The microstructure of the suture in the pecan endocarp

The endocarp of pecan is hard to split before the seed germinates. Little is known about the tissue involved in splitting and if seed germination and dormancy affect the process. The endocarp usually splits along the suture when the seed germinates, but may split in other places when dry seeds are pressured. Regardless of the loading direction and the strength, the endocarp does not crack along the suture. We investigated the structure of the suture region of the endocarp using scanning electron microscopy (SEM), and evaluated whether water in the endocarp affects the bonding force of the suture and germination. Six main types of cells were identified. The endocarp consists of isodiametric sclereids, flat sclereids and crenated oblatoid cork cells. Vascular bundles, ellipsoidal sclereids and dumbbell- or kidney-shaped sclereids are found close to the suture. Ellipsoidal sclereids arranged alternately strengthen the region adjacent to the suture, and ensure that the endocarp cracks precisely along the suture when the seed germinates. The two edges of carpels are bonded together by secondary metabolites and this bond is broken by water necessary for germination. Identification of the metabolites, which can be affected by the moisture content, may reduce natural cracking before or after harvest in commercial orchards.