Abstract:

Objective: Imbibition is the initial and most important stage of seed germination. Nuclear magnetic resonance (NMR) technology was used to explore the initial water absorption site and the spatial and temporal movement of water throughout the seed after hot water treatment from the perspective of time and space, revealing the changes of water phase in seeds. Thisprovides a new method for the study of water absorption in seeds. Method: The seeds of A. julibrissin were treated with hot water at an initial temperature of 80 ℃. The water absorption was calculated by the increase in weight after soaking divided by the initial weight. MRI technology was used to observe the initial water absorption location and the movement of water in the seeds after water absorption, combined with T₂ relaxation detection technology to explore the dynamic changes in phase and state of the water. Result: 1) The water absorption curve of A. julibrissin seeds exhibited an "S" type behavior, where the water absorption of seeds increased slowly in the early stage (0-4 h); the seeds entered the rapid water absorption stage during 4-12 h, where the water absorption increased greatly; finally the seeds entered the slow water absorption stage at 12 h. During the process of water absorption, the MRI result of the seeds showed that the water initially entered the seeds from the micropyle, then migrated inside the seed through three paths, namely: ①The water moved along the vascular bundle at the seed coat toward the chalazal; ② The water moved down towards the chalazal through the gap between the seed coat and cotyledons, entering the lateral side of cotyledons; ③ The water entered the hypocotyl through the radicle, then entered the cotyledons and moved to the chalazal, but its moving speed was noticeably slower than that of the second path. At the initial stage of water absorption (0-6 h), the imbibition area of the right seed coat was significantly larger than that of the left. Later, with the extension of soaking time, the imbibition speed of the left seed coat was faster, and there was no significant difference between the imbibition area of the left seed coat and that of the right seed coat. 2) The NMR spectrum showed that the water content (X) of A. julibrissin and the peak area (S) had a linear relationship, and the linear regression equation was: S=21 132X+695.05, R²=0.999 6. 3) There were three main states of water in A. julibrissin seeds, namely bound water, cytoplasmic bulk water, and extra-cellular free water. 4) At 0 h, bound water accounted for 45.13% of the water in the cell, extracellular free water accounted for 53.51%, and cytoplasmic bulk water was almost absent. During the process of water absorption, the content of bound water decreased significantly, which continued to decrease, accounting for only 2.08% by 48 h; the content of cytoplasmic bulk water increased rapidly at first and then decreased gradually. The content of extra-cellular free water increased continuously. At 48 h, the percentages of cytoplasmic bulk water and extra-cellular free water were 29.12% and 68.80%, respectively. During the whole process of water absorption, the percentages of water phases in three states was always in dynamic change, and the content of extracellular free water was much higher than that in other states. During this water absorption process, the peak of extra-cellular free water and bound water shifted to the left. Conclusion: The micropyle is the initial sites of water absorption in the seeds of A. julibrissin seeds. The water moves through the seed via the seed coat on both sides, the gap between the seed coat and the cotyledon, and the hypocotyl, and moves towards the chalazal. During the process of water absorption, there are three different water phases, namely bound water, cytoplasmic bulk water, and extra-cellular free water in the seeds of A. julibrissin, and they transform into each other during the entire imbibition process, which prepares the seed for germination.

Key words: Albizia julibrissin, water absorption, NMR technology, water phase state