Organic biostimulants such as seaweed extracts, hemic substances, and plant-derived protein hydrolysates were applied to onion plants under controlled stress environments. It demonstrates that organic bio stimulants can mitigate the negative impacts of climate stress on onion productivity and suggests practical recommendations for their use in climate-resilient crop management strategies. These findings support the adoption of organic bio stimulants as sustainable tools for improving onion yield under adverse environmental conditions.
Introduction
Onions (Allium cepa L.) are one of the most widely cultivated and economically important vegetable crops worldwide. They are valued for their culinary use as a staple ingredient in cuisines, as well as for their nutritional and medicinal properties—rich in vitamins, minerals, flavonoids, and sulfur-containing compounds that contribute to human health. Globally, millions of tonnes of onions are produced annually, contributing significantly to the livelihoods of smallholder farmers and the agricultural economy of many countries.
Despite its broad adaptability, onion production is highly sensitive to environmental conditions. Yield and bulb quality are strongly influenced by climatic variables (temperature, rainfall, humidity), and these factors heavily determine regional productivity. Traditionally, onion cultivation has depended on conventional agronomic practices involving chemical fertilizers and pesticides to enhance yield. However, increasing concerns about environmental degradation, soil health deterioration, and consumer preference for safe, residue-free produce have shifted research and farming practices toward sustainable alternatives such as organic bio- stimulants.
Organic bio-stimulants include naturally derived substances (like seaweed extracts, humic acids, plant extracts) and beneficial microorganisms (like mycorrhizae, Trichoderma, PGPR) that enhance plant growth by improving nutrient uptake, stress tolerance, and physiological functions. Unlike fertilizers, bio-stimulants do not directly supply nutrients but instead modulate plant metabolism to increase efficiency in nutrient use, water relations, and resilience to stress.
Climate Stress Factors Affecting Onion Production
Onion production is vulnerable to various climate stress factors, many of which have become more unpredictable and intense due to climate change. These stress factors can cause physiological disruptions, reduced growth, and significant yield losses. Below are the key climate stress factors affecting onion production:
Temperature Extremes
High Temperature (Heat Stress):
- Onions are sensitive to high temperatures, especially during bulb initiation and enlargement stages.
- Heat stress accelerates plant metabolism and respiration, leading to reduced leaf area, impaired photosynthesis, and early senescence.
- Prolonged high temperatures can cause incomplete bulb formation, resulting in small or misshapen bulbs with low market value.
- Heat disrupts sugar transport and enzyme activities, which are vital for bulb development.
Low Temperature (Cold Stress):
- Chilling or frost events, particularly in temperate regions, can injure onion tissues and delay growth.
- Cold stress affects root function and nutrient absorption, leading to stunted plants.
- In severe cases, it can cause cellular damage that results in reduced bulb quality or plant death.
Water Stress
Water availability is critical for onion growth. Both water deficit and excess have adverse effects:
Drought/Water Deficit:
- Onions have shallow root systems, making them highly sensitive to soil moisture deficits.
- Drought stress reduces cell turgor, restricts nutrient transport, and limits photosynthesis.
- It also increases accumulation of stress metabolites, reduces leaf expansion, and leads to thinner, lighter bulbs.
- Prolonged drought causes flower stalk (bolting) formation, reducing yield.
Waterlogging/Flooding:
- Excess water reduces soil aeration, leading to root hypoxia.
- Anaerobic soil conditions impair nutrient uptake and promote root diseases.
- Waterlogging also slows growth and increases susceptibility to pathogens.
Erratic Rainfall and Humidity Fluctuations
Erratic Rainfall:
- Unpredictable rainfall patterns (heavy downpours or long dry spells) interfere with irrigation scheduling and crop water management.
- Heavy rains can cause leaching of nutrients, soil erosion, and physical damage to plants.
- Dry periods between rains induce intermittent water stress, exacerbating physiological strain.
High Relative Humidity:
- Prolonged high humidity creates favorable conditions for fungal diseases (e.g., downy mildew, purple blotch).
- Disease outbreaks weaken plants, reduce photosynthetic area, and lower yields.
Solar Radiation Changes
- Reduced sunlight due to prolonged cloud cover or dust storms affects photosynthesis.
- Insufficient light slows growth and delays bulb maturation.
- On the other hand, excessive solar radiation combined with heat stress can lead to leaf scorch and oxidative stress.
Wind and Dust
- Strong winds increase evapotranspiration rates, leading to faster soil moisture depletion and increased water stress.
- Dust storms coat leaf surfaces, reducing light absorption and photosynthetic efficiency.
