Abstract:
Abstract: 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 kHz, 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 dpa. 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.