| In this study, we selected more than ten wheat cultivars differing greatly in starch accumulaiton and starch characteristics, and set different-rage-shading treatments after flowering and different-stage-shading treatments beginning from flagging to study starch granule size distribution, accumulation peculiarities, changes of related enzymes activities and isolated starch physiochemical characteristics. At the same time, the realtionship between wheat starch composition and dough rheological properties was also analyzed. The main results were as follows.1. The granule size distribution characteristics of wheat grain starch and response to low lightThe granule size distribution of wheat grain starch varied with grain filling stage and genetype. The starch granules formed in the initial grain filling stage grew fastly before 15 days after anthesis (DAA). In the next time after 15 DAA, the starch granule came into being and grew in volume at the same time. And the most larger diameter starch granules were detected in the stage from 20 to 25 DAA. From analysis we deduced that there were three fastigiums of forming starch granules in wheat grain filling stage, namely, before 6 DAA, 15-20 DAA, and 25-30 DAA.At mature stage, most of wheat cultivars' endosperms had a unimodal curve in starch granule number distribution, and a bimodal curve in starch guanule volume or surface area distribution. Few cultivars in our study showed a bimodal curve in starch granule number distribution, and a trimodal curve in starch guanule volume or surface area distribution. The number and surface area ratioes of B-type starch granules were much higher than those of A-Type starch granule in mature wheat grains. But the volume ratioes of the two types wheat starch granules were different between waxy and nonwaxy wheat cultivars. In waxy wheat cultivars, the volume ratioes of A-Type starch granules were lower than those of B-type starch granules. On the contrary, the volume ratioes of A-Type starch granules in nonwaxy wheat cultivars were higher than those of B-type starch granules. There were obvious difference in the starch granule distribution among different genetype wheat cultivars. Compared with strong-gluten wheat cultivars, waxy wheat cultivars' endosperms had much number smaller diameter starch granules, on the other side, the ratio of bigger starch granule in weak-gluten wheat cultivars' endosperm was higher.Size distribution of starch granules in wheat endosperm were influenced markedly by low light. Reduced light density continually after wheat flowering influenced the growing and development of wheat grain starch granules significantly. Time postponed or number cut down of the forming fastigiums of wheat starch granules after continual shading. In mature wheat grains, the ratio of A-type starch granule increased markedly, the ratio of B-type starch granule decreased obviously, and the influencing range of the two type starch granules incresed with the degree of shading.Size distribution of starch granules in wheat endosperm were influenced markedly by different-stage low light after flagging stage and the influence varied with the stage of shading. After shading from flagging to anthsis, the volume ratio of larger diameter starch granules in waxy wheat grain increased in the early stage of grain filling, and decreased during the next time of grain filling. In mature waxy wheat grains, A-type starch granules' number ratio increased, volume ratio decreaesd, and surface area ratio did not change significantly. In mature strong-gluten wheat grains, the ratioes of A-type starch granules increased markedly and those of B-type starch granules decreased obviously after shading from flagging to anthsis. Shading during the early stage of grain filling inhibitted the forming of A-type pyrenoids in wheat grains, so that the ratio of A-type starch granules in mature wheat grains decreased markedly, on the contrary, B-type starch granule ratio increased. Shading during the midterm and latter grain filling stages could increase the ratioes of A-type starch granules and decrease those of B-type starch granules. At the same time, the influencing range of shading from 11-20 DAA was much larger than that of low light from 21-30 DAA.From the results of shading' effects on wheat starch granule distribution, we found that limited photosynthate supplied the existed starch granules firstly for growing and development, nextly afforded to come into being new starch granules during wheat grain filling.2. Low light affected the dynamic changs of starch content and accumulation during wheat grain filling markedlyUnder the low light conditions all through grain filling, wheat starch and its components contents decreased, accumulations reduced and kernel weight decreased. And those changes aggravated with the increasing of shading degree. There were stage differences for the effects of low light on amylose and amylopectin contents under shading conditions from flower to mature. The amylopectin contents decreased greatly in the early grain filling stage, but the amylose contents reduced sharply during the latter grain filling stage. Those changes appeared as the ratio of AM to AP increased in the early stage, and decreased in the latter stage during grain filling.After shading from flagging to flower, the amylsoe (AM) and amylopectin (AP)content and the ratio of AM to AP of strong-gluten wheat cultivars increased in the early stage of grain filling, but the starch content and AM to AP ratio decreased during the latter grain filling stage. Those changes indicated that photosynthate in abundant supply could accelerate the accumulation of starch in wheat grain during early grain filling stage, inhibite the synthesis of starch in the latter grain filling stage and affect the synthesis of AM much larger than AP. Shading from flagging to flower could increase the AP accumulation, decrease the AM accumulation and raise grain weight of strong-gluten wheat. In waxy wheat cultivar, shading from flagging to flower could increase markedly the AP content and accumulation, increase