Ultrastructure of pericarp and seed capsule cells in the developing walnut (Juglans regia L.) fruit

Juglans regia L. is an important nut fruit, but our basic knowledge of it still lags behind that of other plants. In this study, the ultrastructure of the fleshy pericarp and seed coat cells was systematically investigated using transmission electron microscopy during the entire developmental process of the fruit. Our observations showed that during the early stage of fruit development mesocarp cells display the primary characteristics of immature cells, namely numerous organelles, an extensive distribution of plasmodesmata, and the presence of starch grains. As the fruit develops, the vacuole, vesicles and intercellular spaces clearly enlarge, organelles begin to degrade and, at a later stage, phenol grains appear. Although there are no plasmodesmata in the endocarp there is a complex cytoarchitecture in the episperm of the developing seed, with many phenolic substance granules. The mesosperm contains an electron-dense protoplasm with a thin cell wall, enriched with organelles, and has numerous plasmodesmata, although there are almost no starch grains. The companion cells (CCs) are enriched with mitochondria, and the sieve elements (SEs) contain many vesicles in the SE–CC complex of the mesosperm, from which the sieve plate of the SE is built up. The endotesta cells, which are large and loose, form a nourishment-free diffusion space. There is complex vesicle movement in the fleshy pericarp and seed coat. Based on structural and diagnostic changes within the fleshy pericarp and seed coat, the cytological characteristics of substance transport in the walnut fruit are discussed. The main conclusion is that fleshy pericarp parenchyma cells are the nutrient bank, while the parenchyma tissue of the seed coat is the transfer station that transports nutrients to the embryo. Vesicle transportation is the third method of transportation of materials between cells and the transfer cell of the walnut seed coat is characterized by a multitude of mitochondria. This study makes a significant contribution to our understanding of ultracellular events in the developing walnut fruit.