3. Melatonin Rhythms in Plants
In the blood of mammals, melatonin levels exhibit a rhythm with low values during the day and
high levels at night [73–77]. Since this is common to all mammals regardless of their locomotor
activity pattern, melatonin has been referred to as the hormone of darkness or the chemical expression
of darkness [78]. This diurnal pattern of fluctuating melatonin levels in animalis perturbed if light
exposure occurs at night [79–81] while in constant dark conditions the rhythm persists, i.e., the rhythm
is truly circadian.
Considering the ubiquitous nature of the blood melatonin rhythm in mammals, there was also
interest in whether similar variations existed in plants and if melatonin was involved in photoperiodism
in these species. Using a weed (Chenopodium rubrum; syn, Oxybasis rubra; red goosefoot and other
common names) that commonly grows in temperate climates in many areas of the world, Kolar et al. [82]
found that in this species nighttime levels of melatonin exceeded those measured in plants harvested
during the day. Thus, in this plant, as in mammals, the melatonin rhythm could provide information
regarding night length. In contrast to these observations, Tan et al. [83] using the water hyacinth
(Eichornia crassipes (Mart) Solms) found that melatonin levels do exhibit a rhythm but peak levels
occurred near the end of the light period rather than at night. Thus, in this plant species melatonin
clearly does not serve as a messenger of darkness. Also, daytime melatonin levels were higher when
the plants were grown under sunlight (10,000–15,000 μW/cm2) compared to those grown indoors
under artificial light (400–450 μW/cm2). In addition to the observed melatonin rhythm, there was a
similar cycle of N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK); this product peaked shortly
after that of melatonin. Since AFMK is a metabolite of melatonin when it scavenges free radicals
(Figure 1), we surmised that melatonin functions as a radical scavenger in the water hyacinth as in
animals [84–89]; this accounted for the rise in AFMK shortly after that of melatonin. That melatonin
functions as a free radical scavenger in this plant is also supported by the observation that melatonin
concentrations were much higher in plants grown under sunlight than under much dimmer indoor light.