Fired Up and Ready to Grow

Electron micrograph of a Banksia attenuata seed pod (dark grey) and the wax found at its junction zone (lighter gray).

Max Planck Institute of Colloids and Interfaces, Potsdam

Some plants hold onto their mature seeds until an environmental event, such as fire, triggers their release, a phenomenon called “serotiny.” In fire-prone ecosystems of Australia, the wait for a trigger can be up to seventeen years. An international team of scientists recently examined the clam-shaped seed pods of three species of Banksia, a serotinous group of trees and shrubs found in different geographic regions of the country, to determine how they keep their seeds protected from heat, moisture, microbes, and other unfavorable conditions for so long.

In a series of studies, a research team led by doctoral candidate Jessica C. Huss and her advisor, Michaela Eder, of the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, examined waxes found where the two halves of Banksia pods meet, an area known as the junction zone. First, they slowly baked the woody seed pods of B. candolleana and B. serrata, starting at 40°C and increasing the heat 10°C every 24 hours, to establish their opening temperatures for seed release (this temperature was already known for B. attenuata). After the pods opened, the researchers examined the junction zone and its waxes using scanning electron microscopy. To determine the melting temperatures of the waxes, the researchers heated twenty-micrometer-thick slices of the pods to temperatures between 25 and 60°C and observed the waxes’ properties with a Raman microscope.

The scientists found that for all three species, the waxes melt at temperatures lower (45 to 55°C) than what is required to open the seed pods (68 to 75°C). Based on these observations, they suspected that the waxes were melting and filling in tiny cracks in the still-closed pods. They confirmed this concept on a larger scale by dipping strips of pine wood into Carnauba wax and, once the strips were cool, making shallow cuts into the wood—both with and against the grain. Half the strips were heated to 115°C, and then a staining solution was applied to all of them to reveal any unsealed wood. Nearly all the cuts in the heated strips had been sealed by the melted wax.

The researchers suggest that Banksia seed pods use their wax to repeatedly self-seal tiny cracks and deformations in order to protect the seeds until fire releases them. “As an analogy: imagine a food container, whose lid does not close properly anymore, because it had deformed over time,” Huss says. “The food inside would be exposed to all sorts of external influences.” Instead, wax restores a Banksia pod’s structural integrity while the seed remains safe inside. (Journal of the Royal Society Interface)