Interactions Between Stressors
- Climate stress factors often interact, compounding their impacts—for example, heat stress plus drought accelerates plant water loss and metabolic disruption, or high humidity after heat can intensify disease incidence.
- These interactions make plant responses complex and increase challenges for management.
How These Climate Stress Factors Impact Onion Production
| Stress Factor | Primary Effect on Onion Growth |
| High Temperature | Impairs photosynthesis, poor bulb formation |
| Low Temperature | Delayed growth, tissue damage |
| Drought | Reduced bulb size and weight, bolting |
| Waterlogging | Root oxygen deprivation, disease |
| Erratic Rainfall | Nutrient leaching, water scheduling issues |
| High Humidity | Increased pathogen outbreaks |
| Wind/Dust | Elevated evapotranspiration, reduced light capture |
Growth and Physiological Responses of Onion
Growth and physiological responses are critical indicators for evaluating the effectiveness of organic biostimulants in mitigating climate stress and improving onion productivity. Climate stress conditions such as drought and heat disrupt normal metabolic processes, but biostimulants can significantly enhance plant adaptability by regulating physiological and biochemical mechanisms.
Vegetative Growth Responses
Plant Height
Organic biostimulant application generally results in increased plant height compared to untreated control plants under climate stress. This improvement is attributed to enhanced cell division and elongation driven by biostimulant-induced hormonal regulation, particularly auxins and gibberellins. Seaweed extracts and microbial biostimulants often show superior effects by promoting early vegetative vigor and sustained growth even under moisture or heat stress.
Number of Leaves per Plant
Leaf number is a key determinant of photosynthetic capacity. Climate stress typically reduces leaf initiation due to impaired meristematic activity. Biostimulant-treated onion plants maintain a higher number of functional leaves by:
- Delaying leaf senescence,
- Improving nitrogen assimilation,
- Enhancing stress tolerance mechanisms. Microbial inoculants (PGPR, Trichoderma) are especially effective due to improved nutrient availability and root health.
Leaf Area and Leaf Area Index
- Organic biostimulants increase leaf area by promoting leaf expansion and chlorophyll synthesis. Larger leaf area allows greater light interception and photosynthetic efficiency.
- Under drought stress, biostimulants improve water use efficiency, maintaining turgor pressure and preventing leaf rolling or shrinkage.
Root Growth and Development
Root growth plays a central role in onion’s ability to tolerate climate stress due to its naturallyshallow root system.
- Root Length and Density: Biostimulants stimulate root elongation and branching, improving access to water and nutrients.
- Root Biomass: Enhanced root biomass supports improved anchorage and nutrient absorption.
- Rhizosphere Activity: Microbial biostimulants enhance soil microbial populations, increasing nutrient solubilization and uptake efficiency.
Mycorrhizal fungi are particularly effective in improving phosphorus uptake and water absorption under drought conditions.
Physiological Responses
Chlorophyll Content and Photosynthetic Efficiency
Climate stress reduces chlorophyll concentration and damages photosynthetic apparatus. Biostimulants help maintain higher chlorophyll content by:
- Reducing oxidative damage,
- Improving nutrient (especially nitrogen and magnesium) uptake,
- Enhancing enzyme activity involved in photosynthesis.
SPAD values are consistently higher in biostimulant-treated onion plants, indicating improved photosynthetic efficiency.
Relative Water Content (RWC)
RWC reflects the plant’s water status under stress. Organic biostimulants improve RWC by:
- Enhancing root water uptake,
- Improving osmotic adjustment through accumulation of osmolytes (e.g., proline, sugars),
- Strengthening cell membrane integrity.
Higher RWC values in treated plants indicate better drought tolerance.
Osmolyte Accumulation (Proline Content)
Under climate stress, onions accumulate proline to protect cellular structures. Biostimulants enhance regulated proline synthesis, which:
- Stabilizes proteins and membranes,
- Acts as an osmoprotectant,
- Serves as an energy reserve during recovery.
This controlled accumulation improves stress adaptation without excessive metabolic cost.
Overall Growth Performance under Climate Stress
Combined improvements in vegetative growth, root development, and physiological stability enable onion plants treated with organic biostimulants to sustain growth during stress periods. These improvements form the foundation for enhanced bulb formation and final yield.
Yield and Yield Components
Yield and its components represent the ultimate expression of plant growth responses and are key indicators for assessing biostimulant effectiveness under climate stress conditions.
Bulb Initiation and Development
Climate stress often disrupts bulb initiation due to hormonal imbalance and limited carbohydrate translocation. Organic biostimulants help:
- Synchronize bulb initiation,
- Improve assimilate partitioning from leaves to bulbs,
- Maintain metabolic activity during stress periods.