the AM content in the early grain filling stage, decrease AM content in next grain filling stage, and increase AM accumulation and kernel weight. The ratio of AM to AP decreased during grain filling after shading from flagging to flower, which suggested that photosynthate in abundant supply was in favor of being used to AP synthesis.The different stage shadings after flower all inhibitated starch synthesis in wheat grain and degree of the effects varied with shading stages. Shading from 11 to 20 DAA affected the starch synthesis largestly, the effects of shading from 21-30 DAA was the next, but shading from 1-10 DAA influenced the starch synthesis slightly. Shading from 1-10 DAA could decreased the starch content during shading, after got rid of shading the starch content raised in a moment time, but in the next time the starch content of shading treatment was below as compared with the CK. At mature stage, the starch and its components accumulations and grain weight decreased in strong-gluten wheat, but in waxy wheat the AM accumulation decreaed, the AP accumulation changed slightly and grain weight raised a little. Shading from 11 to 20 DAA decreased significantly the content and accumulation of wheat starch and its components, at the same time, decreased the grain weight markedly. The AM and AP contents of strong-gluten wheat cultivars increased firstly and decreased in the next time after shading from 21 to 30 DAA, at mature wheat grains the starch contents and accumulation and grain weight were lower than those in CK. After shading from 21 to 30 DAA, the AP content in waxy wheat decreased, the AM content changed slightly, but the accumulation of AP and AM and grain weight all decreased.The ratio of AM to AP in strong-gluten wheat cultivars increased firstly and decreased subsequently during shading, which indicated that low light inhibitated the synthesis of AP more seriously during the early stage of shading, and with the prolong of shading, low light inhibitated the accumulation of AM much greatly. Finally, the decreasing percent of AM content was larger that of AP in nonwaxy wheat cultivars. The ratio of AM to AP in waxy wheat cultivars decreased firstly and increased subsequently during shading, which indicated that that low light inhibitated the synthesis of AM more seriously during the early stage of shading, which was different from that in nonwaxy wheat cultivars. Those difference might be related to the compositive ratio of AM and AP in waxy and nonwaxy wheat cultivars.The accumulation of AM and AP were significantly different between waxy and nonwaxy wheat cultivars. The AM content of waxy wheat was much lower than that of nonwaxy wheat, but its AP content was significantly higher than that in nonwaxy wheat. The speed of AP accumulate in waxy wheat during the latter grain filling stage was higher than that in nonwaxy wheat, but its AM content began to decrease at 20 DAA, however, the AM content of nonwaxy wheat increased all along during grain filling.3. The enzymatic mechanism of wheat starch biosynthesis and response to low lightDynamic changes of activities of enzymes involved in starch synthesis in waxy wheat grains during grain filling were different from those in nonwaxy wheat grains. The activity of GBSS in nonwaxy wheat cultivar JN17 was significantly higher than that in waxy wheat cultivar NDN50206. The supplying ability of substrates to synthesize starch in the grains of JN17 was stronger than that in NDN50206 grains during the medium grain filling stage, which appeared as the activities of SS, UGPP, and AGPP were higher and the decreasing rang of sucrose was larger in the grains of JN17 than those in NDN50206 grains at that time. On the contrary, the supplying ability of substrates to synthesize starch in the grains of NDN50206 was stronger than that in JN17 grains during the latter grain filling stage. The activities of SSS in JN17 grains was higher than those in the grains of NDN50206 during the early and medium grain filling stages, but during the latter grain filling stage the SSS activies of JN17 were lower than NDN50206's, meanwhile, the AP content in the grains of NDN50206 was always higher than that in JN17 grains, which suggested that the activity of SSS maintained a certain level to be enough to the synthesis of AP in wheat during grain filling. During grain filling, the activity of SBE in JN17 grains was higher than that in the grains of NDN50206, but its DBE activity was lower than the latter's, which indicated that DBE contributed greaterly to the synthesis of AP in waxy wheat grain as compared with nonwaxy wheat cultivars.The sucrose contents of wheat grains at initial grain filling stage decreased obviously after shading throughout grain filling, which suggested that the photosynthate supplying ability of wheat leaves went down under low light conditions. During the medium grain filling stage, the activities of SS, UGPP, and AGPP all decreased under low light conditions, which indicated that the ability of degradation and transform of sucrose in wheat grains weakened. Consequently, the supplying ability of substrates for starch synthesis fell. At the same time, the activities of SSS, GBSS, SBE, and DBE all went down under low light conditions, which resulted in low starch contents. From the results in this study, we could see that, under low light conditions throughout grain filling, wheat could improve the activities of enzymes involved in starch synthesis during the early and latter grain filling stage, thereby increased the utilization efficiency of the limited photosynthate, in this way wheat could maintain a higher starch accumulation. The SBE and GBSS activities in wheat grain decreased greatly after shading, which suggested that the SBE and GBSS were the regulatory sites of light in the biosynthesis of starch in wheat grains.The activities of key enzymes involved in starch synthesis of strong-gluten wheat grains were influenced greatly by different stage shading after wheat flagging stage and the effects varied with the shading stage, enzyme kind, and wheat