The study, led by Tayeb Muhammad and colleagues including Tao Yang, Juan Wang, Baike Wang, and corresponding author Qinghui Yu, focused on the earliest and most vulnerable phase of a crop plant’s life: germination and seedling establishment. This window is decisive for final yield, because seedlings that fail to establish vigor in their first days rarely recover. The researchers compared three groups of tomato seeds: untreated controls, seeds soaked in water alone (hydro-primed), and seeds primed in a melatonin solution. The primed and control seeds were then exposed to cadmium, a toxic heavy metal that accumulates in agricultural soils worldwide, and to polyethylene glycol, a laboratory-standard agent that mimics the osmotic stress plants experience during drought by restricting water availability.

Without protection, both stressors inflicted severe damage. Cadmium and osmotic stress significantly inhibited seed germination, stunted seedling growth, and reduced biomass accumulation. The mechanism behind this damage is well understood in plant physiology: both stresses trigger the overproduction of reactive oxygen species, including hydrogen peroxide, which attack cellular membranes and macromolecules. The team measured this oxidative assault directly, recording elevated hydrogen peroxide levels and increased lipid peroxidation, the destructive degradation of membrane fats that is tracked through malondialdehyde, a classic biomarker of oxidative injury. They also observed increased electrolyte leakage, a sign that cell membranes had lost their integrity and were allowing ions to escape from the cells.

One of the most scientifically interesting aspects of the work is the discovery that the two stressors behave in strikingly different ways within the plant. Cadmium stress primarily targeted the root system, which is unsurprising given that roots are the first point of contact for the metal in the soil solution and the main site of cadmium uptake. Osmotic stress imposed by polyethylene glycol, by contrast, had its strongest impact on shoot growth and above-ground physiological performance, reflecting the plant’s struggle to take up water and maintain turgor in its leaves. This differential targeting pattern matters for breeders and agronomists, because it suggests that no single trait or treatment can fully shield a seedling; a successful protective strategy must strengthen both root and shoot systems simultaneously, which is precisely what melatonin priming achieved.

Melatonin, chemically known as N-acetyl-5-methoxytryptamine, has emerged over the past decade as a master regulator of stress responses in plants, functioning alongside its better-known role as a circadian signal. In this study, melatonin priming significantly improved germination efficiency and seedling vigor under both stress conditions. Primed seedlings displayed enhanced root and shoot elongation, accumulated greater biomass, and maintained higher levels of chlorophyll, the photosynthetic pigment whose degradation is an early casualty of stress. The treatment also boosted proline accumulation, a compatible osmolyte that plants deploy to protect proteins and membranes while adjusting their internal water balance, an adaptation especially valuable under drought.

The protective effect extended deep into the plant’s biochemical defenses. Melatonin-primed seedlings showed markedly lower levels of electrolyte leakage, malondialdehyde, and hydrogen peroxide, indicating that the hormone had blunted the oxidative cascade at its source. The team traced this resilience to a strengthened enzymatic antioxidant system. Activities of four key antioxidant enzymes rose in primed seedlings: superoxide dismutase, which converts superoxide radicals into hydrogen peroxide; peroxidase and catalase, which break hydrogen peroxide down into water and oxygen; and ascorbate peroxidase, a central component of the ascorbate-glutathione cycle that detoxifies peroxide within chloroplasts and other compartments. Crucially, this enzymatic upregulation was mirrored at the molecular level, with increased transcript abundance of the genes encoding these enzymes, showing that melatonin primes the seedlings’ stress-response machinery at the level of gene expression rather than merely acting as a chemical antioxidant itself.

The researchers went further, profiling stress-responsive genes that coordinate the plant’s broader adaptive response. Primed seedlings showed upregulation of SlGST, a glutathione S-transferase involved in detoxifying reactive compounds and sequestering heavy metals; SlDREB, a transcription factor that switches on suites of genes during drought and other abiotic stresses; SlNHX1, a sodium/proton antiporter that contributes to ion homeostasis and vacuolar sequestration of toxic ions; and SlDHN, a dehydrin gene whose products stabilize proteins and membranes during dehydration. Together, these molecular signatures indicate that melatonin priming activates both the ROS-scavenging frontline and the downstream regulatory and protective networks that give seedlings lasting tolerance rather than a fleeting chemical shield.

