3.10 How do plants ‘know’ when to flower?

This intriguing question was posed by one of our readers recently, after having spent a lot more time in the garden than usual during lockdown. She wondered how all the plants of a given variety manage to do it as one each year, wherever they are, whatever the weather. I not only had no idea about the reason, but also had precious little knowledge of botany at all.

Despite Gladys Wilson’s heroic efforts to make stamens and pistils seem interesting to me, I dropped biology at the tender age of fourteen. So, addressing this intriguing question as an adult is something of a voyage of discovery for me as much as for you.

Let’s start by sorting out some of the basic commonalities and differences between plant and human physiology. As the website of Cold Spring Harbour, a highly respected laboratory in New York, puts it:

“Plants have no nervous system to feel the warming temperatures, nor do they have eyes to see that the days are getting longer…… [but  they] use both temperature and day length to figure out when it is time to flower.”

The absence of nerves and muscles is an obvious difference. So is the presence of chlorophyll and absence of haemoglobin as vehicles in the generation of energy. Haemoglobin transports oxygen to our cells via the blood stream so that animals like us can ‘burn’ food for energy; chlorophyll, on the other hand, picks up energy directly from sunlight. It’s thanks to this primary role of plants in trapping energy from the Sun that we get our energy at all. Whether it’s by digesting fruit and vegetables or from eating meat from animals that have themselves fed on plants, we are piggy-backing on the energy that plants have already captured.

An interesting (and vital) aspect of this interdependence is that oxygen is produced as a by-product of chlorophyll’s action – the very substance we and other animals need to enable us to help extract energy from our food. Similarly, carbon dioxide, a waste product of our respiration is a necessary input for the action of chlorophyll in plants. This mutual dependence is a crucial aspect of the balance that makes life possible on Earth – not too much oxygen, not too much carbon dioxide in the atmosphere (at least till recently!)

On the other hand, there are important similarities between the two kingdoms. Plants are made of cells, as we are, and communications between these is carried out, as for animals, through hormones. As with us animals, sex is important for reproduction in plants, though unlike us, it is not essential. Seeds help to spread male sex cells to facilitate the beneficial mixing of genes from different individuals. Many plants can also fertilise themselves, however as each individual plant may carry both male and female versions. Some plants can reproduce without sex at all, as the runners sent out by strawberry plants testify.

To help spread their sex cells and mix their genes, plants need ways of moving their male sex cells around. Female ones remain where they are and become the stable bases for producing the flower, fruit and seeds.  Mobility is not normally an aspect of plant behaviour! To overcome this, some use the wind, some encourage animals to eat and excrete their seeds, and others rely on bees and other insects to transfer their pollen, containing the male sex cells. It’s this latter means that gives rise to the remarkable world of flowers. They are shaped, coloured and scented to maximise their attractiveness to the right kind of insect.

So, key to the timing of flowers is their relationship to the beasts that will transport their pollen. The proliferation of their species depends on it. They need to be up and ready when the conditions are right for the transport of their genes and germination of their seeds. Warmth and light are needed to ensure precious energy is not wasted on unsuccessful procreation. Both the temperature of the surroundings and the length of the day are important factors, ensuring plants aren’t fooled by a few freakish days of winter warmth.

The flowering process

The precise mechanism by which flowers are formed is an area of active research today. Much has been discovered in recent years by focussing in detail on just a couple of model species: one in in the brassica family, Arabidopsis thaliana, the other a type of rice. The big advances have come through research in genetics. Specific genes have now been identified that play crucial roles in the complex processes of flowering. Advances in DNA technology mean that scientists are now able to locate individual genes on the long DNA molecule and work out what each one specifies.

To fill in a bit of background…. genes are sections of the DNA molecule and their structure embodies the information required to make all the parts of a plant or animal or fungus or whatever.  Inside the cells, the information from each gene is used to make the various types of protein a plant or animal needs.

Proteins are vital for all living things as they are both the chemical tools and physical structures needed to sustain life. It’s because all living things share the same fundamental biochemistry that study of such apparently remote creatures as fruit flies, nematode worms and yeast cells can be so relevant to research on human disease. 

The specific gene that sets flowers into bloom was identified in recent years and has been appropriately named Apetala1. When it is activated in the cells of the growing plant, a protein is synthesised which then goes on to trigger flowering. So, if it’s safe for a flower to appear in April but not in January, the gene controlling the process must be dormant for part of the year and active at other times. Key to the timing of flowering is the switching on and off of genes.

Switching on the flowering

Any organism contains many, many genes – around thirty thousand for both plants (and humans surprisingly). Together the whole set of genes (called the genome) specifies every aspect of its being, from breathing and digesting in humans to germinating and flowering in a plant. Clearly, were all genes to be active at all times, chaos would ensue. Leaves, flowers and roots would all be in production everywhere in the plant at all times. Key discoveries in recent years have indicated how the process of expressing genes is controlled. In essence genes are switched on and off, in specific parts of an organism, as and when they are needed.

