In mammals, the tissues that produce gametes and the gamete itself, for example, the oocyte, synthesize melatonin theoretically to protect themselves from damage related to free radical toxicity [134].

The hypothesis is that since the oocyte represents the next generation, during maturation, often associated with a high generation of free radicals, special precautions must be taken to prevent these cells from being oxidatively damaged as such damage can lead to death. or to an abnormal development of the fetus and the newborn, compromising the perpetuation of the species.

Similarly, for the plants the seeds represent the next generation and if the molecules they contain show excessive oxidative stress, the seed cannot develop and the generation is lost. Furthermore, most of the seeds are rich in polyunsaturated fats, which are easily oxidized, for this reason the presence of such powerful antioxidants as melatonin would be important to reduce the likelihood of their damage.
While this information was not a justification for their study, Manchester et al. [133] estimated the concentrations of melatonin in seeds (the next generation) of 15 different plants and all that the authors had described as high levels of indole. These values ranged from 2 to 200 mg / g dry weight with the highest values found in white and black mustard seeds. In walnut (Juglans regia L.), melatonin levels were in the same concentration range (3.5 mcg / g dry weight) [135]. It should be noted that to date the highest levels of melatonin have been measured in pistachio (Pistacia vera), where the reported values are in the range of μg / ml in the methanolic extract [136]. Obviously, seeds and plants generally contain melatonin levels that far exceed those measured in vertebrate blood [137,138] or tissues [139-141], where concentrations are usually in the range of pg / mL and pg / g of protein ,. At least by mammalian tissue standards, melatonin levels in plant organs are much higher.
As noted, the pistachio kernel has unusually high levels of melatonin. This discovery is of particular interest given that pistachio is a desert plant and survives for long periods without water (drought).
Additionally, this plant is highly tolerant of salty soils and can survive in ambient temperatures ranging from -10 ° C to 48 ° C. As will be discussed later in this paper, each of these circumstances, namely, drought, saline exposure, and extreme temperatures, increases melatonin production in plants.
Hence, it is possible that the pistachio tree is able to survive and grow in desert conditions due to its high levels of melatonin. Based on this evidence, desert plants in general may have high levels of melatonin, for example, desert cactus and sage, marigold, lily and willow (Chilopsis), etc. Melatonin levels can also affect regional distribution so that plants that normally grow in extreme latitudes can have high concentrations of melatonin, which can also aid their survival in cold conditions (for example, plants growing on the tundra).

We also hypothesized, for the same reason, that extremophiles (organisms that thrive in physically or geochemically extreme conditions) are probably enriched with melatonin compared to indole levels in mesophiles or neutrophils. So-called Japanese / Chinese herbs have historically been presented as herbs with medicinal properties and several of these have been approved for these uses in Canada. Assuming that any melatonin present in these products could have contributed to their usefulness as treatments for various ailments, Murch et al. [132] conducted a study to estimate indoleamine levels in green and gold leaf varieties of feverfew (Tanacetum parthenium), St. John's wort (Hypericum perforatum), Huang Qin (Scutellaria baicalensis), and Tanacet , a commercial preparation of feverfew that comes in a tablet form. Tanacet meets Canadian requirements for use as a medicinal product. The methodology for the extraction and measurement of melatonin used by Murch and colleagues [132] was that described by Poeggeler and colleagues [142].
The feverfew samples were either fresh, or freeze-dried or dried and all reported melatonin levels in the range of 1-3 μg / g of tissue. Tanacet, the preparation in tablets based on feverfew, instead reported a significantly lower content of melatonin (0.5 μg / g) compared to the levels of this plant. The flowering parts of St. John's wort had a much higher melatonin concentration (4.4 μg / g) while the leaves had levels of 1.8 μg / g. Scutellaria baicalensis leaves had exceptionally high melatonin values at around 7 mcg / g of fresh leaves [132].
If the measures are valid, these medicinal herbs contain much higher levels of melatonin than the edible plants studied by Dubbels et al. [121] and Hattori and colleagues [128]. The authors concluded that the melatonin in these medicinal products may contribute to their anecdotal beneficial physiological effects [132].
As a follow-up to the study by Murch et al. [132], using a solid phase extraction method coupled with high pressure liquid chromatography (HPLC) / mass spectrometry, Chen et al. [143] determined the melatonin levels in 108 Chinese medicinal herbs. Again, the rationale for the study is that, if melatonin is present in significant amounts, it may be a contributing factor to the beneficial effects seen in these preparations. The products from which the extracts were prepared included flowers, seeds, leaves, roots, and stems and all met standards for use in traditional Chinese medicine. Most of the herbs tested contained detectable amounts of melatonin, including 64 they had values that exceeded 10 ng / g dry weight and many had levels greater than 1 μg / g. The results of this study therefore gave greater credibility that the melatonin contained in these products could contribute to their medicinal value.

This group was particularly interested in the possibility that herbs containing melatonin could be used as potential treatments for diseases that have a significant free radical component. Melatonin, as well as several products that are formed when melatonin detoxifies free radicals, play a role in reducing oxidative stress. [127.144-151]. Neither the study by Murch et al. [132] nor by Chen et al. [143], however, has provided evidence that when these herbs are taken, they change the blood levels of indolamine.
Initial reports linked to the presence of melatonin in higher plants did not attract the interest of many plant biologists for at least a decade or more after the first papers were published. During this ten-year interval, however, a series of short reviews were published that summarized early descriptions regarding indole derivatives in plants, pointing to some possible shortcomings in the reported measurements, and most importantly, discussing the functional relevance of melatonin in plants. [152-161]. In general, more credit was given to reports in which gas chromatography and mass spectrometry were used to estimate melatonin levels or to verify the reliability of other tests. Furthermore, considering that melatonin may be unstable in plant extracts this may have caused an underestimation of some of the measured values of actual melatonin concentrations in several plant species.

Extract from the thesis "Melatonin in the plant world (Phytomelatonin): therapeutic properties and future perspectives" by Dr. Giorgio Guerrini

For further information, please refer to the bibliography page.