The solar ballet of the white trillium

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Trillium flowers pointing in the same direction. Credit: Simon JolyTrillium flowers pointing in the same direction. Credit: Simon Joly

The flowers of the white trillium follow the sun, which increases their fecundity, according to a research team in biological sciences at UdeM.

Have you ever noticed, during your walks in the forest, that the flowers of the white trilliums all point in the same direction? In fact, the flowers are pointing south, towards the sun, and this is far from being a coincidence.

In a study recently published in the journal Botany, University of Montreal biology professor Simon Joly and his student Maryane Gradito describe heliotropism in the flowers of the white trillium(Trillium grandiflorum) and show that the phenomenon gives it a reproductive advantage.

Daily and seasonal heliotropism

Heliotropism is the propensity of certain plant organs to follow the path of the sun. We are obviously thinking here of the sunflower, whose flowers follow the sun during the day. But the white trillium does not function quite like the sunflower.

"Although the flowers of the white trillium follow the sun from east to west and from bottom to top during the day, the effect is much less dramatic than for the sunflower," says Maryane Gradito.

In fact, to this daily heliotropism is added a seasonal heliotropism in the trillium, that is to say that the flowers are mainly oriented towards the south during their development and move little thereafter. This explains why trillium flowers seem to point in the same direction in the forest. It is exactly this phenomenon that caught the attention of Simon Joly, who is also a researcher at the Montreal Botanical Garden: "It was while I was cycling along a wooded area in the spring that it struck me: there must certainly be an adaptive advantage for trilliums to adopt such a strategy."

To test this hypothesis, the authors of the study fixed more than 700 flowers in a predetermined orientation (east, west, north or south) with copper wire in seven Laval woodlots. The idea was to test if the orientation of the flower had an effect on the fecundity of the trillium.

The fruits were then harvested one month later to calculate the proportion of fertilized eggs in each fruit. The results showed that orientation had a significant effect on fertility: south-facing flowers had 12% more fertilized ovules compared to north-facing plants.

"By facing the sun, flowers can capture more light, increasing their internal heat and attracting more pollinators, who want to warm up in addition to enjoying the flower’s nectar," says Simon Joly. But the sun’s heat could also be used to promote the growth of pollen tubes or accelerate fruit development. More studies will be needed to demonstrate what is involved in the white trillium."

Heliotropism in temperate zone

What is particularly interesting is to observe this phenomenon in a temperate forest species. Indeed, almost all studies on heliotropism are focused on species in arctic or alpine regions.

The white trillium is one of the first species to bloom in spring, when the air is still cold. Populations carpet the forest floor, even before the trees’ foliage appears.

"These conditions are analogous to the conditions alpine and arctic plants are subjected to, and our results show that heliotropism can be an advantage for an early-blooming spring species like the white trillium," says Maryane Gradito. Once, in our experimental field in early May, it was snowing in the sky, which illustrates the importance of capturing as much heat as possible at this cool time of year!"

Designated vulnerable species

Knowing that the white trillium is a species designated as vulnerable in Quebec, among other things because of grazing by herbivores and the gathering of plants, the researchers want to learn more about the reproduction of this plant in order to contribute to its conservation.

"The white trillium can take up to 10 years to develop its first flower, so it is necessary to know its biology and reproduction to better protect it in the future," says Maryane Gradito.

About this study

Maryane Gradito, Catherine Fauteux and Simon Joly published the article "Heliotropism in Trillium grandiflorum provides increased reproductive success" in the journal Botany on March 22, 2022.

This study was carried out with the financial support of the Natural Sciences and Engineering Research Council of Canada and the collaboration of CANOPÉE: le réseau des bois de Laval.