Exogenous Proline Enhances Nutritional and Sensory Quality of Celery Under Salinity Stress
Celery, a crunchy and nutritious vegetable loved worldwide, faces a major challenge when grown in salty soils. High salt levels disrupt the plant’s ability to absorb water and nutrients, leading to stunted growth and lower nutritional value. A recent study by researchers at Gansu Agricultural University in China explored a promising solution: using proline, a natural compound, to help celery thrive in salty conditions.
How Salt Stress Affects Celery Growth and Quality
Salt stress occurs when excessive sodium accumulates in the soil, creating a harsh environment for plants.
- Sodium (Na⁺) is a mineral that, in high concentrations, competes with essential nutrients like potassium (K⁺)
- calcium (Ca²⁺), making it harder for plants to absorb these elements.
This imbalance, known as ionic toxicity, damages plant cells, slows growth, and reduces the vegetable’s quality. Proline, on the other hand, is an amino acid—a building block of proteins—that plants naturally produce to cope with environmental stresses like drought, extreme temperatures, and salinity.
When applied externally as a foliar spray, proline acts as an osmoprotectant, a substance that helps plants retain water and stabilize cellular structures under stressful conditions.
The researchers tested four groups of celery plants: one grown normally (control group), one treated only with proline, one exposed to high salt levels, and one given both proline and salt.
After a week, they analyzed the plants’ leaves, stalks (petioles), and roots to measure changes in minerals, amino acids, and other key compounds. The results revealed that proline not only helped celery survive salt stress but also improved its nutritional and flavor profiles. Let’s explore these findings in detail.
Balancing Minerals for Healthier Plants
Minerals like potassium, calcium, and magnesium are vital for plant health. Potassium regulates photosynthesis and enzyme activity, calcium strengthens cell walls, and magnesium is a core component of chlorophyll, the pigment responsible for photosynthesis.
When celery is grown in salty soil, sodium levels in its tissues skyrocket. For example, the study found that salt-stressed plants had 81.8% more sodium in their leaves compared to normal plants.
This excess sodium disrupts the uptake of critical minerals. Potassium, for instance, dropped by 22% in salt-stressed leaves, while calcium decreased by 53% in roots.
However, proline application reversed this trend. Plants treated with both proline and salt had 19.4% less sodium in their leaves and 15% less in stalks compared to salt-stressed plants without proline.
At the same time, proline boosted potassium levels by up to 88% in leaves and calcium by 54% in roots.
Trace elements like iron and zinc also increased, with iron rising by 28% in leaves. These changes suggest that proline helps celery roots absorb and retain essential minerals, even under salt stress. This process, known as ion homeostasis, ensures that plants maintain a healthy balance of nutrients despite external challenges.
Regulating Amino Acids for Better Nutrition
Amino acids are organic compounds that serve as the building blocks of proteins. They play a critical role in plant growth, stress response, and human nutrition.
For example, glycine, a simple amino acid, acts as an antioxidant, protecting cells from damage caused by reactive oxygen species (ROS)—harmful molecules generated during stress.
Salt stress disrupted the balance of amino acids in celery, but proline helped restore it. Glycine levels increased by 52% in leaves and 34% in stalks when proline was applied.
Proline also reduced harmful nitrate levels by 60% in leaves and 59% in stalks. Nitrates are compounds that, in excess, can convert to nitrites in the human body, potentially leading to health risks like methaemoglobinaemia (a blood disorder). By lowering nitrate accumulation, proline makes celery safer for consumption.
Interestingly, salt-stressed plants naturally produced more proline as a survival response. However, adding external proline optimized this process, ensuring the plant didn’t overproduce it.
This balance is crucial because excessive proline can sometimes interfere with other metabolic processes. By fine-tuning amino acid levels, proline helps celery maintain its nutritional quality under stress.
Boosting Organic Acids for Flavor and Resilience
Organic acids like citric acid and malic acid contribute to celery’s tangy flavor and play a role in stress adaptation. Citric acid is a key component of the Krebs cycle, a metabolic pathway that generates energy for plant cells.
Malic acid helps regulate cellular pH and acts as an osmoprotectant, helping cells retain water during drought or salt stress.
The study found that proline-treated plants had 57% more citric acid in leaves and 50% more in stalks compared to untreated plants. Malic acid increased by 26% in stalks.
Salt-stressed plants without proline struggled to produce these acids, leading to bland-tasting celery. Proline’s ability to boost organic acids suggests it could be a game-changer for farmers aiming to grow flavorful crops in challenging environments.
These acids not only enhance taste but also improve the plant’s ability to cope with osmotic stress—a condition where high salt concentrations outside the roots make it difficult for plants to absorb water.
Strengthening Structure with Fiber and Lignin
Dietary fiber and lignin are crucial for celery’s texture and health benefits. Dietary fiber, composed of cellulose and hemicellulose, aids human digestion and promotes gut health. Lignin, a complex polymer, reinforces plant cell walls, providing structural support and resistance to pests and diseases.
