📄 agriculture

Efficiency of melatonin as a mitigating agent for saline stress in Formosa papaya in hydroponic systems

This study demonstrates that while hydroponic floating systems inherently offer greater salinity tolerance for Formosa papaya than sand-based systems, the application of melatonin at concentrations of 50 µM or higher effectively mitigates the negative growth effects of salt stress in both hydroponic setups.

Original authors: Brenda Adelino Macêdo Campelo, Vander Mendonça, Francisco Assis de Oliveira, Maria Júlia Silva Oliveira, Jeane Cruz Portela, Rayanne Aires Dantas, Francisco Felipe Barroso Pinto, Antônio Gustavo Luna

Published 2026-09-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Brenda Adelino Macêdo Campelo, Vander Mendonça, Francisco Assis de Oliveira, Maria Júlia Silva Oliveira, Jeane Cruz Portela, Rayanne Aires Dantas, Francisco Felipe Barroso Pinto, Antônio Gustavo Luna Souto, Vanessa Barbosa Brilhante, Francisco Valdegones Selina Caruca, Isadora Karolline Silva, Layza Mayrla Abreu, Karen Geovana Silva Carlos, Breno Leonan Carvalho Lima, Mario Jonas Veras Costa, Vinicius Lima Dias, Giselly Emilly Gonçalves Queiroz

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the arid corners of the world, where the sun beats down and fresh water is a scarce commodity, farmers face a silent enemy: salt. When soil or irrigation water contains too much salt, it acts like a sponge that pulls moisture away from plant roots, leaving them thirsty even when they are surrounded by water. This condition, known as salinity stress, forces plants to shut down their internal machinery, stopping them from growing and producing food. For crops like papaya, which are vital to the economies of tropical regions, this can be devastating. To combat this, scientists have been exploring ways to help plants survive these harsh conditions, often looking to natural substances that might act as a shield. One such substance is melatonin. While most people know it as a hormone that helps humans sleep, plants produce it too, using it to manage stress and protect their cells from damage. The question researchers have been asking is whether giving extra melatonin to a struggling plant can help it withstand the punishing effects of salty water.

A team of researchers in Brazil set out to test this idea using papaya plants, specifically the Formosa variety, which is widely grown for its sweet fruit. They conducted their experiment inside a greenhouse, growing the plants in two different types of soil-free setups. One method involved floating the plants on a bed of foam, with their roots dangling directly into a nutrient-rich water solution. The other method placed the plants in pots filled with washed sand, which was then watered with the same nutrient solution. The scientists wanted to see if the way the plants were grown would change how they handled salt. They subjected the plants to different levels of saltiness in their water, ranging from a mild level to a high concentration that would normally cause significant damage. For the plants facing the high salt levels, the researchers applied different doses of melatonin to their leaves, testing whether this hormone could act as a rescue agent.

The results revealed a clear difference between the two growing methods. The plants growing in the floating system, where their roots were constantly bathed in water, proved to be much more resilient. Even when the water was made very salty, these plants continued to grow well, showing that this method of cultivation naturally helps papaya tolerate high salt levels. In contrast, the plants growing in the sand substrate struggled significantly when exposed to the same salty water. Their growth slowed, their leaves shrunk, and they produced far less biomass, or total plant weight, than their floating counterparts. This suggests that the sand system made the plants more vulnerable to the stress of the salt, likely because the roots had to work harder to find water in the soil compared to the floating system where water was immediately available.

However, the story changed when melatonin was introduced to the struggling plants in the sand. The hormone acted as a powerful buffer against the salt. When the researchers applied melatonin to the leaves of the plants in the sand, it significantly reduced the damage caused by the salt. The plants that received the hormone treatment grew taller, developed thicker stems, and produced more leaves and roots than the untreated plants facing the same salty conditions. The study found that doses of melatonin starting from a specific moderate level were effective in mitigating the negative effects. The hormone seemed to help the plants keep their internal systems running smoothly, allowing them to absorb carbon dioxide and produce energy even under stress. It acted like a protective shield, helping the plants maintain their structure and continue growing when they otherwise would have withered.

The researchers also looked closely at how the plants breathed and processed energy. They measured how much water the plants lost through their leaves and how much carbon dioxide they absorbed. In the floating system, the plants handled the salt with ease, showing little change in their breathing patterns. But in the sand system, the salt caused the plants to close their pores to save water, which in turn stopped them from taking in the carbon dioxide they needed to grow. The application of melatonin helped reverse this trend in the sand-grown plants. It allowed them to keep their pores open enough to function properly without losing too much water, effectively balancing their needs. This physiological adjustment meant that the plants could continue to build new tissue and accumulate weight despite the harsh environment.

Ultimately, the study demonstrated that while the way a crop is grown plays a massive role in its ability to survive salty conditions, there is a chemical tool available to help when the environment is less than ideal. The floating hydroponic system provided a natural advantage, making the plants inherently more tolerant to salt. But for systems where the plants are more exposed to stress, like those in sand, applying melatonin to the leaves offered a viable solution. It did not completely eliminate the stress, but it significantly reduced the harm, allowing the papaya plants to maintain their growth and health. This finding offers a promising strategy for farmers in dry, salty regions who need to grow crops using water that would otherwise be too harsh for standard agriculture. By understanding how to use natural hormones to support plant resilience, it may be possible to turn marginal lands into productive fields, ensuring that vital crops like papaya can thrive even when the water is not perfect.

