Upper left panel shows a Torrey pinecone in dry and wet conditions. Lower left panel is a schematic of the proposed tri-layer model. Lower right panel is a CT scan of an individual cone scale with color; red indicates a denser component and blue denotes less dense material. Upper right panel shows a higher resolution scan of the pore gradient.
Upper left panel shows a Torrey pinecone in dry and wet conditions. Lower left panel is a schematic of the proposed tri-layer model. Lower right panel is a CT scan of an individual cone scale with color; red indicates a denser component and blue denotes less dense material. Upper right panel shows a higher resolution scan of the pore gradient.

Pinecones open and shut their scales in response to weather conditions to disperse their seeds as widely and effectively as possible. When it is wet, and seeds are unlikely to be carried by the wind, the cones’ scales stay shut. When it is dry and windy, however, the scales open to allow seeds to disperse. Now researchers have revealed the mechanism of this clever flexing behavior, which is determined by the material structure of the pinecone's scales [Quan et al., Acta Biomaterialia (2021), https://doi.org/10.1016/j.actbio.2021.04.049 ].

“Pinecones are fascinating biological materials,” says Marc A. Meyers of the University of California, San Diego, who led the effort. “[They] can be as small as the size of a thumb, in the case of the gigantic redwood tree, or as large as a small watermelon, weighing up to 10 pounds.”

Together with colleagues at the University of California, Berkeley, the researchers turned traditional materials science characterization tools onto pinecones from the Torrey pine to shed light on their remarkable properties. Until this detailed analysis, it had been thought that pinecones operate like the bilayer of a simple thermostat with layers of sclereid plant cells and sclerenchyma, cellulose microfibers that give plant tissue strength and stiffness, expanding differently when hydrated. But Meyers and his colleagues found an additional effect at work.

“One side [of the scale] contracts more than the other when the pinecone dries so that it curls outward, leaving the path open for the seeds,” explains Meyers. “But we discovered that the scales are not just two layers with different properties, but a gradient structure, which is both superior in properties and also more complex.”

When the researchers examined cross-sections through scales using X-ray and scanning electron microscopy, they observed a gradient of pores varying in size and density. They describe this structure as a tri-layer system, where the porosity gradient between layers of sclereid cells and sclerenchyma reduces the stress concentration between these two components. The reduction in stress produced during the bending and unbending process allows the pinecone to open and close several times, increasing the likelihood that the pinecone will release all its seeds and improving the chances of producing a successful sapling.

But the finding doesn’t only help us appreciate and understand the reversible deformation of which the pinecone is capable but could also hold clues for synthetic materials, the researchers suggest.

“Our findings may inspire engineers to create artificial smart materials that can change morphology triggered by mild stimulation,” says first author of the study, Haocheng Quan.

The team have already made some progress in mimicking this behavior using synthetic hydrogels with a porosity gradient that responds to moisture in a similar manner, bending and unbending.