Melatonin alleviates acid-induced stress and improves peanut (Arachis hypogaea L.) growth in a dose-dependent manner

This study demonstrates that exogenous melatonin application, particularly at 50–100 µM, significantly alleviates acid-induced stress and enhances peanut growth, photosynthesis, and antioxidant defense in a dose-dependent manner by regulating redox homeostasis and upregulating proton pump genes, with these protective effects validated in both controlled hydroponic and natural acidic soil environments.

The Gist

🥜 The Problem: Peanut Plants in a "Sour" World

Imagine a peanut plant trying to grow in soil that is like a giant, sour lemon. This is what happens in acidic soil (low pH).

For a peanut plant, this environment is a nightmare. It's like trying to run a marathon while wearing heavy lead boots and breathing through a straw. The "sourness" (too many hydrogen ions) messes up the plant's ability to drink water and eat nutrients. It also causes the plant's internal machinery to rust and break down (oxidative stress), leading to stunted growth, yellow leaves, and a poor harvest.

💊 The Hero: Melatonin (The "Nightlight" for Plants)

You might know Melatonin as the sleep hormone for humans. But in the plant world, it's more like a super-herbal supplement or a multivitamin that helps plants fight stress.

The researchers wanted to see: If we give peanut plants this "multivitamin," can they survive the sour soil?

🔬 The Experiment: Two Worlds

The scientists tested this in two ways:

1. The Lab (Hydroponics): They grew peanuts in water with controlled acidity (pH 4.0 vs. pH 6.5). This was like a clean, controlled gym where they could measure exactly how much the plant improved.
2. The Field (Real Life): They grew peanuts in actual acidic soil from a farm in Henan, China. This was the "real-world test" to see if the lab results held up in the messy, complex real world.

They tried different doses of melatonin, ranging from a tiny drop to a heavy dose.

🚀 What Happened? (The Results)

The results were like watching a sick plant get a magic shot of energy. Here is what happened, broken down simply:

1. The "Wake-Up" Call (Germination & Growth)

• Without Melatonin: In the sour soil, the seeds barely woke up. They were slow to sprout, and the little plants were tiny and weak.
• With Melatonin: The seeds sprouted much faster and grew into strong, tall seedlings.
• The Analogy: Think of the acidic soil as a heavy fog that makes it hard to see the road. Melatonin acts like turning on the headlights. It didn't just help them see; it gave them the energy to drive faster.
• The Catch: In the lab, the highest dose worked best. But in the real soil field, a medium dose was the "Goldilocks" zone—not too little, not too much.

2. The "Rust" Fighter (Antioxidants)

Acidic soil makes the plant produce "rust" (called ROS or free radicals) that damages its cells.

• Without Melatonin: The plant's internal "rust fighters" (enzymes like SOD and CAT) were overwhelmed. The plant was covered in rust (lipid peroxidation).
• With Melatonin: It was like giving the plant a team of professional mechanics. Melatonin boosted the plant's own rust-fighting tools, cleaning up the damage and keeping the cells shiny and new.
• The Twist: Interestingly, Melatonin told the plant to stop using one specific tool (POD) that was working too hard and causing more stress. It was like telling a stressed worker, "Stop trying to fix everything yourself; let the new team handle it."

3. The "Pump" Upgrade (H+-ATPase Genes)

This is the most technical part, but here is the simple version:

• The plant has tiny pumps in its roots that push out the bad "sour" ions to keep the inside of the cell neutral.
• Without Melatonin: The pumps were weak and couldn't keep up with the acid.
• With Melatonin: It was like upgrading the engine of a car. Melatonin told the plant's genes to build more pumps and make them work faster. This allowed the plant to push the acid out and keep its internal environment safe.

4. The "Solar Panels" (Photosynthesis)

• Without Melatonin: The leaves turned yellow and stopped making food (chlorophyll broke down).
• With Melatonin: The leaves stayed green and vibrant. The plant could keep making energy from the sun, even in the bad soil.

