关键词: 豆科, 国槐, 水分分布, 种子发育, 核磁共振成像（MRI）

Abstract:

Objective: Seed development is genetically programmed and metabolically regulated. Furthermore, water is closely related to metabolism and therefore has a significant regulatory role in seed development. This study aims to provide insights into the movement pattern of water during seed development, by monitoring the spatial and temporal changes of water in Sophora japonica pods at each developing stage with MRI (magnetic resonance imaging) techniques non-destructively. Method: The pods at different developmental stages (30?150 days after flowers) were collected. The morphology of the pods and seeds were observed and measured periodically. Furthermore, the water absorption curves and the seed germination were determined at each stage. Using MRI technology, temporal and spatial water distributions in pods were analyzed. Result: During development, the pods changed from bright green to yellowish-brown. At the same time, the seed coat gradually changed from green to yellow-green, and finally to black. The color variation pattern of different regions of the seed coat was different. According to the MRI image, at 90 days, the hilum region is darkened in perference to other regions. Accompanied by the color change, there was an increase and then a decrease in the sizes of pods and seeds, with the maximum at 90 days and an almost constant size up to 120 days. The change trends of fresh weight of the peels and seeds were the same at all stages: both increased first and then decreased. The dry weight initially increased and then leveled, with the peels peaking at 120 days and the seeds reaching the maximum at 90 days. The moisture content of the peels first increased and then decreased, while that of the seeds continued to decline. The water absorption capacity of seeds varied during development: seeds at 30 and 60 days were susceptible to breakage during imbibition; seeds at later developmental stages were able to imbibition normally, but the time for imbibition to enter into the plateau phase gradually increased in each stage, and the seeds with 90, 120, and 150 days old entered the water absorption plateau phase at 8, 60, and 108 h, respectively; during this process, the germination speed of excised embryos gradually increased, while that of the seeds gradually slowed down. The MRI technique revealed that the water temporal and spatial distribution pattern differed in each tissue of peel, seed coat, gelatinous endosperm, and seed embryo at different developmental stages. At 60 days, the water content was higher in the peel and endosperm, and there were strong signals in the hilum and lens regions of the seed coat too, and the signals disappeared at 90 days. During 90?120 days the signal in the endosperm decreased dramatically, and the water only existed in the embryo at 120 days. Conclusion: The impermeability of the seeds gradually increases during S. japonica seed development. Water transportation from the hilum region and the lens to the peel is interrupted at 90 days, and the seed enters the mature dehydration phase. At 120 days, the seeds are mature and the water content in the seed is low and mainly concentrated in the seed embryo.

Key words: Legumes, Sophora japonica, water distribution, seed development, magnetic resonance imaging (MRI)