Technical Summary: Efficiency of Melatonin as a Mitigating Agent for Saline Stress in Formosa Papaya in Hydroponic Systems

Problem Statement
Salinity represents a critical abiotic stressor affecting agricultural production, particularly in the semi-arid regions of Northeastern Brazil, where groundwater often exhibits high electrical conductivity (EC). Formosa papaya (Carica papaya L.), a crop of significant economic and nutritional value, is classified as moderately sensitive to salinity, with a threshold between 2.0 and 4.0 dS m⁻¹. Exposure to salt stress induces metabolic changes, including reduced water uptake, stomatal closure, restricted photosynthesis, and inhibited cell division, ultimately compromising growth and biomass. While hydroponic systems offer controlled environments, the efficacy of exogenous melatonin (N-acetyl-5-methoxytryptamine) as a mitigating agent against saline stress specifically in papaya under different hydroponic configurations remains an area requiring further investigation.

Methodology
The study employed a split-plot experimental design conducted in a greenhouse at the Federal Rural University of the Semi-Arid (UFERSA), Brazil. The main plots consisted of two distinct hydroponic systems:

Floating System: Plants grown in 10 dm³ pots with roots in constant contact with a circulating nutrient solution.
Substrate System: Plants grown in 10 dm³ pots filled with washed medium-textured sand.

The subplots comprised five treatment combinations involving salinity levels and melatonin concentrations:

C1: 2.0 dS m⁻¹ + 0 µM melatonin (Control).
C2: 5.0 dS m⁻¹ + 0 µM melatonin (Saline stress).
C3: 5.0 dS m⁻¹ + 50 µM melatonin.
C4: 5.0 dS m⁻¹ + 100 µM melatonin.
C5: 5.0 dS m⁻¹ + 200 µM melatonin.

Papaya seedlings (cultivar Fomosa Tainung No. 1) were transplanted 30 days after sowing. Melatonin was applied foliarly using a sprayer to ensure complete leaf wetting. Evaluations were conducted 48 days after transplanting for gas exchange variables (transpiration, CO₂ assimilation, stomatal conductance, internal CO₂, water use efficiency, and carboxylation efficiency) and 50 days after transplanting for morphological and biomass variables (plant height, stem diameter, root length, root volume, leaf area, leaf number, and dry masses of leaves, stems, roots, and total).

Key Results
The analysis of variance revealed significant interactions between the cultivation system and the salinity/melatonin combinations for most physiological and growth variables.

Hydroponic System Influence: The floating system demonstrated superior tolerance to salinity compared to the sand substrate system. In the floating system, saline stress (5.0 dS m⁻¹) did not significantly reduce plant height, stem diameter, or biomass accumulation in the absence of melatonin. Conversely, the substrate system exhibited severe deleterious effects from salinity, including significant reductions in plant height (35.9%), stem diameter (33.6%), root volume (82.1%), and total dry mass (78.0%).
Gas Exchange: Salinity affected gas exchange differently depending on the system. In the floating system, salinity increased stomatal conductance ( $g_s$ ) by 123.7% compared to the control, whereas in the substrate system, it reduced $g_s$ by 48.2%. Melatonin application generally reduced stomatal conductance in the floating system but mitigated the stress-induced reduction in the substrate system.
Melatonin Efficacy:
- Floating System: The application of melatonin did not significantly improve performance in the floating system, as plants were already highly tolerant to the saline conditions. In some cases, higher melatonin doses (e.g., 200 µM) slightly reduced photosynthesis.
- Substrate System: Melatonin application was effective in mitigating the negative effects of salinity. Specifically, concentrations starting from 50 µM reduced the detrimental impact of salt stress on growth parameters and biomass. The 100 µM and 200 µM doses showed the strongest association with root growth variables (length and volume) and total dry mass accumulation, respectively, in the Principal Component Analysis (PCA).
Biomass Accumulation: In the substrate system, melatonin application restored biomass accumulation levels closer to the non-stressed control. For instance, in the substrate system, the absence of melatonin under salinity caused a 73.67% reduction in leaf dry mass, while melatonin application reduced this loss significantly.

Significance and Conclusions
The study concludes that the cultivation system is a decisive factor in papaya response to salinity. The floating hydroponic system inherently promotes greater tolerance to saline stress, likely due to the constant availability of water and nutrients, which minimizes energy expenditure for uptake. Consequently, the mitigating effect of melatonin was not statistically significant in the floating system.

However, in the substrate-based hydroponic system, where salinity caused severe growth inhibition, exogenous melatonin application proved to be an effective mitigating agent. The authors state that melatonin concentrations above 50 µM successfully reduced the negative effects of salinity on papaya plants grown in sand. The hormone appears to function by protecting chloroplast structure, maintaining photosynthetic activity, and stimulating root system development, thereby enhancing the plant's ability to cope with osmotic and ionic stress. The research highlights that while hydroponic system selection is primary for salinity management, melatonin serves as a valuable physiological modulator for crops grown in substrate-based systems under saline conditions.

More like this