruit number (Jarosz et al., 2025) and increases photosynthetic pigment concentration while reducing oxidative stress (Kashani et al., 2025), directly supporting the leaf area and chlorophyll improvements recorded here. Potassium itself is involved in numerous metabolic processes (Nithin, 2020; Cuin, 2018), directly affecting photosynthetic efficiency (Tränkner et al., 2018), promoting cell expansion (Prado, 2021) and increasing assimilate translocation to plant organs (Sustr et al., 2019).
           
    The enhanced responses under the two-way interaction reflect additive and synergistic effects between the factors. Because potassium is involved in transporting assimilates and nutrients from leaves into the fruits, it also raised fruit TSS and vitamin C concentration, alongside its role in regulating sugar metabolism and hormonal balance (Hosseini et al., 2019). The enhanced leaf mineral content following silicon application may reflect stimulation of the H+-ATPase enzyme (Saaseea, 2018; Nada, 2020), improved transpiration regulation and cellular water status (Koly, 2024) and mechanical reinforcement of the cell wall (Tränkner et al., 2018), together with enhanced antioxidant enzyme activity (Zheng et al., 2019), consistent with similar studies (Nada, 2020; Liu et al., 2017).
           
    The synergistic response observed under the combined Si + Roselle extract treatments (particularly Si2RE2) suggests that the two inputs act through complementary rather than redundant pathways: silicon primarily reinforces structural and water-regulatory functions, while the Roselle extract contributes an exogenous antioxidant and biostimulant load. This complementarity is consistent with reports of enhanced performance when silicon is co-applied with plant-derived biostimulant extracts in related species, where the combined foliar application of potassium silicate and a plant extract produced greater improvements in growth and physiological parameters in Roselle (Hibiscus sabdariffa L.) than either input alone (Abou-Sreea et al., 2022). To our knowledge, the present study is among the first to evaluate this specific combination in strawberry, which represents a key element of its novelty and practical relevance for sustainable, eco-friendly fruit production systems.

    RECOMMENDATION

    Foliar application of Roselle extract five times per year is an effective strategy to enhance the growth, yield and quality of strawberry fruits naturally and safely. This approach also highlights its high value in terms of sustainability, nutritional benefits and environmental friendliness, making it an ideal option for farmers seeking eco-friendly and high-quality production.

    CONCLUSION

    The results showed that the Roselle extract foliar treatment increased growth and productivity as well as physical and chemical properties. So, it is clear that five times foliar spraying of Roselle extract per year is needed for high yield and quality improvement of strawberry fruit. This strategy is also suggested because of its high sustainability and nutritional value as well as for being green process of treatment and a natural biostimulant.

    ACKNOWLEDGEMENT

    The authors would like to thank the Department of Horticulture, College of Agriculture, University of Baghdad, for providing the facilities necessary to conduct this research.
     
    Disclaimers
     
    The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions.
     
    Informed consent
     
    Not applicable. This study involved plant materials only and did not involve humans or animals.

    CONFLICT OF INTEREST

    The authors declare no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

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