Floral Sun-Trackers: Exploring Heliotropic Plants

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We know that many plants rely on light from the sun for energy. But some species are able to follow the movement of the sun to obtain as much light as possible throughout the day in a phenomenon known as heliotropism.

What is Heliotropism?

In the simplest explanation, heliotropism can be described as the ability of a plant to orient itself in line with the direction of the sun. As the sun traverses the sky throughout the day, following the movement of the earth, these plants demonstrate the ability to adjust their growth or movement so that they actively track the sun, maximizing their exposure to light.

Because of this adaptation, these plants can produce more energy and nutrients for themselves through the process of photosynthesis.

You’ll sometimes hear this being described as sun tracking, but it means the same thing. The word heliotropism comes from the Greek, with helio meaning sun and tropism referring to the movement of the plant.

Benefits of Heliotropism

When we think about why plants exhibit heliotropic movements, one of the first benefits that comes to mind is to boost photosynthesis. But this isn’t the only advantage of heliotropism. In fact, it also serves as a way to regulate temperature and boost reproduction. Let’s take a closer look.

Enhanced Photosynthesis

Most plants rely on light from the sun which they then convert into energy and nutrients to support their growth. You could say that plants essentially make their own food.

Think about solar panels and how some are programmed to move in time with the sun, maximizing the energy they can produce. It’s the same concept, and it’s something that’s been inspired by nature. The more light a plant can absorb, the more energy the plant can produce, thereby enabling more efficient photosynthesis. This works by exposing cells called chloroplasts to as much light as possible.

Chloroplasts are one of the main components responsible for photosynthesis and are what give the plant its green color. You might think this would benefit the plant aesthetically, and it does, but in reality, that green leafy hue actually helps the plant soak up even more rays from the sun.

When maximum sun exposure occurs, the plant is able to produce more biomass such as proteins, carbohydrates, as well as more nutrients like amino acids and sugars among other things which ensure healthy growth and metabolism.

The healthier the plant is, the less susceptible it is to environmental stressors. This means being better able to cope with high temperatures, drought, and poor soil.

Temperature Regulation

Every living thing on this planet requires a certain temperature to survive, and plants are no exception. For plants that live in cooler conditions, such as Alpine species, heliotropism ensures that they receive as much warmth from the sun each day as possible. This allows them to thrive in a cool environment that other plants would struggle in.

But even on the flip side, heliotropism is essential. Plants that are able to do this can also limit their exposure to direct sunlight which makes them less susceptible to the effects of heat stress and prevent water loss which occurs through transpiration, a process fueled by the light of the sun. Plants that grow in very dry conditions certainly benefit from this and are able to better retain water and use it more efficiently.

In line with this, the plant may also adjust the position of its leaves to keep them out of direct sunlight, further preventing heat loss through transpiration, so they’re actually able to regulate their own temperature.

Not only does self temperature regulation ensure the overall health of the plant, but it also means more efficient photosynthesis. The chemicals involved in this process work best at certain temperatures, but that’s not all. Photoinhibition occurs when plants receive too much sunlight and this damages the parts of the plant responsible for photosynthesis, thereby decreasing their ability to produce their own nutrients.

Improved Reproductive Success

Many plants require pollinators to take pollen from one flower and deposit it on another of the same species, resulting in the reproductive process of bearing fruit and seeds. But this can only happen if these pollinators are attracted to the plant in the first instance.

Now a lot of plants rely on colorful flowers to draw the attention of pollinators, but heliotropic plants have another tactic besides. By positioning themselves in such a way that they gain maximum warmth from the sun, they themselves warm up and this is attractive to pollinators.

Even more fascinating is that these plants are able to position themselves not only in line with the sun but also the activity of their pollinators so they’re more likely to be visible as pollinators are foraging. This improves the chances of pollinators coming into contact with the stamen and gathering pollen. In turn, this boosts the reproductive success of the plant species and may even help to promote genetic diversity.