Salt stress weakened these components, reducing dietary fiber by up to 17% in stalks. However, proline strengthened these structures. Plants treated with proline and salt had 36% more dietary fiber in leaves and 23% more in stalks compared to salt-stressed plants. Lignin increased by 15% in leaves, making the plant more rigid and durable.
These structural improvements are vital for both farmers and consumers. Firmer, fiber-rich celery has a longer shelf life and offers more health benefits, such as aiding digestion and reducing cholesterol. For farmers, stronger cell walls mean reduced crop losses from physical damage or pathogen attacks.
Enhancing Aroma with Volatile Compounds
Aroma is a key factor in determining celery’s appeal. Volatile compounds—chemicals that evaporate easily and create scent—include terpenes (responsible for herbal notes) and aldehydes (associated with fresh, green aromas). Using a specialized electronic nose (PEN3), the researchers detected 10 volatile compounds in celery.
- Salt stress altered this aroma profile, increasing bitter sulfur compounds by 23%. However, proline treatment reduced these unwanted notes and boosted pleasant aromas.
- For example, aromatic compounds linked to fresh greens increased by 18% in proline-treated leaves.
This finding is significant for the food industry, where consistent flavor and aroma are critical for consumer satisfaction. Proline’s ability to preserve celery’s natural scent under stress could make it a valuable tool for growers supplying fresh markets or food processors.
The Science Behind Proline’s Role in Celery Salt Tolerance
To assess overall effectiveness, the researchers used a scoring system called the affiliation function. This statistical method evaluates multiple variables (e.g., mineral content, amino acids, fiber) and assigns a composite score to each treatment.
Proline-treated plants exposed to salt stress scored highest, with a rating of 0.58 out of 1.0 for leaves and 0.56 for stalks. Salt-stressed plants without proline scored the lowest (0.45), confirming proline’s protective role.
Even plants treated only with proline (no salt) showed improvements, suggesting it could benefit crops in non-stressful conditions too. Proline’s success lies in its dual role as an osmoprotectant and antioxidant.
- During salt stress, it stabilizes cell membranes and enzymes, preventing dehydration and damage—a process known as osmotic adjustment.
- It also neutralizes harmful molecules called free radicals, which accumulate under stress and damage cells through oxidative stress.
Additionally, proline enhances root function, allowing plants to absorb more nutrients despite high sodium levels. These mechanisms align with previous studies on crops like rice and tomatoes, where proline improved growth under drought or salinity.
However, this study is the first to link proline to celery’s flavor, fiber, and aroma quality, offering a holistic view of its benefits.
Practical Tips for Farmers Challenges and Future Research
For farmers dealing with salty soils, proline offers a simple and affordable solution. A 0.3 mM proline solution—roughly 20 grams per liter of water—can be sprayed onto leaves once a week.
This method costs about $20 per kilogram of proline, making it accessible for small-scale growers. Trials showed that proline-treated plants had 30% higher biomass than salt-stressed plants, translating to better marketable yields.
While the results are promising, the study had limitations. The experiment lasted only seven days, so longer-term effects remain unknown. Field trials are also needed to confirm whether proline works as well in real-world conditions as it does in greenhouses.
Future research could explore optimal proline concentrations for different celery varieties or combine it with other stress-relief strategies, like drip irrigation or soil amendments.
Conclusion
The study demonstrates that exogenous proline is a powerful tool for improving celery’s salt tolerance. By balancing minerals, boosting antioxidants, and enhancing flavor, proline helps farmers grow healthier, tastier crops in challenging environments.
For consumers, this means access to nutritious, high-quality celery year-round, even from regions with saline soils. As climate change increases soil salinity worldwide, solutions like proline will be essential for sustainable agriculture.
Frequently Asked Questions (FAQs)
Salt Stress: A condition where excessive salt (like NaCl) in soil or water harms plant growth. Salt stress disrupts a plant’s ability to absorb water and nutrients, causing dehydration and toxic ion buildup. It is a major threat to agriculture, especially in dry or irrigated areas. For example, high salt levels in soil can stunt celery growth, but treatments like proline application help plants cope. Farmers combat salt stress by improving soil drainage or using salt-tolerant crops.
Proline: A natural amino acid produced by plants under stress, such as drought or high salinity. Proline acts as an “osmoprotectant,” helping cells retain water and stabilize proteins during stress. It is vital for improving plant resilience. In the study, spraying celery with proline reduced harmful sodium (Na⁺) and boosted nutrients like potassium. Chemically, proline is written as C₅H₉NO₂.
Mineral Elements: Nutrients like nitrogen (N), potassium (K), and iron (Fe) that plants need for growth. These elements support processes like photosynthesis and enzyme activity. For example, potassium helps regulate water balance in celery cells. Salt stress often blocks mineral uptake, but proline treatment in the study helped celery absorb more nutrients.
