Synergistic effect of seaweed-derived alginic acid and potassium sulfate fertilizer on the alleviation of salinity stress in Flame Seedless grapevine

Technical Summary: Synergistic Effect of Seaweed-Derived Alginic Acid and Potassium Sulfate on Salinity Stress in Flame Seedless Grapevine

Problem Statement
Salinity stress is a critical environmental constraint limiting grapevine (Vitis vinifera L.) productivity, particularly in arid and semi-arid regions. High soil salinity induces ion toxicity (specifically Na⁺ and Cl⁻), disrupts osmotic balance, impairs photosynthetic efficiency, damages cell membranes through lipid peroxidation, and accelerates leaf senescence. While the Flame Seedless cultivar is valued for its market potential, it faces significant yield and quality reductions under saline conditions. Current viticultural practices often lack effective, sustainable strategies to mitigate these effects without relying solely on chemical fertilizers. There is a specific gap in research regarding the application of seaweed-derived alginic acid (ALG) as a biostimulant, particularly in combination with potassium sulfate (PS), to alleviate salinity stress in grapevines.

Methodology
The study employed a factorial completely randomized design (CRD) using 3-year-old Flame Seedless grapevines grown in large pots (80×80×65 cm) with sandy soil enriched with manure. The experiment was conducted over the 2023–2024 season.

• Stress Induction: Salinity stress was applied using four NaCl levels: 0, 50, 100, and 200 mM. Stress was maintained for 15 days after seven months of plant establishment.
• Treatments: The study evaluated 24 treatment combinations involving:
  • Alginic Acid (ALG): Applied foliarly at 0.1% and 0.2% concentrations (Tri-Kelp™ source).
  • Potassium Sulfate (PS): Applied foliarly at 0.2% (K₂SO₄).
  • Combinations: Individual and co-applications of ALG and PS under varying salinity levels.
• Measurements: Data were collected 15 days after the final foliar application (30 days total treatment period). Parameters included:
  • Morphological: Shoot length, stem diameter, root/leaf fresh and dry weights, and leaf area.
  • Physiological: Stomatal conductance, K⁺/Na⁺ ion ratios, electrolyte leakage, and lipid peroxidation (Malondialdehyde/MDA content).
  • Biochemical: Photosynthetic pigments (Chl a, b, total, carotenoids), total soluble sugars (TSS), total phenolic compounds (TPC), total flavonoids (TFC), and free proline.
  • Enzymatic Antioxidants: Activities of superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), and polyphenol oxidase (PPO).
  • Secondary Metabolites: Quantification of stilbenes (trans-resveratrol, delta-viniferin, catechin) via HPLC.
• Statistical Analysis: Data were analyzed using two-way ANOVA, with mean comparisons performed via Duncan's New Multiple Range Test (p ≤ 0.01). Principal Component Analysis (PCA) was used to evaluate trait relationships.

Key Results

1. Growth and Morphology: Salinity significantly reduced growth parameters. However, foliar applications of ALG (0.1% and 0.2%) and PS, individually and in combination, mitigated these reductions. The co-application of ALG 0.2% + PS and individual ALG 0.1% were identified as the most effective treatments for maintaining growth and leaf area.
2. Ion Homeostasis and Membrane Stability: High salinity (200 mM NaCl) drastically lowered the K⁺/Na⁺ ratio and increased electrolyte leakage and MDA content (indicating membrane damage).
   • ALG and PS treatments restored K⁺/Na⁺ ratios, with ALG 0.2% + PS showing the highest efficacy in reducing Na⁺ toxicity.
   • Electrolyte leakage was significantly reduced by ALG 0.1%, PS, and ALG 0.2% + PS.
   • Lipid peroxidation (MDA) was reduced by 7–21 mmol g⁻¹ FW in treated plants compared to saline controls, with ALG 0.1% and 0.2% showing the strongest individual effects in lowering MDA.
3. Photosynthesis and Soluble Sugars: Salinity decreased chlorophyll and carotenoid content. ALG treatments (particularly 0.1%) effectively prevented chlorophyll degradation. ALG 0.2% + PS was superior in maintaining total soluble sugar levels, which are crucial for osmotic adjustment.
4. Antioxidant Systems:
   • Non-Enzymatic: ALG and PS treatments maintained or increased levels of TPC, TFC, and proline. ALG 0.1% was particularly effective in maintaining TPC and TFC levels comparable to non-saline controls under severe stress.
   • Enzymatic: Co-application of ALG and PS synergistically boosted antioxidant enzyme activities. Under high salinity, ALG 0.2% + PS and ALG 0.1% + PS increased SOD, CAT, GPX, and PPO activities by approximately 60%, 50%, 56%, and 50% respectively, compared to controls.
5. Secondary Metabolites: Salinity stress generally increased stilbene biosynthesis. The co-application of ALG 0.2% + PS resulted in the highest concentrations of key metabolites: viniferin (61.88 µg g⁻¹ FW), resveratrol (132.17 µg g⁻¹ FW), and catechin (49.06 µg g⁻¹ FW).
6. PCA Analysis: The analysis confirmed that ALG and PS treatments clustered positively with beneficial traits (antioxidant enzymes, K⁺ uptake, secondary metabolites) and negatively with stress indicators (Na⁺ uptake, MDA, electrolyte leakage), validating their role in stress mitigation.

Significance and Claims
The paper claims that the co-application of seaweed-derived alginic acid and potassium sulfate offers a promising, sustainable strategy for mitigating salinity stress in Flame Seedless grapevines. The study highlights that:

• Synergistic Mechanism: The combination of ALG and PS acts synergistically to regulate antioxidant systems (both enzymatic and non-enzymatic) and maintain Na⁺/K⁺ hemostasis more effectively than individual treatments.
• Stress Alleviation: These biostimulants alleviate salinity stress by reducing oxidative damage (lipid peroxidation), preserving membrane integrity, and sustaining photosynthetic capacity.
• Metabolic Induction: The treatments stimulate the biosynthesis of valuable secondary metabolites (stilbenes, phenolics, flavonoids) which serve as defense compounds against abiotic stress.
• Practical Application: The findings suggest that ALG-based biostimulants, especially when supplemented with potassium fertilizers, can be integrated into viticultural practices to enhance grapevine resilience in saline environments, supporting sustainable production in arid regions.

The authors conclude that while ALG and PS are effective, future research utilizing omics technologies is necessary to fully elucidate the molecular mechanisms and gene expression pathways involved in this stress mitigation.