| Wheat cultivars with different starch components inherently were grown at Tai'an Experimental Station of Shandong Agriculture University during the 2004~2008 growing seasons. The granule size distribution, starch synthesis, and related enzymes activities were roundly investigated, and the results elucidated the relationship between starch component and granule distribution, the relationship between starch component and related enzymes activities, the difference in starch granule size distribution and starch synthesis in different cultivars, and the physiological and chemical mechanism of high temperature effects on starch quality formation. The main results were as follows:1 The characteristics of starch granule size distribution in wheat endosperm1.1 The starch granule size distribution of wheat cultivars differing in amylose contentThe field experiments were carried out at Tai'an Experimental Station of Shandong Agricultural University in two growing seasons from October 2006 to June 2007 and from October 2007 to June 2008. Nine wheat cultivars (Triticum aestivum L.), YM 50, SN 1391 and SN 8355 (high amylose content in grains), and SN 12, JM 20, and SN 8355 (low amylose content in grains), and ANN1, ANN2 and LH9-8 (Waxy wheat) were employed in this study.The results indicated that there were two types of starch granules: smaller B-type granules (<10μm) and large A-type granules (>10μm) in wheat endosperm at maturity. The starch granules diameter changed in the range of 0.3~45μm. Volume distribution of starch granules showed the typical two populations with peak values in 5μm and 21μm, respectively. The number distribution exhibited a single peak curve with peak at 0.5~1μm. And B-type granule number comprised over 99% of the total starch granule numbers, indicating that the number of starch granules was mainly small starch granules. Similar to volume distribution, the surface area distribution also showed double peak curves with peak at 1.5~2.8μm and 21μm, and the B-type granules comprised about 80%, and A-type granules contributed about 20% of the total surface area.In the present study, the B type granules were divided to three classes, small starch granule (<0.8μm), middle starch granules (0.8~2.8μm) and big starch granules (2.8~10μm). The number of small granules account for 34.9~55.5% of total starch granules, the number of middle granules comprised about 40.4~63.3% of total starch granules, the number of big granules occupied 2.9~6.3% of total starch granules (Table 1-2). In three classes test cultivars, the Waxy wheat cultivars contained lowest number of small starch granules, and there was no difference in cultivars with high and low amylose content. The Waxy wheat cultivars had highest number of middle starch granules, and there was no difference in cultivars with high and low amylose content. The number of big starch granules was highest in Waxy wheat cultivars, the second was the cultivars with high amylose, and the third was the cultivars with low amylose content. The results suggested that the amylose content influenced the number distribution, the effect degree varied with the size of starch granules.The volume of B type granules was highest in the Waxy wheat cultivars followed by the cultivars with low amylose content, the cultivars with high amylose content was the lowest. The volume of A type granules was in order in cultivars with high amylose content > cultivars with low amylose content > Waxy wheat cultivars, which indicated that the amylose content affected the volume distribution of starch granules. Similar to volume distribution, the surface of B type granules was highest in Waxy wheat cultivars, the second was cultivars with low amylose, and the third was cultivars with high amylose.1.2 The dynamic changes of starch granules in wheat endospermThe number of starch granules in wheat endosperm exhibited single peak curves or double peak curves. The starch granules initiated at 4 DAA increased in size to a broad band of variously sized granules up to about 20μm diameter by 7 DAA. The diameter band of starch granules continued to grow up and a burst of synthesis created a new population of small starch granules by 10~14 DAA, which reduced the number proportion of big starch granules. At 17 DAA, starch granules in grain mainly increased in volume. At 24 DAA, the number of starch granules less than 0.6μm increased sharply and that more than 0.6μm reduced evidently, indicating that a new distinct class of small granules was synthesized at this stage, but the magnitude of increase reduced. At 21 DAA, the highest diameter of starch granule ceased basically. From 28 DAA to maturity (35 DAA), it is mainly the diameter enlargement of smallest granule, not the other granule.2 Activities of enzymes involved in starch synthesis and starch accumulation in grains of two wheat cultivars with different amylose contentTwo wheat cultivars, JM20 (lower amylose content in grains) and LM21 (higher amylose content in grains), which both carry three waxy subunits were employed in this study during the 2004~2005 growing season. The results indicated that the activities of sucrose syntheses (SS), adenosine