When people are asked to draw the flower of a sunflower plant, almost everyone draws a large circle surrounded by yellow petals.
“In fact, that structure is the flower head, or the flower head, which can be made up of hundreds of flowers, also called florets. The surrounding “petals” are florets of different structure and function from those closer to the center, ”explains the professor. Horticulturalist Paula Elomaa from the Faculty of Agriculture and Forestry, University of Helsinki, Finland.
A giant inflorescence is beneficial because it is effective in attracting pollinators. As pollinators move around the inflorescence, they pollinate hundreds of individual flowers on their journey.
The order of the florets in a flower head is not random. Instead, they are patterned into regular spirals, the number of which follows the familiar Fibonacci sequence of mathematics. Fibonacci numbers are the sum of the two previous numbers in the sequence: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144 …
In the capitulum, the number of winding spirals to the left and to the right is always two consecutive Fibonacci numbers. Sunflower flower heads can have up to 89 sinuous spirals on the right and 144 spirals on the left, while the gerbera, another widely studied plant in the Asteraceae family, has fewer spirals (34/55).
The geometric regularity of nature has fascinated both biologists and mathematicians for centuries.
“The gerbera is a favorable subject of study, because we can use transgenic plants grown in greenhouses as tools to study the functions of individual genes, for example, during the development of the plant. In the case of sunflowers, the transfer of gene is not yet a routine procedure. The gerbera genome, which is close to the size of the human genome, is currently being sequenced. Experience has shown that the gerbera is an excellent model plant “, says Elomaa , who has contributed to Finnish gerbera research from early childhood in the late 1980s.
Now, for the first time, researchers have been able to examine at the molecular level how floral primordia are spiraled into the growing point, or meristem, of gerberas. They had at their disposal a technical solution whose use in plant science could only be dreamed of a few decades ago.
Using X-ray tomography, the researchers scanned three-dimensional images of different stages in the development of the meristem. Using confocal microscopy, they studied meristems as small as less than a millimeter to determine where the plant hormone auxin, which determines the position of primordia, is located.
Finally, the researchers applied mathematical modeling to the data obtained in cooperation with Professor Przemys? Aw Prusinkiewicz of the University of Calgary. The end result was a three-dimensional computer model that emulates the pattern of a real flower head.
Researchers have found that the gerbera meristem is patterned at the molecular level already at a stage where no primordia or other changes are discernible even by an electron microscope.
“During growth, auxin levels reach their maximum simultaneously in several places of the meristem. The number of these clustered spots, called auxin maxima, increases rapidly as the diameter of the meristem grows, following Fibonacci numbers. A new maximum of auxin is always formed between two neighboring maxima and moves so as to be always closer to the oldest of the neighbors. This is why the spirals are regular even in meristems which are not entirely symmetrical. “
The results demonstrate that the expansion growth of the meristem is the factor that affects, for example, the eventual number of florets in the flower head.
“Among other things, this effect is seen in the seed yield, an important factor for sunflower, as it is specifically grown as food and feed. It is also possible that the same model explains the number and structure of floral organs. In the future, this information will be applied, for example, to the strawberry. In strawberries, the size of the fruit is regulated by the number of pistils, ”notes Elomaa.