Types of Heliotropism

While heliotropism does relate to the movement of plants according to the direction of the sun, there is more than one type.

Positive Heliotropism

Positive heliotropism is when plants turn their direction to face the sun in order to maximize the amount of light and warmth they receive. This is also known as sun tracking and is probably the type of heliotropism you would first think of.

In this case, most plants exhibit this behavior in order to boost their photosynthesis processes, enabling them to create more energy for themselves. Interestingly, at the end of the day, the plant may adjust back to its original position and start the process over the next morning in line with the circadian rhythm of the plant.

In environments where the sunlight conditions vary, heliotropism makes it easier for the plant to get the most sunlight compared to those that remain in one position. This interesting adaptation goes even further when we consider that the plant has specialist receptors that allow it to respond to the varying light conditions.

Negative Heliotropism

The second kind of heliotropism is known as apheliotropism or negative heliotropism and works in reverse to its positive counterpart. Instead of moving their structures to face the sunlight, these plants will actively move out of its light.

One of the main reasons for this type of heliotropism is temperature regulation, and is often seen in plants that live in very hot environments or those that need to move out of direct sunlight during the hottest parts of the day. Not only does this help the plant to remain cool, but it also prevents loss of water through transpiration.

If plants are exposed to too much sunlight this can damage their structures and their ability to efficiently perform photosynthesis. Just like plants that perform positive heliotropism, these species also rely on their circadian rhythm to move at specific times of the day, ensuring that they’re able to survive in even the harshest environments.

Phototropism vs. Heliotropism

If you’re a keen gardener, you may have noticed that some plants will grow in the direction of light, but this doesn’t have to be sunlight. Plants that respond to any kind of light are known as phototrophic plants and are only ever seen to be growing towards the light as opposed to the positive or negative movement of heliotropic plants.

You may also notice that phototropic plants typically turn their stems and leaves towards the sun, whereas this movement in heliotropic plants is usually limited to the leaves and flowers. Moreover, the way in which these plants do this is quite different, with heliotropic species relying on many mechanisms in response to changes within their cells while phototropic plants rely on a hormone known as auxin and how it responds to light.

When heliotropic plants are seen in action, the movement is more gradual compared to the rapid responses we see in their phototrophic counterparts. Think about how seedlings may grow towards the light; this is phototropic and is a reversible process, whereas heliotropism may not always be reversible.

Moreover, heliotropic plants rely on their circadian rhythm to determine their movement, and while some phototrophic plants rely on this too, it isn’t always the case.

With all those differences aside, the main aim of both processes is to control sun exposure. However, phototropic plants are looking to maximize this, while heliotropic plants could be trying to maximize or minimize their sunlight exposure.

Plant Species that Exhibit Heliotropism

The most well-known heliotropic plant is the sunflower, and I’ll explore this in a little more detail in the coming sections. But there are many other plants that also behave this way, including morning glory, buttercups, and the Alpine poppy.

1. Sunflower (Helianthus annuus)

The clue is in the name; sunflowers are so-called because of their heliotropic behavior that causes them to follow the movement of the sun throughout the day. Even as young buds, these flowers start by facing east, towards the sun in the morning, and gradually adjust their position throughout the day to maximize their exposure. Interestingly, once the plant matures, it tends to remain facing east as it no longer requires help in growing.

By doing this, the sunflower is able to increase its efficiency when it comes to photosynthesis, but it’s also a very effective method for attracting pollinators. This is another reason that mature plants remain in one position as they’re more likely to attract pollinators as they reach their stage of reproduction.

Sunflowers are able to grow in various environments and light conditions, and this is largely due to their heliotropism. When the sun goes down, these plants will reset their position ready for sunrise the following morning. 

2. Daisy (Bellis perennis)

Daisies are very common plants and you’ll often see them scattered across grass. Take a look at how their flowers face the sun in the morning but, as the day progresses, they gradually move their position.

