The study, which was published in the Journal of The Royal Society Interface, involved researchers from Kings Park and Botanic Garden in Western Australia, the Max Planck Institute of Colloids and Interfaces (MPIKG), TU Dresden, and the University of Natural Resources and Life Sciences, Vienna.
Brush fires can be destructive, but they can also bring forth new life. For example, various species of the genus Banksia, which is common in Australia, must be heated before seeds can be released from the pods.
The mature seeds of Banksia plants often remain in the two-valved pods for long periods of time. Some seeds even remain in the pods for up to 17 years in some species since wildfires occur at irregular intervals. During this time, seed pods face different environmental challenges that can cause tiny cracks.
These cracks can let in moisture or pathogens that may destroy the seeds. However, Banksia plants protect the seeds via special waxes between the two valves of the seed pod. The waxes then melt at high environmental temperatures to seal these tiny openings.
The self-sealing mechanism of three Banksia species
For the study, the researchers analyzed three different Banksia species, namely Banksia serrata (found in Eastern Australia), and B. attenuata and B. candolleana, which are common in the south-west.
The genus Banksia, which has 173 species of evergreen trees and shrubs, features inflorescences that are made up of numerous individual flowers pollinated by birds, insects, or marsupials. Out of the 173 Banksia species, all but one occur naturally only in Australia. The plants are named after Sir Joseph Banks, the first European to collect specimens of the plants in 1770.
The seed pods of a lot of Banksia species must be exposed to very high temperatures, like during brushfires, before they open.
Prior to the study, it was unknown how the temperature-related opening mechanism worked.
Jessica Huss, a doctoral student at the MPIKG and first author of the study, noted, “It was commonly thought that the two valves of the seed pods were held together by resins that melt when heated, thereby opening the valves.” Huss added that because of the study, the team of researchers discovered the presence of waxes, instead of resins, in the junction zone between the valves.
The research team used Raman spectroscopy to get a detailed chemical characterization of the substances in the seeds. Further examinations showed that the waxes in the Banksia pods melt between 45 and 55 degrees Celsius. However, the seed pods didn’t open until the temperature was between 54 and 76 degrees Celsius. This proved that the waxes didn’t trigger the opening of the valves.
In an earlier study, the researchers presented evidence that “the opening mechanism is based on the mechanical properties and differing orientation of the cellulose [fibers] in the tri-layered pod.”
Huss commented that the tissue dries as the seed ripens. During this process, the fibers shrink to differing degrees, which creates pre-stresses. As heat softens the innermost pod layer, the pre-stresses are released so the two halves of the seed pod to split open.
But one question remains: What are the functions of the waxes if they’re not used for the opening mechanism of the pods?
Huss explained that the seeds remain on the plant for an extended period of time in some Banksia species. The plant’s pod are constantly exposed to heat, rain, and UV radiation along with the beaks of hungry birds. The researcher posited that the waxes are there to protect the pods.
Huss said that there are various regions in Australia which often experience temperatures from 45 to 55 degrees Celsius. It’s possible that the waxes melt on hot days and that it then repeatedly seals small fissures. (Related: Magic mushrooms likely developed their psychedelic properties as a defense against predators.)
Using a simple model, the researchers tested if the waxes could seal fissures in wood. They tested their theory using small panels of pinewood covered with a thin layer of carnauba wax. After applying cuts on the wood, the researchers melted wax on some of the wood panels.
A staining test revealed that the liquid wax sealed the cuts in under 15 minutes. Even if it was possible to stain the cavities in the panels that weren’t heated, those in the heated panels didn’t have staining.
Michaela Eder, another study author, concluded that the self-sealing mechanism could be a widespread adaptation in the Banksia plants. Eder noted that the self-sealing temperature-based system could be used to create dimensionally stable wood in outdoor structures.
Learn more about other fascinating scientific phenomena like the self-healing seed pods of banksias at WeirdScienceNews.com.