Developmental changes in element-specific localization during seed maturation of Lotus japonicus

In dicotyledonous seeds, mineral elements are accumulated in specific tissues that support germination and early seedling growth. Although element-specific localization patterns in mature seeds have been reported, the dynamic changes in spatial distribution during seed maturation remain poorly understood. In this study, we investigated developmental changes in elemental localization during seed maturation using in-air micro-PIXE elemental imaging. Seeds of Lotus japonicus (accession Miyakojima MG-20) were collected at multiple maturation stages, and the spatial distributions of potassium (K), phosphorus (P), and calcium (Ca) were analyzed. At early and mid maturation stages, K and P were distributed relatively uniformly throughout the seed, whereas Ca began to accumulate preferentially at the seed surface during mid maturation. With further maturation and differentiation of internal structures such as cotyledons and the root / hypocotyl, P showed preferential accumulation in specific tissues. Ca exhibited a distinct transition, accumulating in both peripheral and internal tissues and forming a characteristic linear pattern in cotyledons of fully mature seeds. In contrast, K remained broadly distributed throughout maturation. These results demonstrate that mineral accumulation during seed maturation is dynamically regulated in coordination with tissue differentiation and follows element-specific accumulation strategies.