genotype. After shading from flagging to flower, the supply level of sucrose increased during the early grain filling stage. At the same time the activities of AGPP and UGPP in wheat grains improved, so that the ability of sucrose-transform in wheat grains increased. Meanwhile, the activities of SSS and SBE in wheat grains enhanced, which facilitated the biosynthesis of AP. As a result, the AP contents of the two strong-gluten wheat cultivars increased obviously in the early grain filling stage. At the latter grain filling stage, the activities of AGPP and DBE decreased, which inhibited the biosynthesis of AP in the two wheat cultivars. After shading from 1 to 10 DAA, the sucrose contents in the grains of the two strong-gluten wheat culticars decreased, the ability of translating sucrose to substrate of starch synthesis went down, and the key enzymes involved in the biosynthesis of AP appeared different extent decrease, which resulted in the decrease of AP contents in strong-gluten wheat cultivars. After shading from 11 to 20 DAA and shading from 21 to 30 DAA, the key enzymes involved in the biosynthesis of starch of the two strong-gluten wheat cultivars all decreased, so that the biosynthesis of starch were inhibited after shading. As a result, the starch content and accumulation decreased. The activities of GBSS in strong-gluten wheat cultivars decreased after different-stage shading, which was the important physiological reason for the decrease of AM content under low light conditions.The activities of key enzymes involved in starch synthesis of waxy wheat grains were influenced greatly by different stage shading after wheat flagging. After shading from flagging to flower, the activities of SS in waxy wheat cultivar NDN50206 increased during the early and medium grain filling stage, so that the ability of sucrose degradation improved, as a result, the sucrose content decreased rapidly. At the same time, the activities of UGPP and AGPP increased, so that the supplying ability of substrate for starch biosynthesis improved. Before 20 DAA, the activities of SSS and SBE in waxy wheat grains enhanced and the activities of DBE did not change significantly. In this way, shading from flagging to flower could improve the ability of AP synthesis in waxy wheat grains during the early and medium grain filling stage. During the latter grain filling stage, the activities of enzymes involved in waxy wheat starch biosynthesis of shading treatment went down much quickly than CK. In mature waxy wheat grains, the AP content increased after shading from flagging to flower. After shading from 1 to 10 DAA, the activities of SS and UGPP in waxy wheat grains increased, AGPP activity decreased slightly, SSS activity enhanced, DBE activity improved before 15 DAA, and the activity of SBE went down. As a result, the AP content of this shading treatment in waxy wheat grains decreased. After shading from 11 to 20 DAA, the activities of SS and AGPP in waxy wheat grains increased, UGPP activity did not change obviously, sucrose content decreased, SSS activity enhanced, but the activities of SBE and DBE decreased markedly. As a result, the AP content of this shading treatment in waxy wheat grains decreased significantly. After shading from 21 to 30 DAA, the sucrose content and activities of SS, UGPP, AGPP, and SSS all decreased markedly, DBE activity increased firstly and then decreased during shading, and SBE activity increased. As a result, the AP content of this shading treatment in waxy wheat grains also decreased. The activities of GBSS in waxy wheat grains were influenced greatly by different stage shading after flagging and its changing trend was similar to the changes of AM. The correlative analysis results indicitated that AM content was in significantly positive correlation with the acticity of GBSS in waxy wheat grain.4. Relationships of wheat starch composition and starch physiochemical characteristics or dough rheological propertiesThere were greater difference among different genotype wheat cultivars in swelling, pasting, thermic, and crystal characteristics. The above characteristics of wheat starch could be influenced by low light and the effects varied with the shading stage, shading rage, wheat genotype and kernel position. The correlative analysis results indicitated that there were close correlations between wheat starch composition, which included AM or AP content and granule size distribution, and starch physicochemical characteristics. The swelling power of wheat starch was negatively correlated with AM content (AMC) to a certain extent and significant positively correlated with the B-type starch granule volume ratio (BV). AMC was significant negatively correlated with peak viscosity (PV), through viscosity (TV), breakdown (BD), peak time (PT), or pasting temperature (PT). On the contrary, AMC was significant positively correlated with final viscosity (FV), setback (ST), or setback ratio (SR). BV was significant positively correlated with PV, TV, BD, peak time(PT), or PT, and significant negatively correlated with FV, ST, or SR. T0, TP, or TC was significant negatively correlated with AMC, and positively correlated with BV. There was not a significant correlation between starch composition and⊿H or craystal raio of wheat starch.The dough rheological properties of wheat were different among wheat cultivars and they could be affected by low light. From the results in our study, we found that there were a certain correation between wheat starch compositon and wheat dough rheological properties. Wheat starch composition were significant correlated with water absorption, but they were not significant correlated with other farinograph parameters. The water absorption parameter was significant positively correlated with BV, but negatively correlated with AM. At the same time, we found that AM was significant positively correlated with resistance or ratio of the extensograph parameters. BV was negatively correlated with the extensograph parameters, but they were not significant. |
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