Detection Of Water Distribution In The Processes Of Wheat Grain Filling And Maize Germination With The Nuclear Magnetic Resonance

Wheat grain filling is regarded as one of the most important stage in the formation of yield. It is also one of the major processes that affect wheat quality. Therefore, it can help advance the theoretic foundation of crop yield and quality increasing through investigating the grain filling process of winter wheat, especially water distribution and transmission in this process. Soil water content is an important environmental factor and soil water deficit can affect the performance of photosynthetic leaves and then impede the grain-filling process. However, many current available technologies are invasive and destructive to plant in water distribution detection, which made it difficult to truly unravel the progresses of water transmission in intact wheat plants. In this study, both proton-density-weighted magnetic resonance imaging(MRI) and T2 relaxometry of nuclear magnetic resonance(NMR) were used to non-invasively and non-destructively probe water distribution and transmission in wheat plant. The effects on the grain filling rate and the yield of wheat caused by soil water deficit were discussed. The results can help advance the theoretic foundation of cultivation and drought resistance research of wheat.Maize is the third largest food crop in China and also an efficient forage and economic crop. Maize germination is a key process that starts with the rehydration of a quiescent seed and ends with the onset of elongation of the embryo. It plays an important role in crop breeding and production. Water is an indispensable trigger condition in seed germination. Analysis of uptake and distribution of water during germination is necessary since changes of water content during this period influence subsequent development and growth of maize. In this study, both techniques of T1 weighted MRI and NMR T2 relaxometry were used to detect the water distribution and transmission during germination of maize seed in a non-destructive manner. The pattern of water absorption in maize seed was discussed. The results can help advance the theoretic foundation of maize cultivation and breeding. Some main conclusions have been drawn as follows.(1) The results of analyses with T2 relaxation spectrum of NMR of winter wheat plant and germinating maize seed showed that water content of the sample investigated was proportional to the signal amplitude. According to NMR T2 relaxation spectra of wheat spike, water in wheat spike could be classified into three different components according to its phase state, i.e. free water(1 ms<T2<15 ms), immobilized water(15 ms<T2<100 ms), and bound water(T2 >100 ms).(2) According to NMR T2 relaxation spectra of winter wheat during grain filling, water of wheat spike in different phases showed a similar tendency of increasing first and decreasing later. Bound water increased gradually along with the accumulation of milky materials in the endosperm and reached a peak at the wax ripeness stage. At the full ripening stage, bound water began to decrease slightly in the process of respiration. The maximal rates of grain filling and dry matter accumulation were achieved at 15-27 dpa(days after anthesis), but total water content of a wheat spike only decreased by one-tenth of its maximum during this period. Water content decreased rapidly with wheat senescence and grain dehydration after 30 dpa. In general, wheat spike maintained its water content at a high level at the rapid grain filling stage, which indicated the importance of high water content during assimilate accumulation. Proton-density-weighted MRI revealed that water distribution during grain filling of winter wheat increased gradually until 15 dpa, and decreased with the accumulation of dry matter in the endosperm from then on. Water content of the central region of the endosperm was higher than the outer region, which indicated that the milky materials were transported from the near coat endosperm to the central part. In the longitudinal images of wheat spikes, milky materials were shown accumulating from top to the bottom.(3) Soil water deficit during the grain filling stage of winter wheat could shorten the functional period of leaves and aggravate their senescence. At the early stage of grain filling, soil water deficit could accelerate the transport process of storage substance of wheat grain into the vegetative organs, but it could decrease grain filling rate untimely at the middle-late stage of grain filling and then lead to a shorten grain filling duration and a diminished accumulation of dry matter. The result of NMR T2 relaxation spectra showed that the water content curve of winter wheat spike looked like a "bell shape". The maximum water content of grain and final grain weight were positively correlated. When the soil moisture content increased, the maximum water content of grain rose and the final weight of single grain spike increased dramatically.(4) Analyses of transverse relaxation time(T2) revealed that water absorption rate followed three distinct stages of imbibition, protrusion, and germination. There was a remarkable increase of water absorption rate during the imbibition stage(0-18 h), then an 8-hour plateau stage in the initial protrusion stage, and then a second substantial increase of water absorption rate. This plateau stage was shorter than the traditional observed period of stagnation, which was probably due to the magnetic effect on plant germination during the NMR detection. When water absorption rate came to 51% at germination stage(absorption for 56-124 h), radicle protruded from the episperm and absorption of water increased rapidly. After the protrusion of coleoptile, water absorption rate oscillated remarkably, which was probably caused by the periodic water absortion by plant roots for more active metabolism. The results of T1 weighted MRI showed that the water absorption was not homogeneous from the episperm to the inside of maize seed. At the early stage of imbibition, there was a considerable amount of water absorbed by embryo and the volume of maize seed increased at the same time. Since water barely accumulated in interior endosperm, water content of endosperm remained stable until 28 hours after starting imbibition. Although water sucked up into episperm gradually in the whole germination period, there was no evident movement of water directly across the episperm into the interior endosperm, which means endosperm not only impeded the diffusion of water and oxygen but also hindered radicle protrusion.In general, NMR can help to reveal the water dynamics and distribution in the processes of maize seed germination and in the living body of winter wheat more continuously and precisely. The study also realized the measurement of water variation, which could not be detected directly by traditional methods. The results of this study will lay a theoretic foundation for the study on maize germination, grain filling, and drought resistance of winter wheat.