Affiliation Function Method: A mathematical tool used to evaluate plant traits under stress. It assigns scores (0 to 1) to measure how well a plant performs compared to others. The formula is: Affiliation Value = (X – X_min) / (X_max – X_min), where X is the measured value. In the study, this method ranked treatments to show proline improved celery quality under salt stress.
Osmotic Regulation: A process where plants balance water and solutes inside cells to survive drought or salinity. Plants accumulate compounds like proline to maintain cell pressure. For example, celery under salt stress uses osmotic regulation to avoid wilting. This process is critical for crops in dry or salty environments.
NaCl: Sodium chloride, common table salt. In agriculture, high NaCl levels in soil (salinity) harm plants by disrupting nutrient uptake and causing toxicity. The study used 100 mM NaCl to simulate salt stress, which increased sodium in celery but decreased other minerals like potassium.
Nitrate: A compound (NO₃⁻) plants absorb from soil to make proteins. Excess nitrate in vegetables can harm human health if converted to nitrites. The study found proline reduced nitrate buildup in salt-stressed celery, making it safer to eat.
Lignin: A tough polymer in plant cell walls that provides structural support. Lignin helps plants stand upright and defend against pests. In the study, proline increased lignin in celery under salt stress, improving strength and stress tolerance.
Volatile Components: Chemicals that evaporate easily, contributing to aroma and flavor. In celery, these include compounds like terpenes. The study used an electronic nose (E-nose) to detect changes in volatiles, showing proline enhanced aromatic quality under salt stress.
Principal Component Analysis (PCA): A statistical method to simplify complex data by identifying key trends. In the study, PCA highlighted how proline affected mineral levels in celery parts. For example, calcium and magnesium were major factors in leaf quality.
Flame Atomic Absorption Spectrometer: A device that measures metal concentrations (e.g., iron, zinc) in samples. It works by vaporizing metals and analyzing light absorption. The study used this tool to track minerals like copper (Cu) and manganese (Mn) in celery.
Kjeldahl Nitrogen Analyzer: Equipment that measures nitrogen content in organic materials. Nitrogen is crucial for plant proteins and chlorophyll. In the study, this analyzer showed proline increased nitrogen levels in celery under salt stress.
Ionic Homeostasis: Balancing ions (like Na⁺ and K⁺) inside cells for proper function. Salt stress disrupts this balance, but proline in the study helped celery maintain it by reducing sodium and boosting potassium.
Antioxidant Activity: The ability to neutralize harmful free radicals caused by stress. Proline boosted antioxidants in celery, protecting cells from salt-induced damage. Antioxidants also benefit human health when consumed.
Osmotic Stress: Damage caused by imbalance of water and solutes in cells. High salt outside cells draws water out, leading to shrinkage. Proline helps plants like celery retain water, easing osmotic stress.
Dietary Fiber: Indigestible plant material (e.g., cellulose) that aids digestion. Celery is rich in fiber, which the study found increased with proline treatment under salt stress, enhancing nutritional value.
Pearson’s Correlation Analysis: A statistical method measuring how two variables relate (from -1 to +1). In the study, it showed strong links between amino acids like lysine and glutamine in celery.
Organic Acids: Compounds like citric acid that influence taste and stress tolerance. Proline increased oxalic and citric acid in celery, helping it adapt to salt stress by balancing pH and storing energy.
Abiotic Stress: Non-living environmental stressors like salt, drought, or heat. The study focused on salt stress, a common abiotic challenge in farming. Proline helped celery withstand this stress.
Hydroponic System: Growing plants in nutrient-rich water instead of soil. The study used hydroponics to control salt and proline levels, ensuring precise experimental conditions for celery.
Photosynthesis: The process where plants convert sunlight into energy. Salt stress damages chlorophyll, reducing photosynthesis. Proline treatment in celery helped maintain this process under stress.
Free Amino Acids: Building blocks of proteins not yet linked into chains. Salt stress alters amino acid levels, but proline increased glycine in celery, aiding stress adaptation and nutrition.
Electronic Nose (E-nose): A device mimicking human smell to detect odors. The study used E-nose sensors to analyze volatile compounds in celery, showing proline improved aroma under salt stress.
Oxidative Stress: Cell damage caused by reactive oxygen molecules (ROS). Salt stress increases ROS, but proline boosted antioxidants in celery to counteract this, protecting cells.
Salinity Tolerance: A plant’s ability to grow in salty conditions. The study showed proline enhances salinity tolerance in celery by improving nutrient uptake and reducing sodium toxicity.
Reference:
1. Gao, Y., Zhang, J., Wang, C., Li, N., Pu, K., Wang, T., Zhang, M., Zhang, L., Li, J., & Xie, J. (2025). Comprehensively assessing the effects of exogenous proline on nutritional and flavour quality of celery (Apium graveolens L.) under salt stress. Scientia Horticulturae, 339, 113847. https://doi.org/10.1016/j.scienta.2024.113847
2. Reference