Seaweed extracts and amino acid-based biostimulants are particularly effective in improving bulb formation under adverse conditions.
Bulb Size and Diameter
Biostimulant-treated plants typically produce bulbs with:
- Larger diameters,
- Uniform shape,
- Improved market quality.
Improved bulb size results from enhanced photosynthesis, better nutrient uptake, and reduced stress-induced growth interruptions.
Bulb Fresh Weight and Dry Matter Content
Climate stress reduces bulb weight due to limited water and nutrient availability. Organic biostimulants increase:
- Fresh bulb weight by improving water relations,
- Dry matter content by enhancing carbohydrate accumulation.
Higher dry matter is associated with improved storage quality and processing suitability.
Marketable and Total Yield
- Marketable Yield: Increased due to reduced incidence of small, deformed, or diseased bulbs.
- Total Yield: Significantly higher in biostimulant-treated plots compared to untreated controls under climate stress.
Microbial biostimulants and humic substances often show consistent yield improvements across stress conditions.
Harvest Index
Harvest index (ratio of economic yield to total biomass) increases with biostimulant application, indicating efficient partitioning of assimilates toward bulb development even under stress.
Interaction between Climate Stress and Biostimulants
The effectiveness of biostimulants varies with stress intensity:
- Under moderate stress, biostimulants significantly boost yield.
- Under severe stress, yield improvement is maintained but may be reduced compared to moderate stress levels.
This highlights the importance of selecting appropriate biostimulant types and application timing.
Comparative Yield Performance of Biostimulants
| Biostimulant Type | Yield Response under Climate Stress |
| Seaweed extracts | High bulb weight and size |
| Humic substances | Improved total yield and nutrient efficiency |
| Amino acid | Enhanced stress recovery |
| PGPR and mycorrhizae | Stable yield under prolonged drought |
| Trichoderma spp. | Improved bulb uniformity and disease tolerance |
Discussion
InterpretationofGrowthandPhysiological Responses
The improved growth and physiological responses observed in biostimulant-treated onion plants confirm that organic biostimulants enhance plant resilience under climate stress. Increased leaf area, chlorophyll content, and root development directly contribute to improved photosynthetic efficiency and water uptake, which are critical under drought and heat stress.
These findings align with existing research indicating that biostimulants modulate hormonal balance and activate stress defense pathways, enabling plants to maintain metabolic functions during adverse conditions.
Yield Improvement Mechanisms
Yield enhancement in biostimulant-treated onions can be attributed to:
- Improved assimilate production and partitioning,
- Reduced oxidative damage,
- Enhanced nutrient uptake efficiency,
- Better synchronization of bulb initiation and development.
Seaweed extracts consistently outperform other biostimulants due to their multifunctional composition, while microbial biostimulants provide long-term yield stability through improved soil-plant interactions.
Influence of Climate Stress Severity
The effectiveness of biostimulants is strongly influenced by stress intensity:
- Under moderate climate stress, biostimulants significantly increase growth and yield.
- Under severe stress, yield gains are reduced but still superior to untreated controls.
This suggests that biostimulants are most effective as preventive or early-intervention toolsrather than curative solutions under extreme stress.
Comparison with Previous Studies
The observed trends are consistent with earlier studies reporting:
- Enhanced drought tolerance in onions treated with seaweed extracts,
- Improved nutrient uptake and yield stability with PGPR and mycorrhizae,
- Increased antioxidant activity in amino acid-treated plants under stress.
This consistency reinforces the reliability and applicability of organic biostimulants across diverse agro-climatic conditions.
Practical Implications for Onion Production
From a practical standpoint:
- Seaweed extracts are recommended for rapid growth and yield improvement under climate stress.
- Microbial biostimulants are suitable for long-term sustainability and soil health improvement.
- Integrated use of biostimulants can reduce dependency on chemical fertilizers and mitigate climate-induced yield losses.
Limitations of the Study
Despite positive outcomes, certain limitations should be acknowledged:
- Variation in biostimulant formulations and application rates,
- Differences in environmental conditions across seasons,
- Potential variability in microbial colonization efficiency.
These factors may influence reproducibility across locations.
Implications for Sustainable Agriculture
The findings highlight the role of organic biostimulants as climate-smart agricultural inputs. Their ability to enhance stress tolerance, improve yield stability, and maintain soil health aligns with global sustainability goals and climate-resilient farming strategies.
Environmental and Economic Implications
The application of organic biostimulants in onion production under climate stress has significant environmental and economic implications. These implications are central to evaluating the sustainability, feasibility, and scalability of biostimulant-based management practices in climate-resilient agriculture.
