The effect of light and soil moisture on the accumulation of three flavonoids in the leaves of Mexican oregano (Lippia graveolens Kunth)

Mexican oregano (Lippia graveolens) is used as an aromatic spice and also in traditional medicine, primarily for the treatment of gastrointestinal and respiratory ailments. High demand for L. graveolens has led to exploitation of wild populations. Cultivation of L.graveolens would reduce the pressure on wild populations, but the effect of ex situ propagation on its medicinal properties requires evaluation of the stability of the chemotype. Here, we report the differential accumulation of three flavonoids with biological significance (naringin, naringenin, and pentahydroxyflavanone-hexoside) in leaves of L. graveolens plants propagated for 11-months under greenhouse conditions. The effect of light intensity and soil moisture on flavonoid content was studied, and flavonoid concentrations in greenhouse-propagated plants were compared with concentrations in wild individuals in wet and dry seasons from a semi-arid region of Mexico. Pentahydroxyflavanone-hexoside was most abundant of the three compounds, and was unaffected by light intensity or soil moisture. In contrast, the concentration of naringenin was four times greater under high than under low light intensity (p < 0.05). Over growing time in greenhouse, there was a positive relationship between the concentration of naringenin, which is a biosynthetic precursor, and its downstream products, naringin and pentahydroxyflavanone-hexoside, under high light intensity (r = 0.72 and 0.89, respectively; p < 0.05). Flavonoid content was highly variable in wild samples, although naringenin concentration was higher than in greenhouse-grown plants (100 vs 45.8 mg kg -1 fresh tissue, respectively; p < 0.05), while pentahydroxyflavanone-hexoside and naringin were twice as concentrated in the greenhouse-grown plants as in the wild samples. Accumulation of naringenin during ex situ propagation can be stimulated by a combination of high light intensity (around 640 µmol m -2 s -1 ) and soil moisture at 80% of field capacity. Under these conditions, naringenin levels are still lower than in wild individuals, but the concentrations are less variable, potentially making pharmacological production more predictable and sustainable.