The practical implications of the work are considerable. Cadmium contamination of agricultural soils is a global problem driven by industrial emissions, phosphate fertilizers, and wastewater irrigation, and drought is intensifying across many of the same regions. Tomato, one of the most widely grown and economically important horticultural crops, is particularly sensitive during establishment. Seed priming is attractive as a technology because it is inexpensive, requires no genetic modification, applies no chemical load to soil, and can be integrated into existing seed conditioning pipelines before sowing. Compared with hydro-priming, melatonin priming conferred a clear and consistent advantage, suggesting that the hormone acts as a genuine elicitor of stress memory rather than simply accelerating germination. The study builds on a growing body of evidence from wheat, maize, rice, rapeseed, and other crops showing that melatonin seed treatments can improve germination under salinity, drought, and heavy metal stress, and extends this evidence to the combined challenge of cadmium and osmotic stress in tomato specifically.

The authors, whose work was supported by the Research Fund for Young Tianchi Talent program of the Xinjiang Uyghur Autonomous Region, published their findings in Volume 160 of Plant Biosystems, article number 245, on 20 August 2026. As climate change expands the footprint of drought and legacy contamination continues to constrain arable land, strategies that harden crops at the very start of their lives are likely to attract growing attention. This study demonstrates that a molecule best known for regulating sleep can, at the scale of a seed, set a young tomato plant on a fundamentally more resilient developmental trajectory, strengthening antioxidant defenses, reprogramming stress-responsive gene expression, and preserving the growth and physiological functions on which a season’s harvest ultimately depends. Future work will need to confirm these benefits under field conditions, but the message from the laboratory is clear: what happens to a seed before it meets the soil may determine how well the plant survives what the soil has in store.

Subject of Research: Melatonin seed priming as a strategy to alleviate cadmium and drought stress in tomato seedlings

Article Title: Seed priming with melatonin alleviates cadmium and drought stress by regulating growth, physiological functions and defense mechanisms in tomato

Article References: Muhammad, T., Yang, T., Wang, J., Wang, B., & Yu, Q. (2026). Seed priming with melatonin alleviates cadmium and drought stress by regulating growth, physiological functions and defense mechanisms in tomato. Plant Biosystems, 160(5), Article 245. https://doi.org/10.1007/s44473-026-00261-y

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00261-y

Keywords: melatonin, seed priming, tomato, cadmium stress, drought stress, oxidative stress, antioxidant enzymes, reactive oxygen species, germination, seedling vigor, stress-responsive genes, plant stress physiology

Soil salinity is no small problem. As global temperatures rise and irrigation practices intensify, salt-laden soils are expanding across prime agricultural regions, reducing crop yields and threatening food security. When wheat seedlings encounter high concentrations of sodium chloride, they face a double assault: first, an osmotic stress that makes it harder for roots to draw water from the soil, and second, an ionic stress in which excess sodium disrupts the delicate balance of potassium and other essential nutrients inside plant cells. The resulting damage cascades through cellular machinery, impairing photosynthesis, destabilizing membranes, and triggering the accumulation of reactive oxygen species that can shred lipids, proteins, and DNA. Plants that survive this onslaught do so by mobilizing an arsenal of compatible solutes, amino acids, and sugars that help restore osmotic balance and shield vulnerable molecules from harm. Metabolomics, the comprehensive cataloging of these small molecules, has emerged as one of the most powerful tools for understanding how plants mount this chemical defense.

The Turkish research team chose their study subjects with deliberate care. Altay-2000, a modern bread wheat variety, represents the hexaploid lineage that dominates global grain production, carrying a full AABBDD genome of 42 chromosomes. Kunduru-1149, by contrast, belongs to the tetraploid durum wheat lineage, with 28 chromosomes, and was officially released in Türkiye in 1967 after being selected from a local Anatolian landrace. Durum wheat has flourished in the region since antiquity, and Kunduru has long been prized for its remarkable resilience under drought, temperature extremes, and other environmental hardships. The Bahri Dagdas International Agricultural Research Institute supplied commercial seed of both varieties, and the researchers set about comparing how each responded to escalating salt concentrations.

The experimental design was straightforward but rigorous. After breaking seed dormancy through a three-day cold treatment at minus 4 degrees Celsius and surface-sterilizing the seeds with sodium hypochlorite and SDS, the researchers germinated the wheat on moistened filter paper in darkness. Once roots emerged, seedlings were transferred to a hydroponic system containing quarter-strength Hoagland nutrient solution, a standardized recipe providing nitrogen, potassium, calcium, phosphorus, magnesium, and sulfur at carefully calibrated concentrations with the pH adjusted to 5.8. Salt was then applied abruptly at five concentrations: zero, 0.25, 0.50, 0.75, and 1.00 percent sodium chloride. Five replicates were maintained per treatment, each consisting of twenty seeds, and the plants were grown for ten days under a twelve-hour light and dark cycle at 25 degrees Celsius before harvest. Growth was documented photographically every day, and at the end of the experiment, shoot and root lengths were carefully measured.

