All gardeners will recognise the tendency of growing plants to turn towards a strong light source. Whether it's tomato seedlings on a windowsill or climbing beans in the garden, most species will slowly twist their stems to let as much sunlight as possible fall onto their leaves.
This movement during growth is known as phototropism, and it happens far too gradually to perceive in real time. However, the results can easily be seen a day or two after changing the position of plants in containers, for example.
But aside from this common growth behaviour, some plants take their sun-seeking activities to an entirely new and speedier level. With an effect most famously seen in sunflowers, these plants closely track the position of the sun across the sky over the course of the day, rather than just adjusting their growth in the general direction of the brightest light.
A sun-loving plant like this will start the day facing east to catch the earliest rays, and over the daylight hours it'll rotate toward the west to follow the sun down in the evening. And then, during the night, it'll turn back eastward and be ready to start the process over again the next day.
This phenomenon is known as heliotropism, from the Greek for 'sun turn', and while it's an effect that's been observed for millennia, it's only recently that it's been fully understood by science.
Two Kinds of Heliotropism
Most plants that follow the light in this way turn their leaves and flowers toward the sun to maximise the amount of light they receive, and therefore increase the level of photosynthesis. This is called positive heliotropism, but there are a few plants that do the precise opposite.
With negative heliotropism, the plants turn their faces away from the sun to receive more shade, avoiding the overheating effects of direct sun in hotter climates. Interestingly, some plants only show negative heliotropism during periods of drought, and behave normally at other times.
How Does Heliotropism Happen?
Traditionally, scientists thought that the movement was a straightforward response to the changing direction of sunlight falling on the plant. More recently, experiments have shown that heliotropism has a more tenuous relationship with direct light. In sunflowers, the movement is actually caused by varying growth rates in different parts of the stem, and these rates change according to the plant's own daily timetable.
For at least a little while, the movement will happen in line with the plant's circadian rhythms, regardless of light levels. Indeed, sunflowers which are replanted 180° from their natural position will track the 'wrong' way across the sky for a few days, until their internal clocks reset and normal heliotropic movement starts again.
What Are the Benefits of Heliotropism?
The most widespread and logical explanation for heliotropism is that the plants follow the sun purely to maximise photosynthesis, in much the same way as solar panel arrays can pivot through the day to increase their energy generation. This is certainly part of the story, and goes a long way towards explaining why so many arctic and alpine species use heliotropism to maximise the warmth they receive.
However, recent research has found another explanation for solar tracking that's just as important to the long-term success of a plant species.
Heliotropism in Young Plants Only
One clue that there was more to the story is that in sunflowers and other species, heliotropism is found only in younger, fast-growing plants. As they reach maturity, the stems toughen up and the flowers become fixed mostly facing east, catching direct sun only in the mornings. Why would this happen, when a greater overall level of sunlight would be captured by facing broadly to the north, like most other sun-seeking plants?
The answer is that by catching the first rays as the sun rises, the flower warms up more quickly at the start of the day. It puts out a stronger perfume before its nearby competitors, and so attracts more early-riser bees and other insects to do their essential pollinating work.
Recent research by the University of California and others showed that plants behaving this way received five times more attention from pollinators than test subjects forced to face west. More pollination means higher reproduction levels, and so gives a clear advantage for the spread and success of a species.
This second explanation completes the heliotropism picture. Younger plants follow the sun to receive as much light as they can while they're in their energy-hungry growth stage. They then use their 'awareness' of the sun's daily cycle to maximise pollination by facing east at the reproductive stage of their life cycle.
Which Plants are Heliotropic?
The most famous example of heliotropism is found in sunflowers, and the effect combines with their sun-like appearance to give the species its common name. However, many other members of the Asteraceae or daisy family also follow the daily pattern to some extent, including dandelions, poppies, buttercups, and tulips.
What Does Heliotropism Mean for a Gardener?
For the most part, heliotropism is just one of those quirky details that increases the interest and pleasure to be found in gardening. But there's one important point to bear in mind. If you're planting sunflowers hoping for a spectacular summer display, make sure you orient the bed correctly, remembering that the mature flowers will usually face eastward. Get the positioning wrong, and the most you'll see could be a row of impolitely turned backs, forever waiting for sunrise.