Abstract:

Objective: Quercus palustris is mainly propagated by seeds, but it is very difficult for seed storage due to its sensitiveness to the desiccation. This study aims to explore the changes of water phases during desiccation of Q. palustris seeds and find out the semi-lethal and lethal water contents, so as to provide a theoretical basis for scientific storage and transportation of seeds, and provide a new technique for the study of other recalcitrant seeds. Method: The newly collected Q. palustris seeds were used as research materials in this study. The natural drying method was used to reduce moisture content of seeds. Nuclear Magnetic Resonance (NMR) technology was used to detect the changes of water phases and content in seeds during the desiccation process. Combined with the germination percentages of seeds under different water contents, the effects of water phases on seed dehydration were analyzed. Result: 1) The moisture content of fresh Q. palustris seeds was 38.0%, and the germination percentage was 96.00%. With the decrease of moisture content, the germination percentage decreased continuously. The seeds died completely when moisture content decreased to 10.0%. According to the fitting curve calculation, the semi-lethal moisture content was 17.39%. 2) The NMR spectrum showed that there was a significant linear relationship between the water content (x) of Q. palustris seeds and the peak area (y) of the NMR relaxation spectrum, and the linear regression equation was: y=21132x+698.05, R²=0.9996. 3) From left to right, the peaks in T₂ relaxation spectrum of transverse relaxation were bound water (T₂₁), immobile water (T₂₂), and free water (T₂₃). 4) The proportion of free water in fresh Q. palustris seeds was the highest, accounting for 66.39%, while bound water and immobile water were 4.28% and 29.07%, respectively. In the process of desiccation, the binding capacity, peak area and peak proportion of water in each phase were in dynamic changes. During the desiccation process, the bound water was stable and hard to be removed. The peak time of immobile water generally shifted to the left, and the binding ability was enhanced. The free water had stronger fluidity, and the peak time shifted to right during the desiccation. When the water content was reduced to 25.0%, the peak area of free water decreased significantly, while the immobile water and bound water increased significantly, and the peak proportion also increased. When water content decreased to 20.0%, the peak area of bound water and its proportion increased, while the peak area of immobile water decreased. Subsequently, the peak area of water in each phase decreased. Conclusion: With the extension of dehydration time, the germination percentage of Q. palustris seeds decreases, and the water content during its storage and transportation should not be lower than the semi-lethal water content of 17.39%. In the process of desiccation, there are three water phases in the seeds, namely bound water, immobile water and free water. Desiccation has a great effect on the quality of Q. palustris seeds. During the desiccation, the water binding ability of seeds becomes stronger to slow down the loss of water and maintain the stability of cell membrane.

Key words: Quercus palustris, recalcitrant seeds, desiccation, nuclear magnetic resonance