Detection Of Water Distribution In Germinating Seed And Growing Body Of Winter Wheat Based On The Nuclear Magnetic Resonance

Winter wheat is one of the main food crops in the arid and semi-arid lands in northern China. Draught often affects the germination, emergence, full stand and strong of winter wheat. Meanwhile, the moisture in plant will respond to the surroundings timely and change with the plant growth period. The moisture variation influence the plant metabolic activity, assimilates accumulation, and final yield. Therefore, investigating winter wheat germination process, water distribution and variation during its lifetime for a continuous long time can help reveal the water absorption mode of seed germination and the processes of development and senescence of winter wheat more directly and precisely. The results will help advance the theoretic foundation of water consumption of winter wheat growing healthy and establish more rational irrigation schedules. Nonetheless, many current technologies are invasive and damaging to the plant in water distribution detection, which made it difficult to truly unravel the progresses of seed absorption and water transmission in plants. The nuclear magnetic resonance(NMR) technique can be used to probe water distribution and variation in wheat seed during the germination process and detect the moisture contents in living wheat organs in a noninvasive and nondestructive way.By detecting and analyzing magnetic resonance imagings and the T2 relaxation parameters of NMR of the germinating winter wheat seed for continuous 72 h, and the T2 relaxation parameters of NMR of wheat plant organs during grain milk and ripening stages, some main conclusions have been drawn as follows:(1) Images of longitudinal and transverse sections of wheat seed were collected every hour after the start of imbibition and used to observe the water distributions in different seed tissues. When the seed soaked, its volume increased rapidly at the beginning of seed imbibition(0-6 h). Some localized hydration was already evident in the embryo, coat, and nucellar projection. During the germination prepare phase(6-22 h), water content of the coat was higher than the endosperm and there was a clear boundary between the bran and the endosperm. Although water accumulated in the coat surrounding the endosperm, there was no evident movement of water directly across the coat and into the underlying starchy endosperm. The water content of nucellar projection was also higher than the endosperm. Water diffused into the endosperm gradually. This proved that the entrance of water into wheat seed was the nucellar projection rather than the coat and embryo. This finding is different from current views in some textbooks. In this process, the water content of embryo increased and the volume was also going to increase at the same time. It showed that embryonic cells began to divide and elongate and the volume of root sheath became remarkably larger than before. In the stage of germination, the root sheath emerged from a breaking hole in seed coat at 22 h and the radicle growed smoothly out at 24 h. Whereafter, the volume of shoot sheath started to increase and the seed sprouted at 27 h. From then on, water imbibed through the radicle and hole in the coat. Water content of endosperm increased, in which activated reaction taken place. Stored nutrients in endosperm began decomposing in the aleurone cells near the embryo and moved to the central area gradually. The above water transfering process could not be visually detected from the outside.(2) There were three distinct phases during the uptake of water by a mature dry seed of wheat. In the first phase, water uptak was rapid initially and then became stable. In the second phase, water absorption rate increased slowly, which was different from the traditional view of stagnation period. This phenomenon was probably due to the temperature change effect and magnetic biological effect on plant growth during the NMR detection. In the third phase, water absorption rate oscillated clearly, which was probably caused by the periodic water absorbion by plant roots. When roots growed over time, their ability to absorb water was enhanced. Thus, the amount of water involved in metabolic activity became larger. When the seed needed more water for metabolism, the root had to absorb enough water to meet the metabolic demand. Consequently, the root water absorbion rate reduced appropriately until the seed required plenty of water for metabolism next time. Hence, the water absorption rate of wheat seed oscillated sharply, but with an overall increase in the third phase.(3) In order to investigate water distribution in a living winter wheat plant, the T2 relaxation parameters of NMR were used to measure the moisture contents(MC) in leaf, stem, and spike of winter wheat. This study showed that the amplitude of a nuclear magnetic resonance T2 relaxation spectrum is linearly correlated to the mass of water in the plant sample. According to the nuclear magnetic resonance T2 relaxation properties of wheat leaf, stem, and spike, their T2 relaxation spectra can be divided into two components, from the peak areas of which the fresh weight can be estimated through linear regression. Hence, the moisture contents of various organs could be obtained. To verify the moisture content estimation function, the organ moisture contents of seven wheat varieties were measured with both NMR and traditional oven-drying methods. The root mean square error(RMSE) was adopted to measure the accuracy. The RMSE values of two kinds of measured moisture contents of leaf, stem and spike were 5.3%, 3.5% and 3.3%, respectively.(4) The plant moisture detection method established above was applied to monitor the long-term and diurnal changes of the moisture contents in living wheat plants at different growth stages. The results showed that moisture content of leaves, stems and spike decreased during grain milk and ripening stages. The diurnal variation of leaves moisture contents decreased first and then increased. At grain milk stage, leaves moisture contents reduced from their initial state at 8:00 to the minimum at 14:00-16:00 and then recovered to their initial state at 20:00. However, at ripening stage, leaf moisture contents began to reduce at 8:00, but they could not recover to their initial state at 20:00.In conclusion, NMR can help to reveal the water dynamics and distribution in germinating seed and growing body of winter wheat more continuously and precisely. The study also achieved the water variation measurement which could not be detected directly. The results will lay a theoretic foundation for the study on germination control, water consumption of winter wheat healthy growing and rational irrigation schedules.