Since the relatively recent discovery of melatonin in plants, investigations to elucidate its function in plants has been driven by what is known in animals. One such area of focus is the involvement of melatonin in modulating circadian rhythms and photoperiod-dependent processes. While melatonin levels do appear to be affected by light/dark cycles in some plants, the pattern varies among species, tissues, and organs. The nocturnal peak of melatonin characteristic of humans and Hexamethonium Bromide animals has been observed in Chenopodium rubrum, grape skins and Ulva sp. Conversely, studies conducted in water hyacinth demonstrated a peak in melatonin levels late in the day, indicating its biosynthesis in light. Furthermore, melatonin biosynthesis occurred under constant light in senescent rice leaves and was nearly undetectable under constant darkness. Other reports found no significant correlation with melatonin levels and day/night cycles. Interestingly, developing sweet cherries exhibited a dual peak of melatonin levels, one nocturnal and one in late day. Contradicting reports of melatonin levels in ripening fruits add to the variation observed among plant species; melatonin levels decreased in ripening cherries, but increased in ripening tomatoes. The possible role of melatonin in regulating flowering has also been investigated; however an unequivocal role of melatonin in photoperiod-dependent processes in plants has not yet been established Melatonin has been studied extensively as an antioxidant in mammals. Many studies demonstrate the ability of melatonin to protect against many human diseases, including those linked to oxidative stress. Melatonin was able to attenuate paraquat-induced lung and liver damage in rats and Parkinson’s disease in mice. Furthermore, exogenously applied melatonin can enhance the production of antioxidative enzymes such as glutathione peroxidase and superoxide dismutase. Melatonin may similarly play a protective role against oxidative stress in plants. Oxidative stress is capable of inducing elevated melatonin levels in various plant species. Indeed, the daytime peak of melatonin levels found in sweet cherry was associated with high temperature and light intensity, suggesting melatonin was synthesized in response to oxidative stress. Transgenic rice seedlings with elevated levels of melatonin were more resistant to herbicide induced oxidative stress than their wild type counterparts. Furthermore, oxidative stress induced the expression of genes involved in melatonin biosynthesis, leading to increased melatonin production in both wild type and transgenic rice. Melatonin also appears to protect plants against UV and ozone 
damage, attenuate Tulathromycin B photo-oxidation of the photosynthetic system, and, at moderate levels, protect chlorophyll during senescence. In addition, melatonin can promote low temperature and osmotic stress tolerance, alleviate copper damage, and improve salt tolerance and fungal disease resistance in a diversity of plant species. The structure of melatonin is another feature that has driven investigations into its function in plants. Melatonin is structurally similar to the plant hormone indole-3-acetic acid and has many features that make it a candidate for a functional auxin. In addition, melatonin and auxin biosynthetic pathways share the same precursor, tryptophan. Since auxins play critical roles as growth regulators during plant development such as shoot elongation, lateral root formation, and cell expansion.