The research team approached peppermint not as a single commodity but as a collection of locally adapted populations, each shaped by the altitude, temperature regime, and soil chemistry of its home region. Iran spans an enormous environmental gradient, from the warm, low-lying plains of Alborz province near the capital to the cooler, elevated territories of East Azarbaijan and Zanjan in the northwest. Because essential oil biosynthesis in mint species is highly sensitive to environmental conditions, the researchers reasoned that accessions collected across this gradient would display meaningful differences in both the quantity of oil they produce and the relative proportions of its constituent monoterpenes. Their sampling strategy, drawing directly from the fields of established commercial growers, ensured that the plants assessed reflected real-world cultivation rather than greenhouse idealization.

The measurements that followed were comprehensive. The team recorded morphological traits such as plant height, branching, and biomass accumulation, examined leaf anatomy under the microscope, and determined dry shoot yield on a per-hectare basis. Essential oil was extracted and analyzed to quantify both its total content as a percentage of dry matter and its overall yield per hectare, while gas chromatographic profiling resolved the oil into its major components, most importantly menthone and menthol, the two monoterpenoids that define peppermint’s sensory and commercial identity. Alongside this phytochemical work, the researchers measured the concentration of macroelements and microelements in the plant tissue, together with selected heavy metals, providing a picture of what these plants draw out of their soils as well as what they manufacture within them.

The yield results alone tell a striking story of local variation. The highest dry shoot yield, 3410 kilograms per hectare, came from the Karaj accession, demonstrating that the lowland Alborz environment favors raw biomass production. But biomass is only half of the peppermint equation, because the value of the crop lies in the oil concentrated within its leaves and flowering tops. Here the ranking shifted decisively. The highest essential oil contents, 2.36 percent and 2.31 percent of dry material, were detected in accessions from Khosroshahr in East Azarbaijan province and from Zanjan province respectively, both northwestern, higher-altitude sources. When oil content was combined with harvested biomass to calculate essential oil yield per unit area, the Khosroshahr accession led with 68.5 kilograms per hectare, followed by Zanjan at 56.01 kilograms per hectare, confirming that these northwestern plants translate their chemical richness into genuine productivity.

The composition of the oil proved just as variable as its quantity. Menthone, the ketone precursor in the menthol biosynthetic pathway with the formula C10H18O, reached its highest proportions in the Zanjan accession at 27.23 percent and the Kermanshah accession at 24.31 percent. Menthol, the monoterpene alcohol C10H20O that delivers peppermint’s signature cooling effect through its activation of cold-sensitive receptors, peaked in the Khosroshahr accession at 47.57 percent, with the Hersin accession close behind at 46.52 percent. For buyers of peppermint oil, these numbers matter enormously, because menthol content is the primary determinant of oil quality and price. An accession that converts nearly half of its oil into menthol is a fundamentally more valuable raw material than one dominated by menthone, and the fact that this difference tracks with geographic origin suggests that environment and genotype together shape the enzymatic machinery of monoterpene metabolism.

The study’s most conceptually interesting finding concerns altitude and temperature. At lower altitudes, exemplified by Alborz, elevated temperatures appeared to increase branch production, pushing plants toward vegetative exuberance and the impressive dry yields recorded at Karaj. At higher altitudes such as Khosroshahr and Zanjan, the cooler conditions enhanced plant quality, and the researchers observed that sufficient branching in these accessions rendered them superior and more suitable for consumption. In other words, the lowland plants excel at making plant material while the highland plants excel at making the chemistry that consumers actually want. This trade-off between quantity and quality is a classic pattern in aromatic plant science, reflecting the temperature sensitivity of the terpenoid pathway, and it gives breeders and growers a clear framework: match the accession to the objective, choosing highland material when oil quality is paramount and lowland material when bulk biomass is the goal.

Beneath the yield and chemistry data sits a quieter but potentially far-reaching component of the work: the measurement of heavy metal uptake. Plants inevitably absorb elements from their soil, and aromatic and medicinal crops raise particular concerns because accumulated metals such as cadmium and lead can enter herbal products and essential oils destined for human use. At the same time, plants that tolerate and accumulate metals are of interest for phytoremediation, the use of vegetation to extract contaminants from polluted ground. Heavy metal stress is known to generate reactive oxygen species in plant tissues, including hydrogen peroxide, superoxide anion, and hydroxyl radicals, which damage cells but also trigger defensive responses that can alter secondary metabolism, including the production of monoterpenoids and other specialized metabolites. By documenting how peppermint accessions differ in their uptake of macroelements, microelements, and selected heavy metals, the study provides a baseline for both food-safety screening and the potential deployment of mint in contaminated landscapes.

The element analysis also connects to the broader physiology of oil production. Essential oil synthesis in peppermint depends on adequate supplies of nitrogen, phosphorus, potassium, and magnesium, and on trace micronutrients that serve as cofactors for the enzymes of the plastidial and cytosolic terpenoid pathways. Accessions that accumulate nutrients more efficiently may be better positioned to sustain oil gland activity through the growing season, which is one plausible contributor to the superior oil yields of the Khosroshahr and Zanjan plants. Understanding these nutrient dynamics at the accession level, rather than treating peppermint as an undifferentiated crop, opens the door to precision fertilization strategies tailored to specific plant material and specific soils, reducing input waste and improving oil consistency for processors.

For Iran, one of the historic centers of mint cultivation and a country with a rich tradition of medicinal plant use, the study is a reminder that agricultural biodiversity is an economic asset hiding in plain sight. The Khosroshahr accession, with its exceptional oil content, highest oil yield, and top menthol percentage, emerges as a standout candidate for commercial propagation and for use as a parent in breeding programs aimed at combining high menthol with robust agronomic performance. The Zanjan accession, with its high menthone and strong oil yield, offers complementary chemistry. Meanwhile, the Karaj accession’s biomass dominance illustrates the value of lowland material for applications where total plant matter, rather than oil, is the product. Preserving and characterizing such landrace diversity, the authors’ work implies, is essential insurance for an industry facing changing climates and shifting growing regions.

More broadly, the research exemplifies a trend in crop science toward treating phytochemical quality as a heritable, environment-modulated trait that can be mapped, predicted, and optimized. The peppermint plant, a sterile hybrid that has spread around the world through cuttings, carries within its populations a surprising range of chemical phenotypes, and studies like this one show that those phenotypes respond systematically to altitude and temperature. As demand for natural menthol continues to grow, and as concerns about heavy metals in herbal products intensify, the ability to select the right accession for the right site, and to understand what that plant will draw from its soil, becomes a competitive necessity. The Iranian peppermints profiled in Plant Biosystems demonstrate that the future of this ancient flavor may depend less on new chemistry than on paying closer attention to the old plants still growing in the fields where they have always been grown.

Subject of Research: Phenotypic and phytochemical diversity and heavy metal uptake in peppermint accessions

Article Title: Phenotypic and essential oil profile diversity, and heavy metal uptake potential of peppermint (Mentha × piperita) accessions

Article References: Sarkhosh, M. R., Abbaszadeh, B., Ardakani, M. R., Habibi, D., & Ilkaee, M. N. (2026). Phenotypic and essential oil profile diversity, and heavy metal uptake potential of peppermint (Mentha × piperita) accessions. Plant Biosystems, 160(4), Article 227. https://doi.org/10.1007/s44473-026-00162-0

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00162-0

Keywords: peppermint, Mentha × piperita, essential oil, menthol, menthone, heavy metals, Iran, altitude, leaf anatomy, phytoremediation, Lamiaceae, crop diversity