However, unlike the sunflower, these pretty blooms don’t reset themselves once the sun goes down. They’ll close up their petals for the night and then lean to face the sun once it rises again in the morning.

The very name daisy is thought to come from the term ‘day’s eye’ since the plant only remains open during the day and this is something that Geoffrey Chaucher referenced in his work.

While there are several theories as to why the daisy behaves in this way, it’s commonly agreed that they most likely do it in order to gain warmth from the sun and attract pollinators as a result of this. 

3. Morning Glory (Ipomoea tricolor)

As its name suggests, the morning glory is most well-known for its beautiful flowers that bloom in the morning. Over the course of the day, the petals will gradually close. But during this time, these plants will actively follow the path of the sun. Depending on the species, the flowers may close at different times of the day.

If you’re looking for a variety that will stay open for longer, then the heavenly blue is known to remain open until early evening. It’s best to plant this species in a south-facing garden for even greater sun exposure; they require between six and eight hours of direct sunlight every day.

In any case, morning glory is an attractive garden plant because of the amazing range of colors that it comes in and its pretty heart-shaped leaves. Because of its heliotropism, morning glory is able to produce more energy for growth and as such, grows very quickly, climbing up fences, trellises and other structures.

4. Alpine Buttercup (Ranunculus adoneus)

Found primarily in the Rocky Mountains, the Alpine buttercup is one of the first plants to flower and, when conditions are right, it may even bloom again in summer. One of the things worth noting is that the flowers have very prominent green centers which promote greater seed production.

It’s thought that this is one of the effects of the plant’s heliotropic abilities which also serve as a way of boosting photosynthesis. On top of this, being able to track the movement of the sun means that the Alpine buttercup is able to regulate its temperature and stay warm, despite living in very cold conditions.

Additionally, it’s thought that heliotropism benefits this plant in terms of pollination. Studies have shown that individuals facing the sun at a 45-degree angle are more likely to attract pollinators, which aid the reproductive success of the plant. Interestingly, young female flowers are more active in their movement compared to older flowers. 

Research reveals that it is the peduncle of the buttercup that actually tracks the sun and not the flower itself. Even when flowers were removed with the peduncle left behind, the plant was still able to track the movement of the sun.

5. Arctic Poppy (Papaver radicatum)

Not a lot grows in the Arctic compared to other parts of the planet, and that’s mainly because of the harsh conditions here. But the Arctic poppy is perfectly adapted for life here and is able to track the movement of the sun using heliotropism.

It was noted in studies that not only are these plants sun trackers but that their petals are so positioned to direct sunlight toward the ovaries, therefore increasing reproductive success because of the improved warmth.

The Arctic poppy is the most northern flowering plant on earth, and its white petals actually help to promote warmth once it has been gathered from the sun. Studies have shown that those with white petals are up to 1.5 degrees warmer than those with yellow petals, proving that this strategy really does work.

6. Compass Plant (Silphium laciniatum)

With great cultural significance to Native American communities, the compass plant is a well-loved species with a rich history in traditional medicine. What’s more, it’s pretty yellow flowers are super attractive to pollinators which contribute to its reproductive success.

The name compass plant is thought to have two origins, the first in relation to the species heliotropic activity. But it’s also believed that the pioneers thought that the leaves of this plant always pointed in a north-south direction. This is true, but the flowers are totally opposite and follow the sun from east to west each day, more commonly seen in young plants.

The flower of the compass plant is delicate but incredibly large, sometimes growing up to 5 inches (12.7 cm), while the entire plant can reach up to 12 feet (3.7 meters). This growth is likely in relation to their heliotropism but can also be attributed to the deep tap roots that allow the plant access to water even when there’s not much rainfall. This makes them extremely tolerant to drought.

They grow so well in the prairie ecosystem that these perennial plants return year after year, forming dense cover across the landscape.