Structure Formation Characteristics Of Starch In Wheat Grain And Their Responses To High Temperature

Starch and its component contents in wheat grain are the main quality indexes of flour. Starch quality is affected by both genetic traits and environmental factors. Previous studies pointed that environmental factors were the main factors changing starch structure and physicochemical properties, especially for high temperature during grain filling period. In order to study further the formation process of starch granules and the structure formation characteristics of amylopectin and the relationship between grain starch structure and physiochemical properties, an experiment was conducted at Jiangsu Provincial Key Lab of Crop Genetics and Physiology of Yangzhou University from 2013 to 2015, using waxy wheat Yangnoumai 1 and Ningnoumai 1 and their non-waxy parents as the materials. In addition, a pot experiment was conducted in an artificial intelligent greenhouse to investigate the effects of temperature during grain filling stage on starch structure. The present study is of critical importance for providing theoretical reference for starch quality formation in wheat grain. The main results were as follows:1. Amylose and amylopectin have already been formed in wheat grain on the 5th day after anthesis (DAA). There are three types of starch granules in wheat grain. C-type granule (< 2μm) number was the main type, followed by B- type (2～10μm) and A-type (>10μm). During grain filling, starch granule size and the number of A-type granule increased significantly on the 10th DAA. The number of C-type granule increased rapidly from the 15th DAA to 20th DAA, which decreased the number of A-type granule significantly. At maturity, the order number of three types of granule in waxy and non-waxy wheat was C-type> B-type> A-type. The number of C-type granule in two waxy wheat varieties was significantly higher than that in their non-waxy parents, while the number of A-type granule was significantly lower than that in their parents.2. The formation process of amylose and the long and short chain formation in amylopectin were different between the starch of waxy wheat and non-waxy wheat. During grain development, the content of amylose increased in non-waxy wheat and reached the maximum level (28%-34%) at maturity, while the content of short chain in amylopectin was significantly decreased and the content of long chain changed insignificantly. The content of amylose in waxy wheat was the highest in the preliminary formation stage of grains, accounting for 18%-19%, and then the content of amylose declined to about 1% at maturity. The content of short chain in amylopectin in waxy wheat increased during the grain development, but the long chain content did not change significantly. The test of iodine stain of starch grain showed that amylose was distributed in the central position of starch granule in waxy wheat.3. The XRD patterns of waxy and non-waxy wheat starches exhibited A-type spectra. The maximum diffraction intensity of all varieties appeared at 18°2θ. The ratio of diffraction intensity at 20° 2θ to that at 18° 2θ in waxy wheat was lower than that of their parents. The relative crystallinity in waxy wheat was significantly higher than that of non-waxy wheat. NMR spectra showed that the area of crystallization was significantly bigger and the degree of crystallinity in C1 region in Ningnuomai 1 was significantly higher than that in Yangnuomai 1, which proved that the waxiness of Ningnuomai 1 was stronger.4. The differences in the physiochemical properties between waxy and non-waxy wheat were observed. Swelling power, solubility and transmittance of starch in waxy wheat were higher than those of non-waxy wheat. Waxy wheat starch had higher peak temperature, final temperature, gelatinization enthalpy and the less easiness of retrogradation. The restraining retrogradation ability of Ningnuomai 1 was stronger than that of Yangnuomai 1.5. The number of C-type granule in non-waxy wheat Yangmai 15 increased, while the number of C-type granule in waxy wheat Yangnuomai 1 decreased after high temperature (day/night 35℃ /15℃) stress at different grain filling stages. The short chain content of amylopectin increased and the content of amylose decreased after high temperature stress during grain filling, especially for the treatment of high temperature from the 16th to 20th DAA. The effects of high temperature stress from the 6th to 10th DAA had less effects on the structure formation of amylose and amylopectin in non-waxy wheat Yangmai 15, but more significantly inhibited the formation of amylose in waxy wheat and in the end amylose content was only 0.01% at maturity. Relative crystallinity, gelatinization enthalpy, swelling power, solubility and transmittance decreased, while retrogradation degree increased after high temperature stress at different gain filling periods, especially the treatment of high temperature stress at 35℃ from the 16th to 20th and 21st to 25th DAA. These results proved that the period from 16th to 20th DAA and the period from 21st to 25th DAA were the key periods for the amylopectin structure formation. High temperature stress after anthesis changed the structure of amylopectin, inhibited the formation of crystallization area, and affected the physiochemical properties of starch. The grain yield decreased after high temperature stress at different grain filling stages. The loss of grain number per ear and grain weight of high temperature stress from the 6th to 10th DAA, and the loss of grain weight of high temperature stress from the 16th to 25th DAA, were all the main reasons for the decline in yield.6. Relative crystallinity, short chain content, long chain content and volume distribution of B-and C- type starch granules were negatively correlated with retrogradation degree at the probability level of 0.05 or 0.01. Relative crystallinity, short chain content and volume distribution of B- and C- type starch granules were positively correlated with solubility and transmittance at the probability level of 0.05 or 0.01. Long chain content had not significant correlation with gelatinization enthalpy, retrogradation degree, solubility, swelling power and transmittance. These indicated that the higher number distribution of small- and medium-sized granules, the higher relative crystallinity, the higher enthalpy needed to break the micro-crystal structure of starch granules, the higher ability of water uptake and transmittance during the process of gelatinization, and the less easiness of retrogradation. As the amylose content increased in starch, the gelatinization enthalpy, swelling power, transmittance, and water uptake ability increased, and starch retrogradation was easier after gelatinization.