diphosphorate glucose pyrophrylase (AGPP), soluble starch syntheses (SSS), granule-bound starch syntheses (GBSS) and starch branching enzyme (SBE) all changed in pattern of a single-peak curve during grain filling. The SS, AGPP, GBSS, SSS and SBE activities of cultivar, which had higher amylose content, were higher than those of cultivar, which had lower amylose content (Fig.2-1-A, Fig.2-2). The accumulation rate of amylopectin was significantly or very significantly correlated with the activities of SS, AGPP, SSS and SBE; and the accumulation rate of amylose was highly significantly correlated with the activities of SS, AGPP and GBSS. The accumulation courses of both amylopectin and amylose were well fitted by the Logistic Equation by relating amylopectin and amylose content against days after anthesis. The simulation parameters revealed that the higher content of amylopectin and amylose resulted from earlier initiating time and greater accumulation rate, but accumulation duration probably played a less important role (Table 2-1). The amylose content in grains was mainly determined by GBSS activity as well as the activities of SS and AGPP. The change of GBSS activity as reflected by expressed amount of waxy gene was closely associated with the accumulation rate of amylose, indicating that amylose content in grains was determined by GBSS activity, especially at late grain filling stage. 3 Comparison of starch accumulation and sink strength in superior and inferior grains between compact and loose spike wheat cultivarsTwo loose spike wheat cultivars, SN1391 and SN12, and two compact spike wheat cultivars, LM21and JM20, were chosen in this study during the 2005~2007 growing seasons. The results showed that the accumulation of amylose and amylopectin in superior grains were significantly higher than those in inferior grains in two spike type cultivars. But the magnitude of difference in amylose and amylopectin accumulation between superior and inferior grains in compact spike wheat cultivars was large relative to that in loose spike wheat cultivars. The accumulation courses of starch were well fitted to the logistic equation by relating starch contents against days after anthesis. Logistic simulation revealed that the higher starch accumulation in superior grains resulted from earlier initiating accumulation time and greater accumulation rate. There was a larger difference in starch accumulation rates between superior and inferior grains in compact spike wheat cultivars relative to that in loose spike wheat cultivars, which may be one of the reasons that caused a larger difference in starch accumulation between superior and inferior grains in compact spike wheat cultivars. Endosperm cell numbers in superior grains were significantly higher than those in inferior grains. The magnitude of difference in endosperm cell numbers between superior and inferior grains varied with cultivars, the greater difference was detected in compact spike cultivars, whereas smaller in loose spike cultivars. The sucrose content in superior grains was lower than those in inferior grains during grain filling, which indicated that the sucrose content (substrate of starch synthesis) may not be a main limiting factor for starch synthesis in inferior grain. The activities of enzymes involved in starch synthesis including SS, AGPP, SSS and GBSS in superior grains were significantly higher than those in inferior grains. The magnitude of difference in the above indexes between superior and inferior grains was larger in compact spike cultivars than that in loose spike cultivars. The results here suggested that grain sink strength, determined by both endosperm cell numbers and activities of enzymes involved in starch synthesis, was the main factor responsible for starch accumulation in wheat grain. The larger difference in the grain sink strength was found to result in larger difference in starch accumulation between superior and inferior grains in compact spike cultivars as compared with loose spike cultivars. 4 Effect of postanthesis high temperature on grain yield and starch quality formation of two wheat cultivars differing in heat tolerance4.1 Effect of postanthesis high temperature on grain yield and yield components of wheat cultivarsTwo contrasting winter wheat cultivars, JM20 (weak heat tolerance) and LM21 (strong heat tolerance) were used in this study during the 2004~2006 growing seasons. Three treatments of high temperature were made in the field with plastic sheds in the early (5~9 d after anthesis, T1), middle (15~19 d after anthesis, T2), and later grain filling (25~29 d after anthesis, T3), respectively, in comparison with the control plants that grew naturally.The results showed that there was significant effect of high temperature stress on grain yield and yield components of wheat. High temperature reduced grain yield, grain numbers per spike, and kernel weight, but not spike number, and the damage degree of high temperature on grain yield and kernel weight at later grain filling stages was higher than middle and early-filling stages. And high temperature at early grain filling stages reduced kernel number