Hazelnuts have been valued for ages as a delicious and nourishing treat. They can be found in woodlands across Missouri, making them a popular wild delicacy among many individuals. Each autumn and winter, squirrels and other small rodents indulge in the nuts, as do various birds and white-tailed deer.
American hazelnuts possess both male and female blossoms – a characteristic that is not particularly unusual. However, the female hazelnut blossoms exhibit some peculiar traits. In the barren forest during late February, they present a striking appearance. Positioned at the tips of short, woody limbs, the spiky stigmas of female hazelnut flowers resemble outstretched crimson fingers or diminutive red fireworks.
These fiery red protrusions have always seemed peculiar to biologist Susanne Renner, given that hazelnuts do not necessitate the attraction of pollinators for reproduction.
Hazelnuts depend on the wind to carry pollen grains from male blossoms, referred to as catkins, to eventually land on female stigmas. This process does not require any bees to facilitate the union. Therefore, why would wind-pollinated flora allocate resources to create floral structures adorned with an expensive red pigment?
“Numerous wind-pollinated trees exhibit red stigmas, but until now, we were unaware of the reasons,” stated Renner, an honorary professor in the biology department at Washington University in St. Louis and co-author of a recent study published in Molecular Biology and Evolution.
Renner, along with researchers from Beijing Normal University, examined the stigmas of flowers in a genus within the walnut family, Juglans. Some species in this family produce red stigmas while others generate greenish ones. The two types can cross-pollinate in their natural setting, leading to a hybrid zone near Beijing where some trees have red stigmas and others have green stigmas.
The researchers found that the red stigmas of wind-pollinated trees accumulate anthocyanin, the same pigments evident in autumn leaves once the production of green chlorophyll diminishes and eventually ceases.
The presence of anthocyanin enhances pollen germination and growth by functioning as an antioxidant, neutralizing reactive oxygen species that could potentially cause sun damage, according to the researchers.
This adaptation could be crucial in the sun-illuminated thin walls of the stigmas, Renner suggested.
“The red hue safeguards some of the most vulnerable areas of the tree’s reproductive system,” Renner explained. “This includes stigma papillae, style transmission cells, and pollen tubes. We propose that this mechanism may be particularly vital in wind-pollinated species that often possess relatively large stigmas.”
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