The hard shell of the walnut perfectly protects the sensitive seedling from the dangers of the environment. But it also presents it with a major problem, as it could never break through the woody wall on its own. Until now, it was a mystery how the nut cracks itself to allow the baby tree to sprout. Now a team led by Sebastian J. Antreich from the Technical University of Munich has discovered how the walnut opens its shell. The key is the thin strip that connects the two half-shells in the centre. As the team reports in the journal «Advanced Functional Materials», it consists of two different tissue structures. These react differently to contact with water. When the nut gets wet again and again, they expand to different degrees and create cracks due to the tension between them. After a few wet-dry cycles, the cracks expand to such an extent that the shell falls apart.
The nut is a difficult compromise for the plant, because the security of the hard shell comes at the expense of growth in the first phase of life. It is therefore crucial that the shell opens at exactly the right time. In order to decipher how this works, the team scrutinised the chemical and physical properties of the walnut using several methods. The two half-shells consist of cells with thick walls of regularly arranged cellulose fibres and lignin - the two most important components of wood. Protrusions of the cells also interlock and form a very hard, wood-like material. The seam between them, on the other hand, consists of a recognisably different material. As Antreich's team reports, it is essentially made up of two types of tissue. In the upper part of the nut, towards the tip, cylindrical cells with thin walls embedded in a surrounding material dominate. In the lower part, on the other hand, cells with thick, woody walls predominate.
The chemical analysis of these tissue types showed that the material between the thin-walled cells in the upper part of the nut is pectin. These are molecules composed of sugar building blocks that absorb water very well and increase their volume in the process. The material at the lower end of the nut, on the other hand, is more like a stiff shell. The team then exposed the nut to alternating moisture and dryness and observed what happened under the microscope. The pectin-rich seam layer at the upper end of the nut expanded and shrank again. The resulting tension caused cracks to form in the suture material, which slowly worked their way in from the sides. In nature, the cracks eventually meet and separate the shell halves from each other - so that the seedling can finally prise them apart and escape to freedom.