The growth data revealed a sobering picture. Across both varieties, shoot length declined significantly as salt concentration climbed, a relationship that held up under formal two-factor statistical analysis with a p-value of 0.00021. Yet, somewhat surprisingly, the researchers found no statistically significant difference between Altay and Kunduru in their overall growth response to salinity, and the interaction between genotype and salt concentration also failed to reach significance. This meant that, at least in terms of visible growth under these experimental conditions, neither variety demonstrated a clear physiological advantage over the other when confronted with rising salt.

But the metabolomic analysis told a far more nuanced story. After freeze-drying and grinding shoot tissue in liquid nitrogen, the team extracted polar metabolites using a methanol-chloroform protocol and subjected the resulting samples to one-dimensional proton NMR spectroscopy on a 500-megahertz Bruker Avance III instrument, acquiring roughly four thousand scans per sample with water suppression. Spectra were processed against the Chenomx reference library, yielding a quantified matrix of 43 metabolites spanning amino acids, sugars, organic acids, and other primary metabolic compounds. Principal component analysis revealed that the first two components explained 84.1 percent of the total variance, suggesting that the metabolic profiles were dominated by a handful of powerful, coordinated shifts.

The researchers then applied a supervised modeling technique known as partial least squares discriminant analysis, or PLS-DA, to test whether metabolic profiles could reliably distinguish between salt treatments within each variety. For Altay, the model achieved a cross-validated classification accuracy of 100 percent, with an R-squared value of 0.942 and a Q-squared statistic of 0.821, supported by exact permutation testing with a p-value of 0.0024. The Kunduru model performed nearly as well, reaching 88.9 percent accuracy with similarly strong statistical backing. These results confirmed that both varieties underwent profound and reproducible metabolic reorganization as salt concentrations rose, even though the overall growth trajectories were statistically indistinguishable between the two genotypes.

When the researchers compared the two varieties directly at each individual salt concentration, the pairwise models showed visually striking separation, with cross-validated Q-squared values exceeding 0.97 at four of the five salt levels. Yet the team exercised considerable scientific caution here, noting that with only three biological replicates per genotype per condition, the number of possible balanced label assignments was extremely small, capping the power of permutation testing. As a result, these pairwise comparisons were flagged as exploratory rather than confirmatory, illustrating an important lesson in modern metabolomics: impressive-looking model performance can coexist with formally inconclusive statistics when sample sizes are limited.

Despite these statistical limitations, the analysis did yield several recurrent compounds that emerged as candidate salt-responsive metabolites. Asparagine, proline, alanine, lysine, serine, fructose, and dimethylamine were repeatedly highlighted across different comparisons. Proline, in particular, is a classic player in plant stress biology, widely regarded as a compatible solute that helps cells maintain osmotic pressure and protect proteins and membranes from salt-induced damage. Asparagine participates in nitrogen storage and transport, potentially reflecting a shift in carbon-nitrogen balance under stress. Changes in sugars and organic acids, including fructose, sucrose, glucose, malate, and succinate, point to a broad reorganization of primary metabolism as plants redirect energy and resources toward survival. The researchers emphasized that these compounds should be regarded as hypotheses for further investigation rather than validated biomarkers of salt tolerance, since physiological endpoints such as biomass retention, sodium-to-potassium ratios, and survival under field conditions were not measured in this initial study.

What makes this work particularly noteworthy is its grounding in one of the world’s great centers of wheat biodiversity. Türkiye sits within the Fertile Crescent, the cradle of wheat domestication, and the Anatolian Plateau has harbored wheat varieties for millennia. Kunduru-1149, derived from a landrace native to this ancient agricultural landscape, is considered among the most genetically diverse of the local durum cultivars and has previously shown remarkable adaptation to abiotic stresses. By applying modern metabolomic tools to this historically significant germplasm, the researchers are helping to bridge the gap between traditional agricultural knowledge and cutting-edge plant science. The metabolic profiles they have generated may serve as a roadmap for future breeding programs seeking to develop wheat varieties that can thrive on salt-degraded soils, an increasingly urgent goal as climate change and unsustainable irrigation practices continue to expand the world’s salinized farmland. Larger studies with physiological validation will be needed before any of these candidate metabolites can be confidently deployed as markers of salt tolerance, but the foundation has now been laid for a deeper understanding of how one of humanity’s oldest crop companions copes with one of its most pervasive environmental challenges.