利用低场核磁共振及其成像技术分析水稻浸种过程水分传递

Analysis on moisture transport in process of rice soaking using low field nuclear magnetic resonance and its imaging

为研究水稻种子浸种过程中内部水分流动情况,可视化内部水分传递过程,利用低场核磁共振及其成像技术,监测沈农9816号、七山占及秀子糯3个品种水稻种子48 h浸种过程。每6 h时间间隔利用自旋回波(spin echo,SE)脉冲序列获取样品的质子密度加权像,利用硬脉冲自旋回波(carr-purcell-meiboomgill sequence,CPMG)序列获取样品的横向弛豫时间T2反演谱,从而分析浸种过程对水稻种子内部水分分布的影响。试验结果表明:核磁共振是一种有效的水分检测技术,可以实现浸种过程中种子内部水分的快速、准确、无损的检测。利用水稻种子的质子密度加权像,能够直观检测到种子内部水分分布情况,动态的监测到种子内部水分流动过程,分析发现水分最初是从胚进入种子内部,继而通过种皮的渗透,最后到达胚乳部分。根据T2反演谱信号幅值计算得到的水稻种子吸水率,发现3个品种在相同浸种时间的各个监测点均反映出秀子糯吸水率最高,沈农9816号吸水率最低,试验结果验证了支链淀粉的吸水性优于直链淀粉。研究结果可以为水稻种子浸种过程中水分传递的理论模型构建提供数据支持。

In order to study the internal water flow process of rice seed in presoaking and visualize the internal water transport process, an experiment of monitoring three varieties of rice seeds, namely SYAU No. 9816, Qi-shan-zhan, and Xiu-zi-nuo, during a 48 h presoaking process with TD-NMR and MRI technology has been conducted. To learn the effect of the presoaking process on rice seed water distribution and water absorption, during the experiment period, the PDWIs of all samples have been obtained with SE pulse sequence, and the T2 spectral deconvolution images have been obtained with the CPMG pulse sequence every 6 hours. During the observation period, after presoaking for 0 hour, 6 hours, 12 hours, 18 hours, 24 hours, 30 hours, 36 hours, 48 hours, spin-echo（spin echo, SE） pulse sequence had been used to obtain proton density-weighted images of all the samples. CPMG（carr-purcell-meiboomgill sequence, CPMG） sequence had been used to obtain the transverse relaxation time T2 inversion spectrum of all the samples, so as to analyze the impact of seed soaking of the internal water distribution of rice seeds. NIUMAG MRI image processing software had been used on the 256 pixel × 256 pixel grayscale images acquired by the magnetic resonance imaging software for unified grayscale, pseudo color images, filtering and other processing, to adapt the images into forms that were more suitable for observation. The average value of the 3 CPMG pulse sequence value generated by the analysis software after repeated application of NMR signal at different corresponding time and the peak value had been calculated and the average value had been imported into NMR inversion software to obtain T2 spectral deconvolution. The experiment results showed that： MRI is an effective water detection technology, with whose help the internal water of rice seeds during presoaking can be monitored efficiently, accurately, without any loss. The use of T2 relaxation spectrum and proton density-weighted images helped to obtain a sample of internal hydrogen proton density and distribution, so as to reflect the moisture content and water flow information of the sample. PDWI can show the distribution of hydrogen proton within the samples. As in a PDWI, the part will be brighter if the density of hydrogen proton in that part is higher and the hydrogen protons within rice seeds mainly come from water, the brighter the part of the image is the higher level of water content in that part of the seeds it is. Therefore, with PDWI, the internal water distribution of rice seeds can be detected and explicitly shown. By observing the grayscale and the signal intensity curve of the three different kinds of rice after 48-hour presoaking time at various sampling points, the results found embryos had the highest moisture content, the seed coat came second, and endosperm had the lowest moisture content. A serial of pseudo-color maps of rice seeds obtained every 6 hours during presoaking can manifest the internal water dynamic flow process within the seeds. The map analysis showed that the water first penetrated the semi-permeable membrane of the embryos, went inside of the seed from the embryo end, penetrated the seed coat into the seed, and finally reached the endosperm. According to the T2 spectral deconvolution signal amplitude spectrum, water absorption of the rice seeds can be calculated. After each period of presoaking at various monitoring points, Xiu-zi-nuo showed the highest level of water absorption, Qi-shan-zhan came the next, and SYAU No. 9816 had the lowest level of water absorption. The paper analyzed the causes of the difference in water absorption： amylopectin has better water absorption than amylase, and Xiu-zi-nuo has the highest amylopectin content within the three different seed types, while SYAU No. 9816 has the highest amylose content. This conclusion was perfectly verified by the MRI results. The experiment results may provide valuable data for the module construction of water transport theory during rice seed presoaking.