The studies that have been done on plants, though fewer in number than in animal models, have found melatonin present in the roots, leaves, flowers, fruits, and seeds. Many plants used for medicinal purposes have been found to have high levels of melatonin. It is speculated that melatonin may act as an antioxidant in plants to protect them against environmental oxidative stress and perhaps even control flowering, but more studies need to be conducted in order to get a clearer picture of the physiological role of melatonin in plants.
Since melatonin is found in the fruit of other plants, it is thought that it may also be present in grapes. The health benefits of melatonin, if present in grapes, could act synergistically with the polyphenols (i.e. resveratrol) and other health beneficial compounds in grapes, which would explain in part some of the widely studied health benefits of wine. One study was able to find melatonin present in Vitis vinifera grapes, though the levels varied across varieties and winemaking techniques. Extremely little is known about the role melatonin plays in grapes, and almost nothing is known about its abundance during the day and night.
The study under review today is one of the very first to examine these questions. Published this spring in the Journal of Pineal Research, the authors goals were three fold. First, they sought to test whether melatonin levels in Vitis vinifera grape skins fluctuate during the day; second, to examine whether or not daylight itself was responsible for changes in melatonin levels throughout the day; and finally third, whether there was a time of day that melatonin and other polyphenols levels were all at a maximum, in order to potentially create a more healthy bottle of wine. Though brief, this study provides fascinating insight on the role of melatonin in grapes, and provides an interesting time schedule for harvesting grapes, in order to create the most health-beneficial wine possible.
Grapes were harvested during the 2010 season at a commercial vineyard of Malbec grapes (selected clones without rootstock), in sandy soils and drip irrigated plots, which was located in Gualtallary, Tupungato, in the province of Mendoza, Argentina. The 11-year old vines were trained on a vertical trellis and pruned as Guyot. Rows were arranged in a north-south direction, and were spaced 2 meters apart, with 1.2 meters between plants in each row. The vines were protected by antihail nets, which were made from black polyethylene, which produced about 17% shade on the plants.
The experiment was set up as a randomized complete block design (for those not familiar with statistical methods, this type of a design is fantastic), with each row acting as a block. Grape sampling was completed on April 26th, 2010, which coincided with the commercial harvest date. In each block (row), 50 grapes were collected from 10 clusters at different times of the day (and placed in black nylon bags to avoid light exposure after harvest). Times of harvest were 02:00, 06:00, 09:30, 13:00, 17:30, and 23:30 hr. All samples were kept away from light and on ice while in the field, and upon arrival in the laboratory, were subsequently frozen and stored at -18oC until further analysis.
For the light exclusion treatment, clusters were covered with black ethylene vinyl acetate (EVA) in the inner side, and a white paper covering this EVA in order to avoid an increase in cluster temperature throughout the day. Temperature and photosynthetic active radiation was measured throughout the day.
Melatonin was measured via HPLC-ESI-MS/MS. Total phenolic compounds were measured via UV-Vis. Also measured during this experiment was malondialdehyde, which is a product of lipid perioxidation.
- Melatonin levels in the skins of Malbec grapes were low during the night, and reached a strong peak at dawn. Throughout the day, melatonin levels decayed to undetectable values at noon and during the afternoon.
o In humans, melatonin (which is released by the pineal gland) increases in the serum during the night, and decreases during the day.
o For the grapes that were kept in the dark by EVA covering, melatonin levels were higher than those exposed to natural sun radiation.
§ Temperatures of covered versus not covered grapes showed no significant differences.
- Anthocyanins and polyphenols levels did not fluctuate during the day at harvest.
- Malodialdehyde (MDA), an indicator of oxidative damage, fluctuated during the day and night, and reached a maximum level at 17:30 hrs.
o MDA minimums coinciding when melatonin levels were detectable in grapes. Since MDA levels were lowest (i.e. least damage done) when melatonin was present, it may suggest a protective antioxidant role for the grapes and plant.
§ This thought it further supported by the finding that MDA levels in the covered clusters were significantly lower than those clusters exposed to natural radiation.
Discussion and Conclusions
Despite being a relatively short study, it proved to be the very first to show that melatonin levels in grapes fluctuate during the day and night in the Malbec variety under field conditions. These fluctuations could be a result of some sort of circadian rhythm for the plant, with the lowest levels occurring during the afternoon hours of the day. In addition to a sort of circadian rhythm, this study also showed that melatonin may play an important antioxidant role for the grapes. Melatonin levels are seemingly replenished at night, and act as a protector against UV rays during the day, which cause an overall degradation/decrease in melatonin levels. If there was no melatonin replenishment in the evening/early morning hours, there would be no protection against UV damage during the day, and the fruit would not survive. This line of thought may be supported by the results of the light exclusion study, in that those clusters that were covered during the day showed higher levels of melatonin than those that were exposed to natural radiation. Since there was significantly less UV radiation on the covered clusters, very little melatonin was degraded. This suggests an antioxidant role for melatonin in grapes.
This study was also able to show that other compounds, such as polyphenols and anthocyanins, do not fluctuate during the day. If polyphenol and anthocyanin levels do not change throughout the day, then the best time to harvest is clearly when melatonin levels are at their highest. Therefore, in order to create the most health beneficial wine, in regards to cardiovascular benefits and antioxidative/anticancer benefits, grapes should be harvested when melatonin levels are at a maximum, thereby maximizing all three health beneficial components. In this example, in order to get a wine rich in polyphenols, anthocyanins, and melatonin, the ideal time to harvest would be at dawn.
As a side note: I wonder if this is part of the reason why wine makes me more sleepy than say if I were drinking beer or spirits. I can have the same amount of alcohol, but with wine, I often feel like lying down and taking a nap. The presence of melatonin may also function as a sleep-inducing agent as well (not considering the depressant characteristics of the alcohol portion of the beverage), though this is 100% speculation on my part and has yet to be studied.
This study provides results that are “groundbreaking in viticulture”, and provide insight into the mechanisms behind melatonin fluctuations in plants. Much more research needs to be completed; however, this study provides a solid baseline by which future research should be measured against. This study is beneficial not only for viticultural applications, but for other plants as well which may be useful for their nutritious and potentially pharmaceutical properties.
I’d love to hear what you all think about this topic. Please feel free to comment below!
Source: Doi: 10.1111/j.1600-079X.2011.00884.x