The sunset is certainly one of the most spectacular moments of the day, but often the attention that we reserve to it is only due to the curious games of colors that is able to give us. Instead there are organisms for which the sunset is really a key moment of the day, an event able to regulate the most important vital processes: these are plants.

This should not surprise us too much; Thinking about it, in fact, there was an age when also man regulated his existence according to the sunset. Before the invention of the clock and the birth of the conventions that today guide our lives, time was marked by the alternation of day and night. We worked until sunset, we dined when it was dark... And so on.

Plants, in spite of what one might think, have preserved over the centuries the ability to perceive with extreme precision time, and many of the most important processes for a plant, such as flowering, are often regulated by events such as the sunset and the dawn, and from the intervals of time that pass between one and the other. Flourishing at a time when the interval between sunset and sunrise is too long, for example, could mean that you do not have enough light hours to do photosynthesis and generate energy, with disastrous consequences.
Plants therefore use different systems that, interacting with each other, are able to "understand" time with extreme precision. These are mainly proteins capable of assuming different conformations based on the presence or absence of a given type of light; the presence of one or the other conformation represents a signal that is read by the plant along different paths.
The main proteins of this type are cytochromes, cryptochromes and phytochrome. The latter, in particular, is the photoreceptor dedicated to the perception of the presence or absence of light.

As we said, phytochrome is a small protein pigment with a molecular mass of 125 kDa; it is present inside the plant cells in two mutually interchangeable conformations, called PR (phytochrome red) and PFR (phytochrome far-red).
The form considered inactive is the PR, and it is present in large quantities in the absence of light. This form is able to perceive the light at wavelengths between 650 and 670 nm, ie the red light. In spite of what would be reasonable to think, the red light component is more abundant during the day than during the sunset, although in our eyes it is masked by other colors. During the day, therefore, the PR works as a receptor for red light, activating itself progressively and transforming itself into PFR.
The PFR represents the active form of the protein, and its relative abundance in reality with compared to PR acts for the plant as the signaling of the day. PFR was wrongly defined as an active form because it was thought to be the only one of the two forms to induce certain activities in plant cells, but today we know that both forms can interact with numerous signaling pathways.
However, the PFR performs its functions during the day but at dusk the light is becoming scarce, and the red component of the light gradually gives way to the infrared component (or far-red). The PFR acts as a receptor for infrared light, and its abundance causes the protein to convert back into the PR form. The new increase in PR compared to the PFR signals to the plant the beginning of the night.

Through this complex system, and with the help of other similar systems, the plants accurately perceives not only the time of day in which it is located, but also the total duration of each of the two main phases, night and day. Understood the main mechanism it is not difficult to understand how it is possible to measure precisely the duration of the intervals.
The transition from PR to PFR and vice versa is neither immediate nor complete, but takes time and is constantly in place: even if one were to reach a condition of equilibrium, this would be a dynamic equilibrium; there would always be a small part of PR, for example, able to spontaneously transform into PFR, just like the opposite. Obviously, the greater the amount of PR produced, and the more stable the system, the longer the duration of the night will be.
The plant is therefore able to perceive the different concentrations of the phytochrome, making an estimate of the duration, in this case, of the night. High percentages of PR will mean long night.
In turn, the more or less marked length of the night is associated with a certain moment of the year and therefore with a particular season.
If we took an autumn flowering plant, which therefore requires a long night to develop inflorescences, and irradiated it with red light in the middle of the night, part of the PR would be converted to PFR, giving the plant the wrong information of the end of the night. The presumed early end of the night, in this case, would be able to block the flowering.

Clearly, when using expressions such as "the plant perceives the length of the night", it does not refer to perceptions, and consequent actions, aware of the plant.
Despite periodically someone trying to support the contrary, the plants show no sign of conscious choice, but all the patterns that also regulate complex activities have been well characterized in a precise cause-consequence scheme.

However, it is still legitimate to ask how plant organisms have evolved mechanisms of this type over time. The most widespread theory is that the appearance of the phytochromes was not initially linked to the perception of the day-night cycle, but rather to that of the shadow-light alternation.
The first plants were in fact very low, if not even creeping. In such an environment, the first plants that have had the opportunity to rise above the others have acquired an important competitive advantage: they could access the light before and better than the plants below.
Faced with such an innovation, all plant organisms have undergone an evolutionary drive that encouraged the erect and vertical growth, so as to be "first" to light.
It is therefore thought that the origin of the phytochromes is linked to the need to perceive the states of shadow: the presence of shadow signals to the plant the presence of another taller plant nearby, stimulating it to vertical growth rather than other activities such as flowering. In support of this theory it is possible to observe the absorption scheme of the normal chlorophylls: they absorb the red light very effectively; the light coming from the sun, therefore, immediately yields its red component to the first layers of vegetation, leaving a prevalence of infrared light to the lower layers. The perception of this infrared light signaled to the plants that it was necessary to grow in height or, at most, to move away from the source of shadow.
The development of such an answer is almost trivial: between two plants placed in shadow condition, the one that manages to get away from it will have greater success; the distancing mechanism is therefore rewarded and strengthened.

With the passage of time the mechanism has been perfected and its functions expanded, up to the current condition. And if it is true that plants can not develop conscious thoughts and actions, it is equally true that, probably, they are able to appreciate sunrises and sunsets much better than we do.

References

• Harry Smith (2000). Phytochromes and light signal perception by plants - an emerging synthesis.
• W. Rüdiger, F. Thümmler, E. Cmiel, and S. Schneider (1983). Chromophore structure of the physiologically active form (Pfr) of phytochrome.

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