It appears that genes are regulated in several different ways. One way involves a kind of ‘head’ gene from which a particular type of protein is made (called a transcription factor). This protein then moves on and attaches itself to other genes, switching them on in the process. These then go on to produce all the proteins needed for a task – creating a flower, for example. In this ‘bootstrapping’ way, it takes just one specific gene to produce a specific protein that goes on to switch on all the other genes needed to produce all the other proteins.

So it is with the amazing process of producing a flower.. The single gene Apetala1 provides the code needed to produce a protein that then goes on to switch on over 1,000 further genes. The proteins produced from these multiple genes then carry out the complex set of actions required to create all the components of a flower at a particular time at a spot on the plant.


The precise moment at which the Apetala1 gene gets activated, launching the flowering process, depends on a blend of internal and external factors, including the length of the day and the temperature. Internally, plants, like humans and other animals, have a kind of “clock”, based on the 24 hour period of the earth’s rotation. Exactly how this clock ‘ticks’ is an area of active research with several possibilities under investigation. Like any timing mechanism – a pendulum clock or digital watch, for example –  there must be some kind of back and forth oscillation which takes a regular period of time. Various biochemical processes in the cells of a plant are candidates for this – like the length of time it takes to read off a length of DNA or for particular molecules to pass through the membrane of a cell. 

This internal clock has a so-called “circadian” (24-hour) rhythm which is used to time many different processes, such as rooting, leafing and flowering. So an intriguing question is: how does this clock help the plant sense the amount of daylight? It has been discovered in recent years that a particular kind of protein known as Constans builds up in the leaves of plants, roughly twelve hours after dawn. Measuring this twelve-hour span is where the clock plays its part. This Constans protein is not a very stable structure. It’s OK in the light, but as soon as it gets dark it rapidly degrades. In wintertime, when days are short, daylight doesn’t last as long as twelve hours, so the Constan protein degrades as soon as it is produced, twelve hours after dawn. It’s only when the days are longer than twelve hours –  Springtime – that it remains intact long enough to build up.

Making the flower

So what happens in the evening of the longer days of Spring and Sumer, when the Constans protein is in abundance? The Constans in the cells of the leaves, acts as a switch, turning on a gene containing the code needed to make the crucial signalling protein, named delightfully: ‘florigen’. This influential protein is able to move out of the leaves, through the phloem of the plant to the regions of the shoot where flowers are to form. It’s the travelling molecule, comparable to the hormones that regulate activity in our bodies.

These potential flowering regions, known as meristems, contain generic cells, like stem cells in animals, capable of developing into any kind of specialised cell, as needed for the development of the flower. In this region, where the flower is to appear, the florigen protein activates the ‘head’ gene mentioned above: Apetala1. Activating this gene enables all the further genes needed to create the many parts of a flower to switch on in turn.

Thus the answer to our gardener’s simple question seems to be that flowers of a given variety tend to flower at the same time because they share the same genes and experience the same daylight. The mechanism is somewhat complicated – with proteins that detect light, measure time, and activate genes that go on to produce further proteins. And that’s just the daylight part of the story – there’s still the influence of temperature, ensuring flowering is held back in a late Spring. But that’s enough complexity for one session!

Further detail

Telling the time of day

The story above starts with the onset of dawn and initiation of the cascade of events that leads to flowering. But there’s one earlier step that needs filling in. How does a plant detect the dawn?

The time at which flowers appear depends on both the internal clock mechanism of the plant and external factors such as the temperature and length of day. The plant is able to detect the rise and fall of daylight through the action of a class of proteins that literally change shape when light shines on them. One such is phytochrome.

This diagram shows the structure of the light sensitive part of the phytochrome molecule as a series of lines joining up the atoms of which it is composed. The letters indicate atoms – N for nitrogen, O for oxygen, H for hydrogen, S for sulphur. Carbon atoms are present at every vertex where lines join.

The left hand picture shows the arrangement of atoms when the molecule is in daylight; the right hand picture, when they are in darkness. Although these shapes look quite distinct, close inspection shows that it is merely the rotation of one pentagon at the top left hand side around a single line that causes the manifest difference between the structures. Light falling on the molecule makes this rotation happen.

Phytochrome’s ability to respond to light makes it a vital component of the plant’s timing mechanism. It underlies the flowering mechanism described above by determining whether dawn has broken, thus setting off the process that produces the Constans protein which will determine whether flowering is triggered off.

The light sensing ability of the phytochrome molecule helps the plant in other ways too. For small seeds sown near the surface it helps determine the moment to germinate. With their tiny food reserves they need to be sure sufficient light for photosynthesis will be present as soon as they emerge. Phytochrome also helps plants respond to the seasons by detecting the average length of the day as the year progresses. A vital protein helping regulate the myriad actions needed throughout the cycles of the day and the year.

© Andrew Morris 21st September 2020

Botany is not my specialism, so if you notice any errors or misrepresentations, please let me know andrewmorris110@gmail.com

To receive an alert when a new blog is posted contact andrewmorris110@gmail.com