Unlock the Walnut: How a Pectin‐Rich Suture Tissue and Moisture‐Driven Crack Formation Induce Shell Splitting and Facilitate Seed Germination

Abstract

Hard shells and seed coats have evolved to protect seeds but often hinder germination. This study delves into strategies of walnuts to overcome these protective barriers and enable seed germination. Using chemical imaging, microscopic analysis, and 3D reconstructions, the structure and composition of the walnut suture and its difference from the shell tissue are explored. Depending on its location on the shell, the suture tissue gradually changes from small, thin‐walled, and pectin‐rich cells to larger, thick‐walled, and lignified cells. To understand the influence of these different tissue compositions on shell splitting, single as well as cyclic rewetting experiments are performed. During drying and rehydration, walnut shells deform remarkably, primarily driven by hygroscopic suture expansion. The different swelling behavior between the suture and shell is linked to variations in cell wall architecture and triggers the formation of cracks within the splitting zone. Multiple cycles of drying and rehydration subsequently enlarge these cracks, facilitating the opening of the shell during seed germination. This study advances the understanding of the intricate mechanisms that control walnut shell splitting by structural adaptations, which ensure the reproductive success of walnut trees. These insights into biomechanical crack formation could inspire functional biomimetic material design or seed engineering technologies.