Low-field nuclear magnetic resonance of maize seed germination process under salt stress

Abstract: Taking the non-waxy corn Zhengdan 958 seeds as the test object, a Low-Field Nuclear Magnetic Resonance (LF-NMR) experiment was performed under different NaCl concentrations (0, 50, 100, 150 and 200 nmol/L), in order to explore the influence of salt stress on the water distribution and activity during the germination of corn seeds. Two parts were included in the LF-NMR experiment, Magnetic Resonance Imaging (MRI) and nuclear magnetic resonance signal test. The proton density image was obtained by the Multi-slice Spin Echo (MSE) pulse sequence, and the nuclear magnetic signal was obtained by Carr-Purcell-Meiboom-Gill sequence (CPMG) pulse. The samples were incubated for 6 days at 28℃, providing for the analysis of nuclear magnetic resonance proton density images and NMR relaxation spectrum. The results showed that: The germination of maize had undergone the swelling (0-2 d), sprout (3-4 d), and budding (5-6 d) during the entire experimental period. Specifically, the overall moisture content increased, while the activity became stronger, as the gray value increased, where the grayscale image became brighter and whiter in a certain area. Therefore, the gray value of images can be used to represent the overall or local moisture content of seeds. Furthermore, the Region of Interest (ROI) of proton density image was extracted further to count the average gray value. It was found that the moisture content of endosperm and embryo part increased rapidly during the seed swelling stage, whereas, the growth rate decreased after 2 d. There was also a fluctuating increase in the moisture content of endosperm, whereas, the water in the embryo site fluctuated horizontally. The rate of water decreased as the salt concentration increased. Since the pseudo-color images can be used to visually show the germination process of seeds, they were used to explain the distribution and migration of overall moisture. However, more accurate data was needed to analyze the division and change rules of moisture in different phase states. The germination rate of seeds decreased from 90% to 0, as the degree of salt stress increased. When the salt concentration was 50 nmol/L and even below, the water signal amplitude of different phases was less affected by the salt stress, but when the salt concentration reached 100 nmol/L, the content of free water was suppressed at a low level. Particularly when it reached 150 nmol/L, the rate of change was greatly delayed in the content of bound and semi-bound water. After the 1th d, the T2 relaxation spectra showed that, a small area of the side peaks which did not have obvious regularities with time appeared on the left of the bound water, the relaxation time was within the range of bound water, and the proportion of total moisture was below 10%; the seeds would not germinate and no side peaks appeared when the salt solution concentration reached 200 nmol/L. Side peak signal represents water whose hydrogen bond is more stable than in starch, and the special hydrogen bond might come from the biochemical reaction that occurred after the seed was released from the dormant state. The appearance of the side peaks closely related to whether the seeds germinate showed that the swelling stage is not completely a physical phenomenon, and the chemical reaction in this process prepares for the subsequent germination and exposure. Side peaks indicate a kind of bound water that marks the germination of seeds. The findings can provide the theoretical reference for the follow-up research on the response mechanism of maize seed germination to salt-alkali stress. The LF-NMR test can be used to simulate the salt-alkali environment, futher to quickly identify the salt-tolerant ability of seeds or plants.