In order to only release their valuable seeds under suitable conditions, pine cones close their scales when wet. Contrary to previous assumptions, this passive mechanism is not only caused by a swelling layer of tissue. The latest analyzes show that all layers in the cone are able to absorb water and thus contribute to the closure of the cone scales.
Trees such as pine, fir or spruce are not only characterized by their coniferous growth, but also by the formation of cones. These woody outgrowths are used by the conifers for reproduction and contain seeds that are pollinated by the pollen from the male cones. The seeds can then be carried by the wind or animals to a new location. The pine has developed a particularly sophisticated mechanism for this: the pine cones open when it is dry and close when it is wet. In this way, the pine seeds are only released under favorable conditions, namely when it is as windy as possible and the seeds are carried far. This avoids wasting seeds that would not make it far from the tree of origin in rain and humidity.
A passive and an active layer?
The opening and closing of the pinecone’s scales is particularly interesting because the movement is passive and thus uses no metabolic energy. Until now, researchers assumed that the movement of the pine cone was based on the interaction of two layers of tissue in the scales: an upper, rigid layer that absorbs little or no water, and a lower one that swells and lengthens when exposed to moisture. She pushes the scales up and the cone closes. As the bottom layer dries and shrinks, it pulls the scales back down – the cone opens.
A research team led by Carmen Eger from the University of Freiburg now contradicts this simplified model in a new analysis and shows that the locking mechanism is based on a somewhat more complex structure. In order to investigate the influence of moisture on the various layers of tissue, they carried out measurements on the wetting behavior and water absorption of Pinus wallichiana pine scales. In addition, the researchers created a height profile of the cone tissue during opening and closing, which provided more detailed information about the three-dimensional changes in the tissue structures of the pine cones.
Teamwork in the cone tissue
The results indicate that multiple tissue layers are indeed part of the passive movement of the pine scales: “The scales of the pine cones bend due to a complex interaction of different tissues, each contributing to the movement,” report Eger and her colleagues. This is possible because after the primary uptake through the epidermis, the water is distributed to all tissue layers of the cone scale. The bending of the upper layer, and thus the closing of the cone, is not only due to a swelling of the lower layer, as assumed by the previous model.
In fact, the upper layer, which consists of the so-called sclerenchyma fiber strands, is also able to absorb water and react to it flexibly. These fibers are rigid when dry, but can absorb a surprising amount of water in sustained wet conditions, the research found. It is only this softening that allows the upper tissue layer of the cone scales to bend – and thus the cone to close to protect the seeds. “Bending of the cells and the fiber strands of the sclerenchyma thus complement each other to trigger a synergistic and powerful movement,” explain Eger and her team.
The now better understood closing mechanism of the pine cones can also serve as an inspiration for bionic valve systems in the future. Similar to pine cones, these should react to moisture and be used, for example, to regulate the climate in building envelopes.
Source: Albert-Ludwigs-University Freiburg im Breisgau, specialist article: Advanced Science, doi: 10.1002/advs.202200458