Abstract:

【Objective】 Water is important for seed germination, and nutrients in seeds require water to undergo physiological and biochemical reactions. This study used a nuclear magnetic resonance technology to explore water absorption of Robinia pseudoacacia seeds after hot-water treatments in order to explore changes in water phases during seed imbibition and germination. This study also provides a theoretical basis for water absorption. 【Method】Seeds of R. pseudoacacia were treated with hot water at an initial temperature of 85 ℃. Water absorption was calculated through dividing the increase in weight after soaking by the initial weight. The low nuclear magnetic resonance (L-NMR) was used to collect transverse relaxation time (T₂) signals of R. pseudoacacia seeds during imbibition, and a T₂ relaxation spectrum was obtained by inversion. Changes in water phases and abundances of each phase in the seeds during water absorption were analyzed. 【Result】The water absorption rate of seeds treated with hot water was markedly higher than that of seeds in the control group; the seeds entered the rapid water absorption stage from to 0-12 h, after which their water absorption rate decelerated, and saturation was reached at 36 h. The NMR spectrum showed that the water content (x) of R. pseudoacacia and the peak area (y) had a significant linear relationship, and the linear regression equation was y = 21 132x +698.05; R² = 0.999 6. There were three phases of water in R. pseudoacacia seeds during water-absorbing germination: bound water (T₂₁, >0.1-1 ms), immobile water (T₂₂, >1-100 ms) and free water (T₂₃, >100-1 000 ms). During imbibition, the peak time of bound water did not change significantly, and the relaxation range and peak area generally increased first and then decreased. However, the proportion of the peak area decreased. After 24 h of water absorption, the proportion remained below 4%, and the proportion of bound water in the seeds was very small. Bound water disappeared completely when the radicle passed through the seed coat. During 3-9 h of water absorption, the peak time of immobile water shifted significantly to the right, after which the peak time tended to be stable (approximately 9-96 h). When the radicle passed through the seed coat, peak time again shifted significantly to the right. The relaxation range of immobile water showed an increasing trend. However, it decreased significantly when the radicle passed through the seed coat. The peak area increased rapidly at first (approximately 3-12 h) and then remained stable, but the proportion of peak area decreased slightly, especially when the radicle passed through the seed coat. The peak time of free water changed the most. It reached a maximum at 72 h and then decreased significantly. That is, peak time shifted to the right first and then to the left. The relaxation ranges of free water also first showed an increasing trend and then decreased when the radicle passed through the seed coat. The peak area of free water increased rapidly at first (approximately 3-24 h) and then remained stable. When the radicle passed through the seed coat, it reached a maximum, which was 4.16-fold the minimum value (3 h). The proportion of peak area showed an increasing trend, especially when the radicle passed through the seed coat. 【Conclusion】There are three phases of water in R. pseudoacacia seeds during imbibition, and the most abundant water phase was immobile water. Each phase of water was in a state of dynamic change during imbibition. With the extension of water absorption, nutrients within the seeds began to decompose and transform, decreasing their ability to bind water. In particular, when the radicle passed through the seed coat, the content of free water increased significantly, and the seeds’ metabolism increased markedly.

Key words: Robinia pseudoacacia seed, imbibition, low nuclear magnetic resonance(L-NMR), water phase state