Short: Spraying melatonin on vines increases grape size and tightens up the time window over which grapes ripen so that there’s less variation in °Brix across the whole vineyard on any given day. This was just one study in merlot vines, but melatonin could well become a useful viticultural tool.
Longer: Melatonin is a lot more versatile than the little bottles on “natural supplements” shelves suggest. You’re probably familiar with the hormone as something you take to relieve jet-lag or to help you fall asleep on-schedule if you have insomnia. Animals naturally produce the stuff, and so do plants. We know less about what it does in our leafy companions than our furry ones, but research over the past decade has been outlining some kind of role for melatonin in plant growth regulation. It’s structurally similar to auxins, used for decades to increase yields for table grapes which, together with recent studies using it on other fruits, led some Chinese researchers to try spraying melatonin on wine grapes.
14 year-old merlot vines were sprayed with melatonin twice, ten days apart, before veraison (July). The researchers also tried spraying vines just once; spraying twice had a more significant effect. Melatonin-treated vines bore slightly larger berries, possibly a negative for wine quality. But treated grapes also ripened more evenly, a boon for growers trying to harvest as many fully and evenly ripe berries in a single pass as possible. Untreated grapes came in at 16-25 °Brix and twice-sprayed grapes at 18-23 °Brix (eyeballing their graphs), with alcohol content in the resulting wines about even across the board. The researchers also documented some changes in a whole range of aroma compounds that are a bit too up-and-down to warrant saying much, but they suggest that melatonin probably has positive wine sensory effects, increasing ripe/spicy notes (perhaps in part thanks to more even ripening?)
This is one of those “early studies” that happens long in advance of a technology actually hitting the market. But if these results play out in other varieties and other locations, melatonin might well be a reasonable commercial prospect. It seems to have low (if any?) toxicity, be easy to apply and, I suspect is reasonably easy to produce. And (though I shouldn’t make light of the damage vineyard and farm workers suffer from exposure to toxic sprays), treatment for accidental exposure might just be going home for a nap.