水稻生长过程中籽粒水分状态和横向弛豫特性分析

【目的】基于低场核磁共振(LF-NMR)技术观察水稻抽穗到成熟过程中籽粒水分状态的变化,探讨淀粉、蛋白质的积累效应对前者的影响,为水稻品质形成规律提供数据参考。【方法】对抽穗后63 d内的"越光"有机稻间隔采样,测定籽粒的百粒重、硬度、水分含量、淀粉含量和蛋白质含量,比较水稻在抽穗后不同生长时期的整体品质变化,通过LF-NMR测定的横向弛豫参数定性和定量地分析籽粒中的水分动态,探讨水稻品质形成过程与水分状态的相关联系。【结果】在抽穗后7—14 d百粒重和淀粉含量增长速率最快,两者表现极显著正相关(P0.01)。籽粒硬度分别与淀粉含量和蛋白质含量都极显著正相关(P0.01),与水分含量极显著负相关(P0.01)。水分含量在抽穗后7—56 d从57.16%呈指数型下降到22.39%。淀粉含量在抽穗后42 d内呈"S"型曲线增长至50.47 g/100 g湿基。蛋白质含量在抽穗后7—49 d线性地增长至峰值6.56 g/100 g湿基。总体而言,水稻籽粒在抽穗后49 d左右整体品质已经形成。LF-NMR反演图谱表明在抽穗后7 d内籽粒部分水分往高自由度方向移动。抽穗后7—21 d,反演曲线整体向左迁移,表征流动性最弱"结合水"的T2b峰出现,表征束缚水的T_(22)峰发生峰的分化现象。4种横向弛豫时间T_(2b)、T_(21)、T_(22)和T_(23)随生长时期而减小,水稻籽粒中的整体氢质子自由度在逐渐降低。抽穗后21 d左右,"结合水"的峰比例超过束缚水和自由水的峰比例总和。籽粒内的水分含量和横向弛豫参数(T_(2b)、T_(21)、T_(22)、T_(23)、A_(2b)、A_(22)和A_(23))都随生长时期极显著地变化(P0.01),而且淀粉和蛋白质积累与前者变化具有极显著的相关性(P0.01)。尤其是淀粉充实胚乳细胞,水分子被淀粉颗粒包围或与其中的亲水基团氢键作用,使得整体水分状态向"结合水"的方向迁移。基于对横向弛豫信号的主成分分析,发现抽穗后42 d内不同生长时期的籽粒水分状态差异显著,42 d后整体趋于稳定。【结论】水稻灌浆期间,籽粒内部水分状态与淀粉和蛋白质的积累显著相关;"结合水"的比例逐渐上升,束缚水和自由水的比例显著下降;利用LF-NMR技术可以有效分析抽穗后不同生长时期籽粒的整体水分动态变化。

Analysis of Moisture State and Transverse Relaxation Characteristics of Grains During the Growth Process of Rice

【Objective】This paper is mainly to observe the changes of moisture state of grains during the growth process of rice after heading by LF-NMR, and investigate the effects of accumulation of starch and protein on the moisture state of kernels, which provides reference for formation regularity of rice qualities.【Method】The spikes of “Yueguang” organic rice were sampled at intervals for 63 days after heading (DAH), The 100-grain weight, hardness, moisture content, starch content and protein content were measured, separately, which were used to compare the changes of whole qualities of grains at different growth periods of rice after heading. The transverse relaxation parameters determined by LF-NMR were adopted to qualitatively and quantitatively analyze the moisture dynamics of grains. What’s more, the internal relationships between the formation process of rice qualities and moisture state of grains were discussed.【Result】Both the 100-grain weight and starch content for “Yueguang” organic rice increased most quickly within 7-14 DAH, and they were highly significantly and positively correlated with each other (P＜0.01). Hardness was highly significantly and positively correlated with starch content and protein content, respectively (P＜0.01), and negatively correlated with moisture content (P＜0.01). Within 7-56 DAH, moisture content exponentially declined from 57.16% to 22.39%. With showing an S-shaped growth curve, starch content increased to 50.47 g/100g wet-basis within 42 DAH. Protein content linearly increased to the peak value of 6.56 g/100g wet-basis within 7-49 DAH. On the whole, the whole qualities of rice grains had been formed within 49 DAH. The inversion spectrum of LF-NMR data showed that part of water in the grains moved towards the direction of high mobility within 7 DAH. Within 7-21 DAH, when inversion curves of LF-NMR data gradually moved towards the left, T_2b peak representing for “bound water" with least mobility occurred, followed by the differentiation phenomenon of T₂₂ peak representing for capillary water. Four kinds of transverse relaxation times including T_2b, T₂₁, T₂₂ and T₂₃, gradually decreased with growth periods, which indicated that whole proton degree of freedom in the kernels dropped obviously. The peak ratio of “bound water” was more than peak ratio summation of capillary water and free water at 21 DAH. The moisture content and transverse relaxation parameters (T_2b, T₂₁, T₂₂, T₂₃, A_2b, A₂₂ and A₂₃) of grains varied with growth periods highly significantly (P＜0.01), which were also highly significantly (P＜0.01) related to the accumulation of starch and protein. With endosperm cells filled with starch granules, most of water molecules in the grains were mainly surrounded by starch granules and formed hydrogen bonds with hydrophilic groups of starch, which resulted in the migration of moisture state towards the direction of “bound water”. According to principal component analysis of transverse relaxation signals, there were significant differences among the moisture state of rice grains for different growth periods within 42 DAH, followed by keeping steady.【Conclusion】It was found that during the grain-filling process of rice, the moisture state of grains was significantly related to the accumulation of starch and protein. The ratio of “bound water” gradually increased, and the ratios of capillary water and free water significantly decreased in the grains. LF-NMR can be adopted to effectively analyze the whole moisture dynamic changes of grains at different growth periods of rice after heading.