per spike, high temperature at middle and later-filling stages showed little effects on the kernel number per spike. The results suggested that the decrease of grain yield was result from the reduction of kernel weight under high temperature conditions. The conclusion also showed that there was a difference in reduction extent of grain yield and grain weight, the cultivar with weak heat tolerance had a larger reduction extent of grain yield and grain weight than those of cultivar with strong heat tolerance.4.2 Effect of postanthesis high temperature on grain quality of wheat cultivarsThere was difference in the response of farimograph index to high temperature among different stages. The wet gluten content increased after high temperature stress, and the increase variation of high temperature at early grain filling stages was highest, the second was middle-filling stages, and the third was the later-filling stages. After T1 and T2 treatments, the dough development time and the dough stability time increased significantly compared to control treatment, but decreased after T3 treatment.RVA parameters were influenced by high temperature, the peak viscosity, trough viscosity, break down, final viscosity, and set back decreased after T1, T2, and T3 treatments. The T3 treatment had the largerst damage on the peak viscosity, trough viscosity, break down, final viscosity, and set back, followed by T2 treatment and T1 treatment. High temperature had little effects on the pasting temperature and peak time.4.3 Effect of postanthesis high temperature on starch accumulation, starch component, and starch granule size distribution of wheat grainThe results showed that there was significant effect of high temperature stress on starch accumulation of wheat grain. High temperature remarkably reduced starch accumulation at maturity. The total starch and amylopectin contents of high temperature treatments decreased markedly, but amylose content of heat treatments increased, as compared with control. Then the ratio of amylose to amylopectin in high temperature treatments was significantly higher than that of control. High temperature in the later period of grain filling had a larger effect on the starch accumulation than that in the middle and early grain filling. High temperature increased the volume, number and surface area of A type granule, but decreased the volume, number and surface area of B type granule, compared with control.4.4 Effect of postanthesis high temperature on antioxidant enzymes activities and lipid peroxidation of flag leaves and rootsAfter T1 treatment, the superoxide dismutase (SOD) activities of flag leaves and roots increased, with the time of stress released, the SOD activities of flag leaves and roots sharply decreased. After T2, T3 treatments, the SOD activities were reduced compared to control treatments. The peroxidase (POD) activities of flag leaves and roots in two cultivars significantly increased after high temperature. This indicated that there was difference in response to high temperature among different antioxidant enzymes. After high temperature stress, the catalase (CAT) activities of flag leaves and roots decreased compared to control treatment. The malondialdehyde (MDA) contents of flag leaves and roots increased gradually with the plant senescence. After T1, T2 and T3 treatments, the MDA contents of flag leaves and roots sharply increased relative to control treatment. And high temperature reduced the soluble protein contents in flag leaves and roots. The results suggested that high temperature increased the level of lipid peroxidation and accelerate the senescence of flag leaves and roots.4.5 Effect of postanthesis high temperature on enzymes involved in starch synthesis of grainAfter 5 d high temperature stress, the slight increases occurred in the sucrose content, the activities of SS, AGPP, SSS, GBSS and SBE in T1 treatment of two cultivars. But after removal of the high temperature stress in T1 treatment, the above parameters in wheat grains became lower than those of the control at both 15 d and 20 d. The significant decreases were observed in the sucrose content, SS, AGPP, SSS and SBE activities of T2 and T3 treatments. However, only a little difference existed in GBSS activity between high temperature treatment and control. The grain starch accumulation was found in consonance with the grain sucrose content, and the activities of SS, AGPP, SSS, GBSS and SBE, suggesting that it was poor supply of sucrose and the decreased activities of the enzymes involved in starch synthesis that brought about the declined starch accumulation in grain under high temperature stress.As compared with LM21, JM20 had a larger decline in the starch accumulation and the activities of related enzymes, indicating that the difference between cultivars existed in tolerance to high temperature. Hence, it is suggested that growing cultivar with strong heat tolerance, and applying appropriate schedules of irrigation and fertilization would be effective to cut dawn the influence of high temperature stress during grain filling on wheat production.
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