小麦灌浆过程籽粒水分变化的核磁共振检测

Water changes in wheat spike during grain filling stage investigated by nuclear magnetic resonance

花期至成熟期是小麦产量形成的关键时期,在这一时期麦穗的水分状态随着干物质的积累而呈现出独特的变化特征。为了揭示活体冬小麦灌浆过程的水分变化规律,利用核磁共振技术的无损检测特性,结合核磁共振质子密度加权成像和核磁共振T2弛豫谱分析,对小麦麦穗进行了连续活体检测。核磁共振质子密度加权成像结果表明,灌浆前期籽粒的水分不断增加,至花后15 d籽粒的水分含量达到最大值,此后小麦籽粒的水分逐渐减少。在此过程中,灌浆物质在籽粒中积累的顺序是由外向内、自上而下的。经核磁共振T2弛豫谱分析,麦穗中的水分可分为结合水、半结合水和自由水三种相态。从籽粒形成至完熟期麦穗不同相态的水分都表现为先增大后减小的特点,但涨落步调不尽相同,其中结合水含量的增长期最长,至蜡熟期结束时(花后33 d)才达到最大值。籽粒形成后麦穗总水分含量维持在较高水平,即使在籽粒干质量快增期(花后15 d至花后27 d),籽粒中干物质的迅速积累也并未导致水分含量的明显减小,单穗的总水分含量与最大水分含量相比仅仅减少了十分之一。花后30 d之后,随着颖片及穗轴逐渐变黄衰老和籽粒的脱水成熟,麦穗水分含量才急剧减小。小麦灌浆中期麦穗维持较高水分含量,说明水分在同化物积累过程中的重要作用。除了灌浆中期较高的水分含量,蜡熟期的快速脱水亦有利于营养物质的贮存并减少呼吸消耗,对于小麦产量的形成和稳定亦具有重要意义。

Water plays a predominant role in physiology and biochemistry of a wheat spike during grain filling stage. It is one of the major factors that affect wheat yield and reproduction. Magnetic resonance imaging(MRI) and nuclear magnetic resonance(NMR) T2 relaxometry were used in order to non-invasively and non-destructively study water distribution and migration of wheat spike from anthesis to maturity. An NMR imager with a 0.5 T permanent magnet and a 20 mm inner diameter open bored RF probe was used for both MRI and NMR T2 relaxometry measurements. Armed with gradient coils which could produce a maximum of 0.04 T/m magnetic field gradient at x, y and z direction, the NMR imager could result in a vertical or transverse proton density weighted image by using multiple spin echo(MSE) pulse sequence. Repetition time, echo time, and spectral bandwidth were set at 5000 ms, 9.4 ms and 20 k Hz, respectively. Slice thickness was 3 mm and a resolution of 0.22 mm was achieved within 42 min. An external reference tube with pure water in it was set vertically along the wheat spike for pixel signal intensity calibration when MRI measurement was conducted on transverse direction. NMR T2 relaxation measurements were conducted by carr-purcell-meiboom-gill(CPMG) scheme. Repetition time and echo time were 13000 ms and 800 us, respectively. Four repeats were accumulated to increase signal to noise ratio. Proton density weighted MRI showed that water content in wheat grain increased gradually until 15 days post-anthesis(dpa), and from then on decreased with the accumulation of dry matter in the endosperm. 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. Weight measurements showed that dry weight of glume and rhachis of individual wheat spike kept constant after anthesis and its water weight was stable during most development stage, which indicated that the change of signal amplitude in NMR T2 relaxation spectrum of one wheat spike was mainly due to water status change in grains. According to NMR T2 relaxation spectra of a wheat spike, water in wheat spike could be classified into three different components according to its phase state, namely free water, immobilized water, and bound water. Bound water increased gradually along with the accumulated 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, but total water content of a wheat spike only decreased by one-tenth in this period. Water content decreased rapidly with wheat senescence and grain dehydration after 30 d. In general, the wheat spike maintained its water content at a high level at the rapid grain filling stage, which indicated the significance of high water content in assimilate accumulation. On the other hand, rapid dehydration at the ripening stage would also be an important factor in yield stability, since it would reduce energy cost by respiration and therefore preserve nutrients in grains.