🌍 The Big Lesson: It's All About the Dose

The most important discovery was that more isn't always better.

• In the Lab (Perfect Conditions): The plants could handle a high dose of melatonin to fight the extreme acid.
• In the Field (Real Life): The soil is complex. A huge dose of melatonin actually slowed the plants down a bit. The medium dose (5 µM) was the sweet spot.

The Analogy: Think of melatonin like spice in a soup.

• If the soup is bland (normal soil), you only need a pinch. Too much spice ruins the flavor.
• If the soup is terrible (acidic soil), you need a lot of spice to make it edible. But even then, you have to find the right amount so it doesn't become too spicy.

🏁 Conclusion

This paper proves that Melatonin is a powerful tool for farmers growing crops in acidic soil. It helps peanut plants:

1. Wake up and grow faster.
2. Clean up the internal "rust" caused by stress.
3. Upgrade their internal pumps to fight off acid.
4. Keep their leaves green and healthy.

However, to get the best results, farmers need to be careful with the dosage. It's not a "spray it on and forget it" solution; it requires finding the perfect balance for the specific soil conditions. This could help farmers grow more food on land that was previously considered too sour for farming.

Technical Summary

Title

Melatonin alleviates acid-induced stress and improves peanut (Arachis hypogaea L.) growth in a dose-dependent manner.

1. Problem Statement

Soil acidification affects 30–40% of global arable land, posing a severe threat to agricultural productivity. Acidic soils (pH < 5.5) limit macronutrient availability (P, Ca, Mg) while increasing the solubility of toxic ions (Al³⁺, Mn²⁺). For peanuts (Arachis hypogaea), a major legume in tropical/subtropical regions, acid stress causes:

• Inhibited root elongation and nutrient uptake.
• Disrupted photosynthesis and chlorophyll degradation.
• Severe oxidative stress characterized by Reactive Oxygen Species (ROS) accumulation (H₂O₂, O₂⁻) and lipid peroxidation (MDA).
• Disruption of intracellular pH homeostasis.

While plants possess antioxidant defenses, these are often insufficient under severe acid stress. There is a need for effective, sustainable mitigation strategies to improve crop resilience in low-pH environments.

2. Methodology

The study employed a dual-experimental approach combining controlled hydroponics with field validation to investigate the role of exogenous melatonin (MT) in mitigating acid stress.

• Plant Material: Peanut seeds (Arachis hypogaea L. cv. Xinbaihua 25).
• Experimental Design:
  • Hydroponic System: Seeds were soaked in MT solutions (0, 0.5, 5, 50, and 100 µM) and grown in half-strength Hoagland nutrient solution under two pH regimes: Acidic (pH 4.0) and Near-optimal (pH 6.5).
  • Field Validation: A pot experiment was conducted using naturally acidic field soil (pH 4.3–4.5) from Henan Province, China. Seeds were treated with MT (0.5–100 µM) and sown in soil pots under natural outdoor conditions.
• Measurements:
  • Growth: Germination rate, root/shoot length, fresh/dry biomass.
  • Physiology: Photosynthetic pigments (Chl a, Chl b, carotenoids), chlorophyll fluorescence (Fv/Fm).
  • Biochemistry: Antioxidant enzyme activities (SOD, POD, CAT, APX), ROS levels (H₂O₂), and lipid peroxidation (MDA).
  • Molecular: Quantitative real-time PCR (qRT-PCR) to analyze the expression of plasma membrane H⁺-ATPase genes (AhAH1 and AhAH2).

