How do plants know what time it is? – basic science

By Carlos Takeshi Hotta

They ensure survival through small calculations


Plants are very underrated creatures. However, behind an apparent simplicity, they are able to perform elaborate operations, so there are some who confuse this ability with intelligence. To give just one example of this complexity, suffice it to say that they are able to do simple calculations so as not to starve to death overnight.

Plants have a “routine”. Like other living beings, they have an internal clock, the circadian clock, which generates daily rhythms in order to synchronize their bodies with environmental rhythms. You are preparing to harvest the sunlight before dawn; release fragrances to attract pollinators when they are most active; In the afternoon, when the humidity is lower, they avoid water loss and at night they feed on the energy reserves produced during the day.

Dawn and dusk are predictable events, and vegetables know it. In other words, you know what time it is.

During the day, plants produce photosynthesis and produce the carbon skeletons necessary for their survival, growth and reproduction. One of the molecules they make is starch, a polysaccharide that acts as an energy reserve. It is this sugar that gives them energy at night so they can grow and prepare for the new dawn.

The dynamic of strength seems simple: it is scarce at dawn. Over the course of the hours, its amount increases ten to twenty fold due to the assimilation of carbon through photosynthesis. At night the amount of the molecule decreases linearly until it reaches the low values ​​at the beginning. If this polysaccharide is missing for any reason, the plant will suffer from energy stress and its growth will be impaired.

A research group led by Dr. Alison Smith of the John Innes Center in England conducted an experiment to better understand the dynamics of strength. The researchers moved the dawn of plants grown in growth chambers forward by four hours and were surprised to find that they used the molecule more slowly to endure the stand until dawn. Despite the abrupt night, there was no energy stress. The next day, the plants began to accumulate starch faster, so they achieved higher amounts of the reserve polysaccharide in a shorter 4 hour day to survive longer nights. Interestingly, we still don’t know exactly how they do it.

In order for plants to ration starch, they need to know how much they have and how much time they have before dawn, and then calculate the rate at which the molecule is being used. We know how the hours of the day are estimated: the internal clock of plants has a rhythm similar to that of the ambient rhythms of light and dark. The same circadian clock is used to keep track of the shortening of the day period that heralds winter or its lengthening before summer.

In her experiments, Dr. Smith also used plants with defective circadian clocks. If this clock marked a day with less than 24 hours, i.e. a clock in a hurry, the reservation would end before the sun arrived, both on normal days and on extended nights. In other words: if the plants’ sense of time is changed, they cannot ration the molecule properly at night and suffer from energy stress. Plants with a damaged biological clock perform less photosynthesis, use more water and grow less.

The mathematical models designed to try to better understand starch generation underscore the importance of the circadian clock in this process and show that plants must have mechanisms in place to know how much of this polysaccharide they contain, how much they consume or how much energy they have. We still don’t know how to do this and how to incorporate this information. Understanding how plants manage their energy over the hours is one of the big questions in this area. Not only does the answer expand our basic knowledge of these complex organisms, but it can also help us make them bigger and better for our own benefit.


Carlos Hotta studies the biological clock of plants and is an associate professor at the Institute of Chemistry at the University of São Paulo.

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