Abstract:Water absorption influences the germination and metamorphosis of seed. A continuous longterm investigation of germination process and water distribution can help to reveal the water absorption mode of winter wheat (Triticum aestivum L.) seed and lay a theoretic foundation for rational water management during seed germination. The nuclear magnetic resonance (NMR) technique can be used on probe water distribution of seed in a noninvasive and nondestructive way and has been broadly used in the studies on seed and water relationship. The techniques of magnetic resonance imaging (MRI) and T2 relaxation spectrum of NMR were used to continuously investigate the water distribution and variation in a germinating seed of winter wheat for 72h. Images of longitudinal and transverse sections of wheat seed were obtained every hour after the starting of imbibition and used to observe water distributions in different seed tissues. When the seed soaked, its volume increased rapidly at the beginning of seed imbibition (0~6h). Some localized hydrations were already evident in the embryo, coat, and nucellar projection. During the germination prepare phase (6~22h), water content of the coat was higher than that of the endosperm and there was a clear boundary between the bran and the endosperm in the images. 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 that of endosperm. Only the water from the nucellar projection gradually diffused into the endosperm. This proved that it was through the nucellar projection rather than the coat and embryo, the water entered into wheat seed. This finding is different from the description in some current textbooks. In this process, the water content of embryo increased and the volume of embryo was increasing 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 22h and the radicle grew smoothly out at 24h. Thereafter, the volume of shoot sheath started to increase and the seed sprouted at 27h. From then on, seed imbibed water through the radicle and hole in the coat. Water content of endosperm increased and endosperm was activated from bottom to top. Nutrients stored in the aleurone cells near the embryo began decomposing and moved to the central area gradually. The above processes of water transfer and distribution could not be visually detected from the outside with traditional methods. The results of T2 relaxation spectrum analysis showed that there were three distinct phases of wateruptake during the germination of a mature dry seed of winter wheat. During the first phase, water-uptake was rapid initially and then became stable. In the second phase, water absorption rate increased slowly, which was different from the traditionally observed period of stagnation. 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 remarkably. This kind of oscillation was probably caused by the periodic water absorption by plant roots. When roots grew over time, their ability to absorb water was enhanced. Thus, the amount of water involved in metabolic activity became larger. When the seed needed a large amount of water for metabolism, the root had to absorb enough water to meet the metabolic demand. After that, the water absorption rate of roots reduced appropriately until the seed required plenty of water for metabolism again. Hence, the water absorption rate of wheat seed oscillated sharply, but with an overall increase in the third phase. In general, the techniques of NMR can help to reveal the water dynamics and distribution in germinating seed more continuously and precisely. The study also realized the direct inside measurement of water content variation in wheat seed, which could not be detected directly with traditional methods. The results will help lay a theoretic foundation for the study and management of germination and water consumption of winter wheat.