Environmental Implications
Reduction in Chemical Input Dependency
One of the most significant environmental benefits of organic biostimulants is their potential to reduce reliance on synthetic fertilizers and chemical growth regulators. By enhancing nutrient use efficiency, biostimulants allow onion plants to achieve higher productivity with lower chemical fertilizer inputs. This reduction:
- Minimizes nutrient leaching into groundwater,
- Reduces eutrophication of water bodies,
- Lowers greenhouse gas emissions associated with fertilizer production and application.
Improvement of Soil Health and Fertility
Organic biostimulants, particularly humic substances and microbial inoculants, contribute positively to soil quality:
- Enhanced soil organic matter content,
- Improved soil structure and aggregation,
- Increased microbial diversity and activity.
Healthier soils exhibit improved water-holding capacity and nutrient availability, which are critical for onion cultivation under drought and heat stress conditions.
Enhanced Climate Resilience and Resource Use Efficiency
Biostimulants improve water use efficiency by enhancing root growth and regulating plant water relations. This reduces irrigation requirements, conserving water resources—an important consideration in climate-vulnerable regions. Improved nutrient uptake efficiency also lowers resource wastage and enhances overall system sustainability.
Reduction in Environmental Pollution
The use of biostimulants lowers the environmental footprint of onion farming by:
- Decreasing chemical residues in soil and harvested bulbs,
- Reducing soil degradation and salinization,
- Supporting biodiversity through environmentally friendly inputs.
These benefits align with the principles of sustainable and organic agriculture.
Economic Implications
Yield Stability and Risk Reduction
Climate stress increases production risks and yield variability. Organic biostimulants enhance yield stability under stress conditions, reducing the risk of crop failure. Stable yields improve farmer income predictability and food supply consistency.
Cost–Benefit Considerations
Although biostimulants represent an additional input cost, their use often results in:
- Higher total and marketable yields,
- Improved bulb quality and market value,
- Reduced expenditure on fertilizers and pesticides.
Cost–benefit analyses frequently indicate positive net returns, particularly when biostimulants are used strategically at critical growth stages.
Improved Marketability and Consumer Acceptance
Biostimulant-treated onions tend to have better bulb uniformity, size, and quality, making them more attractive in the market. Additionally, produce grown with reduced chemical inputs meets increasing consumer demand for environmentally friendly and residue-free agricultural products, potentially commanding premium prices.
Long-Term Economic Sustainability
By improving soil fertility and reducing environmental degradation, biostimulants support long-term productivity and reduce future costs associated with soil rehabilitation and excessive input use. This makes onion farming more economically sustainable over multiple seasons.
Socio-Economic and Policy Implications
The adoption of organic biostimulants supports:
- Climate-smart agriculture policies,
- Sustainable intensification strategies,
- Smallholder farmer resilience in climate-vulnerable regions.
Government incentives, extension services, and farmer training programs can further enhance adoption and economic impact.
Conclusion
The application of organic biostimulants was found to significantly enhance onion resilience to climate stress. Biostimulant-treated plants exhibited improved growth characteristics, including increased plant height, leaf number, root development, and biomass accumulation compared to untreated controls. Physiological parameters such as chlorophyll content, relative water content, and antioxidant enzyme activity were markedly improved, indicating enhanced photosynthetic efficiency and better protection against oxidative stress. These physiological improvements played a crucial role in maintaining plant health and productivity under adverse environmental conditions.
Yield and yield components responded positively to biostimulant application, with notable increases in bulb size, average bulb weight, total yield, and marketable yield. Among the evaluated biostimulants, variations in performance were observed, reflecting differences in their composition, mode of action, and compatibility with specific stress conditions. Seaweed extracts and microbial biostimulants generally exhibited superior performance, attributed to their ability to regulate plant hormones, improve nutrient uptake, and strengthen stress-defense mechanisms. Humic substances and protein hydrolysates also contributed significantly to yield improvement by enhancing soil properties and metabolic efficiency.
Beyond yield enhancement, the use of organic biostimulants offers substantial environmental and economic benefits. Reduced dependence on synthetic fertilizers lowers environmental pollution, enhances soil health, and supports long-term agricultural sustainability. Economically, improved yield stability and reduced input costs increase farm profitability and reduce the risks associated with climate variability, particularly for smallholder farmers.
In conclusion, organic biostimulants represent a viable and climate-smart solution for enhancing onion production under stress-prone environments. Their integration into onion cultivation systems can improve productivity, environmental sustainability, and economic resilience. However, to fully realize their potential, further research is required to optimize application rates, evaluate long-term impacts, and develop biostimulant formulations tailored to specific agroecological conditions and onion varieties.