Analysis on water absorption of rice seeds during germination process under polyethylene glycol solution using low-field nuclear magnetic resonance

Water absorption is one of the major factors limiting rice seeds germination rate. It is very important to study the drought-tolerance mechanism of rice under drought stress, in order to improve the drought-tolerance capacity of rice and thereby increase the yield. In this paper, 2 varieties of rice seeds were placed in 10%, 20% PEG (polyethylene glycol) 6000 solution and control solution respectively, and moisture variation was analyzed with T2 relaxation spectrum and proton density weighted image of low field nuclear magnetic resonance (NMR) 0, 6, 22, 48, and 72 h later after seed germination. The influence of PEG solution on water absorption was studied, and the regression equation was deduced between relative moisture content and total signal amplitude of nuclear magnetic signal in this paper. The proton density weighted images obtained by the magnetic resonance imaging system can show the absorption and flow patterns of water during the germination of rice seeds directly. The germination test results showed that the drought resistance of H9710 is higher than that of LX1. The proton density weighted image results showed that water molecules enter the seed directly through cracks on the seed surface, and embryo absorbs water molecules and expands downwards firstly. After 24 h, the nutrient in the seeds flows to the seeds embryo to provide adequate nutrition for the growth of the radicle and germ. The seeds absorbed less water and the germination process was inhibited obviously under PEG stress, and the higher the PEG mass fraction, the less the water absorption and the slower the germination process. This is consistent with the results of T2 relaxation spectrum detection. Based on the theory of NMR and multicomponent characteristics of T2 relaxation spectrum, when inversion frequency was 10000 times, the water in rice seeds during germination was categorized into 2 phases, which were short relaxation time standing for bound water and long relaxation time standing for free water. The T2 relaxation spectrum results suggested that the amplitude of NMR signals of the bound water, that of the free water and the total amplitude all increased gradually. The drought tolerance of rice seeds is closely related to the relative water absorption rate after 24 h, and PEG solution inhibits the absorption of water; the higher the PEG mass fraction, the stronger the inhibition. The relative water absorption rate of rice varieties with stronger drought resistance was higher than that with weaker drought resistance. The change of NMR signals directly reflects the moisture distribution and water absorption of rice seeds during germination. In addition, the moisture content of the seeds and the total amplitude of NMR signals were positively correlated(R2=0.983). This indicated that the data obtained by NMR technique were consistent with those obtained by the conventional dry-weight weighing method. Therefore it is a proper method to detect the moisture content in seeds during germination with NMR technique. By this regression equation, the moisture content of each state of water during seed germination can be calculated. These empirical data offer a reference for the study of rice seeds' response mechanism under drought stress during seed germination and the screening of germplasm resources, and also develop a new nondestructive detecting technology of moisture for plants under stress.