Changes in Water Dynamics and Vigor of Recalcitrant Phoebe chekiangensis Seeds during Desiccation by Nuclear Magnetic Resonance and Transmission Electron Microscopy

The vigor of recalcitrant seeds is closely related to seed moisture. Real-time, non-destructive monitoring of changes in water distribution and status during the seed desiccation, utilizing nuclear magnetic resonance (NMR) technology, is crucial for preserving the high vigor of these seeds. In this study, we investigated the changes in the vigor of Phoebe chekiangensis seeds during natural desiccation, focusing on seed germination, seed size, and ultrastructural changes, while also exploring seed moisture dynamics with NMR. Our results indicated that the moisture content of fresh, undehydrated P. chekiangensis seeds was 37.06%. As the seeds dried to 25.09% moisture content, their germination ability decreased by approximately 88%. Magnetic resonance images (MRIs) revealed that the internal water of fresh P. chekiangensis seeds was primarily concentrated in the embryonic axis and the middle of the cotyledons. During desiccation, water loss occurred from the exterior to the interior of the embryonic axis, and from the periphery to the center of the cotyledons. Low-field NMR results demonstrated that fresh, undehydrated seeds contained the highest proportion of free water at 55.47%, followed by immobile water at 37.88% and bound water at 4.36%. As drying progressed, the proportion of free water decreased significantly, while immobile water initially decreased and then increased markedly, and the proportion of bound water also rose. Combined with transmission electron microscopy results, we observed that when the seeds were dried to 28.11% moisture content or lower, the cells in the cotyledons and embryonic axis began to shrink due to free water loss, resulting in plasmic wall separation and a subsequent loss of seed vigor. Correlation analysis further revealed a highly significant relationship between the decrease in germination and the loss of free water of P. chekiangensis seeds.