3. Key Contributions

• Mechanistic Insight: The study elucidates the molecular mechanism by which melatonin regulates proton transport, specifically linking MT application to the upregulation of AhAH1 and AhAH2 genes, which are critical for maintaining cytosolic pH homeostasis.
• Dual Regulatory Role: It demonstrates that melatonin acts not just as a direct ROS scavenger but as a sophisticated modulator of the enzymatic antioxidant network, showing distinct regulatory patterns for different enzymes (e.g., upregulating SOD/CAT/APX while downregulating POD under stress).
• Context-Dependent Efficacy: The research highlights the "hormetic" nature of melatonin, showing that optimal concentrations vary significantly between stress (acidic) and non-stress (neutral) conditions, and between hydroponic and soil environments.

4. Key Results

A. Growth and Biomass

• Acid Stress (pH 4.0): Control plants showed severe growth inhibition. MT application significantly rescued growth in a dose-dependent manner.
  • Optimal Dose: 100 µM MT resulted in the highest biomass, increasing root length from ~6 cm (control) to ~20 cm and root fresh weight from 1.09 g to 3.45 g.
• Near-Optimal (pH 6.5): Low MT doses (0.5 µM) slightly promoted growth, but higher doses (50–100 µM) showed diminishing returns or slight inhibition, suggesting MT is primarily a stress-rescue agent rather than a constitutive growth promoter.

B. Photosynthetic Pigments

• Acid stress drastically reduced chlorophyll and carotenoid content.
• MT treatment restored pigment levels, with 50–100 µM showing the strongest recovery under acidic conditions. Under pH 6.5, low doses (0.5 µM) were sufficient for peak pigment accumulation.

C. Antioxidant Defense and Oxidative Stress

• ROS Scavenging: MT significantly reduced H₂O₂ and MDA (lipid peroxidation) levels. Under pH 4.0, 100 µM MT reduced H₂O₂ by ~50% and MDA by ~69% in leaves compared to the stressed control.
• Enzymatic Regulation:
  • Upregulated: SOD, CAT, and APX activities increased significantly with MT under acid stress, enhancing the plant's capacity to detoxify ROS.
  • Downregulated: POD activity decreased with MT treatment under acid stress. The authors interpret this as a sign of reduced oxidative pressure (less need for POD-mediated scavenging) and a shift toward more efficient redox regulation, preventing cell wall rigidification and growth inhibition.

D. Gene Expression (H⁺-ATPase)

• MT induced a strong, dose-dependent upregulation of AhAH1 and AhAH2 (plasma membrane H⁺-ATPase genes).
• AhAH2 showed the most dramatic response, with expression increasing up to 11.6-fold in roots at 100 µM MT under pH 4.0.
• This upregulation suggests MT enhances proton extrusion, helping to maintain intracellular pH and ion homeostasis against the influx of external protons.

E. Field Soil Validation

• In naturally acidic soil (pH 4.3–4.5), the optimal MT concentration shifted. While 100 µM was best in hydroponics, 5 µM was the most effective dose in soil.
• The 5 µM treatment significantly improved germination, vegetative growth, Fv/Fm (photosystem II efficiency), and yield traits (peg number, pod number, pod weight).
• Higher doses in soil (50–100 µM) were less effective, likely due to adsorption by soil organic matter or microbial degradation, emphasizing the need for dose optimization in field applications.

5. Significance

• Sustainable Agriculture: The study provides a practical, non-genetic strategy (seed soaking with melatonin) to improve peanut cultivation on acidic soils, which are prevalent in many tropical and subtropical regions.
• Physiological Mechanism: It establishes a clear link between melatonin application, H⁺-ATPase gene expression, and the restoration of pH homeostasis, offering a molecular target for future breeding or biostimulant development.
• Precision Application: The findings underscore that melatonin application is not "one-size-fits-all." The optimal dose is highly dependent on the severity of the stress and the growth medium (hydroponic vs. soil), with lower doses often being more effective in complex soil environments.

Conclusion: Melatonin acts as a potent, stress-dependent bioregulator that enhances peanut tolerance to acid stress by coordinating antioxidant defense, protecting photosynthetic machinery, and transcriptionally activating proton pumps to stabilize cellular pH.