“Good morning sunshine!” Sunflowers in full bloom seem to greet the first light of the day. Researchers have now used experiments to shed light on why they always face east after following the position of the sun in their youth. The orientation towards the morning sun optimizes the temperature of the inflorescence so that pollinators are attracted and the development of the pollen vessels and later the seed yield are positively influenced.
Not only do they look like suns – the sunflowers are also known for their fixation on the light source of the day: During development, the young inflorescence continuously adjusts to the direction of daylight: in the morning it faces east, then follows the sun until it goes down in the west. In the course of the night the “face” of the plant then turns again to the east and the cycle begins again. However, the plants end this changing orientation when they reach maturity: when fully in bloom, they only look to the east. A research team led by Stacey Harmer from the University of California at Davis has been researching for some time what this sunflower behavior is all about.
So far, the focus has been primarily on the movement of young plants. The scientists were able to show that when tracking the sun during the day, the east side of the stem grows particularly strongly. At night, however, the west side stretches, which causes a reorientation to the east. These growth processes are triggered by hormones that are under the control of genes that have a rhythmic activity. Apparently this is important for the development of the inflorescence: If this changing orientation is disturbed, sunflowers develop comparatively little biomass. The results so far have also shown that at the end of the growing season – when the flower begins to fully unfold – the process that turns the plant to the east predominates.
Why this line of sight?
But why do sunflowers in full bloom stay in this orientation? Indications that this has to do with reproductive success, the researchers have now investigated through further experiments. They compared fully bloomed sunflowers in a natural eastward orientation with specimens that they had turned to the west. They recorded visits by pollinator insects, the temperature conditions in the inflorescence as well as the development of the plant reproductive organs and the seed yield.
As they report, it was initially confirmed that the east orientation encourages bees to visit in the morning. Apparently this is connected with the warmth as well as with the increased attractiveness of the flowers. With the light on the face, the inflorescences warm up significantly more intensely than with radiation on the “back of the head” with a western orientation. This is an energetic advantage for bees when searching for food in the morning – which is why they literally fly to the warm flowers, the scientists explain.
As they further report, the direct sunlight also illuminates visual signals for the pollinators, which remain hidden from our view: the ultraviolet markings on the petals are particularly clear. It should be noted that the sunflower belongs to the so-called daisy family. The interior, framed by the petals, actually consists of hundreds, sometimes thousands, of individual flowers.
Reproductive success with light on the face
The researchers have now also been able to prove that the morning warmth in this center has a positive effect on flower development and reproductive success: Plants facing east tend to produce larger and heavier seeds, according to the evaluations. In addition, the stamens release pollen earlier in the morning, which coincides with the times when the pollinators prefer to fly to the area. These effects are apparently controlled by the temperature at the flower head: when the researchers used a portable heater to artificially heat the west-facing flower heads, they obtained results similar to those of the east-facing flower heads.
The researchers were also able to document the importance of flower alignment for the success of pollination using a special experimental approach: The researchers surrounded sterile male plants, which can produce seeds but not pollen, with “normal” plants that either face east or west became. With the help of a genotyping – a kind of paternity test – they were then able to show that the pollen of the east-facing plants produced more offspring than that of the west-facing plants. “The bottom line is that the background of the conspicuous eastward orientation of the sunflowers has now become clear: The view of the morning sun ensures more offspring,” sums up Harmer.
Source: University of California at Davis, specialist article: New Phytologist, doi: 10.1111 / nph.17627