| The studies, selecting wheat cultivar Jimai20 with high yield and strong gluten potential as experiment material, using enzymological method, 15N isotope tracing technique, physiological and biochemical analysis method, on the basis of different nitrogen fertilizer level, in combing with techniques of shading and changing the ratio of source to sink, elucidated the rule of soil nitrogen changing under high-yield field, the physiological and ecological mechanism of carbon and nitrogen metabolism and the formation of grain yield and quality of wheat. The principal results were as follows. 1.Effect of nitrogen fertilizer rate on changes in soil nitrogen and on carbon and nitrogen metabolism, yield and quality of wheat 1.1 Nitrate leaching and nitrogen loss in soil under high yield condition During wheat growing, soil nitrate content of 0200cm soil layers trended to increase with nitrogen fertilizer rate increased, but differences occurred among different growth stages and different soil layers. During the stage from sowing to before winter, nitrate of treatments with nitrogen addition accumulated in soil before sowing moved down to deeper soil layer and accumulated under 140cm soil layers. As for treatments with less than 168 kgN·hm-2 added, no peak of nitrate concentratio was found in 0100cm soil layers; for treatments with 168240 kgN·hm-2 added, the peak was found in 40100cm soil layers; for treatment with 276 kgN·hm-2 added, the peak occurred in 100120cm soil layers. It showed that supply with excessive nitrogen fertilizer not only induced nitrogen newly added to move down, but also induced the accumulated nitrogen to move to deeper layers and accumulated in there. On wheat jointing, soil nitrate content of the control approached stability. During the period of before winter to jointing, soil nitrate in upper layer of treatment with 132 kgN·hm-2 added moved down obviously. For treatment with more than 168 kgN·hm-2 supplied, the content of soil nitrate decreased in 020cm soil layers. and the peak appeared in 2080cm and 160200cm soil layers. On wheat anthesis, the supply of topdressed nitrogen affected the soil nitrate content of 0100cm soil layers more than that of 100200cm soil layers. Excessive nitrogen added induced the leaching of nitrate to deep layer. For treatment with 276 kgN·hm-2added, nitrate accumulated in deep soil layer on jointing moved down deeper than 200cm on anthesis. On maturity, soil nitrate content of the control decreased in deep soil layers, but for treatments at high nitrogen fertilizer level, soil nitrate accumulated in 160200cm soil layers. After basal nitrogen added, the loss of NH3 volatilization from soil was less than 10 percent of basal nitrogen added when the nitrogen fertilizer rate were lower than 168 kgN·hm-2; and the loss was 10.67 to 15.92 percent when the nitrogen fertilizer rate were more than 168 kgN·hm-2. 1.2 Effect of nitrogen fertilizer rate on wheat nitrogen utilization under high yield field condition The results of 15N isotope tracing experiments showed that, the basal nitrogen that wheat plant absorbed before jointing were 78.74 to 94.41 percent of that wheat absorbed during all the growth periods; the topdressed nitrogen wheat plant absorbed during jointing to anthesis were 78.64 to 94.55 percent of that wheat absorbed during all the growth periods. The recovery rate in wheat of topdressed nitrogen fertilizer was higher than that of basal nitrogen fertilizer. With the amount of nitrogen applied increased, nitrogen uptake proportion of basal and topdressed fertilizer during early stage of wheat growth decreased, nitrogen uptake proportion during later stage increased. With the amount of nitrogen applied increased, nitrogen recovery rate decreased. 1.3 Effect of nitrogen fertilizer rate on wheat nitrogen and carbon metabolism, and on grain yield and quality 1.3.1 Effect of nitrogen fertilizer rate on wheat photosynthesis characteristics With nitrogen supply increasing from 0 to 240 kgN·hm-2, stomatal conductance of flag leaf increased, intercellular concentratio CO2 decreased, and photosynthetic rate elevated significantly. Compared with the treatment supplied with 240 kgN·hm-2, when supplied with276 kgN·hm-2, intercellular concentratio CO2 increased dramatically, stomatal conductance, stomatal limitations and photosynthetic rate decreased dramatically. It showed that excessive nitrogen fertilizer resulted in decreasing of photosynthesis ability in leaf cells. With no nitrogen added, Fv / Fm and ΦPSⅡof flag leaf appeared the lowest at noon, and during 13:0014:00, photosynthetic rate decreased obviously, showing photoinhibition. With nitrogen supply increasing from 0 to 240 kgN·hm-2, Fv / Fm and ΦPSⅡincreased; but with excessive nitrogen of 276 kgN·hm-2, the extent of the photoinhibition of flag leaf and the heat dissipation increased, photosynthetic rate of flag leaf decreased. 1.3.2 Effect of nitrogen fertilizer rate on non-structural carbohydrate (NSC) metabolism in wheat stem and sheath and on starch synthesis in grain Increasing nitrogen supply from 0 to 240 kgN·hm-2, decreased the activities of fructan exo-hydrolase (FEH) in stem and sheath during prophase and metaphase of grain filling, promoted the synthesis and accumulation of fructan of DP≥4 and DP3 in stem and sheath during prophase and metaphase of grain filling. But with more nitrogen applied, NSC retained in stem and sheath increased, and it was the physiological reason that high nitrogen supply resulted in the decreasing of harvest index. With excessive nitrogen added, photosynthetic ability and sucrose synthesis ability of flag leaf decreased, the supply of sucrose reduced, the content of NSC in stem and sheath reduced too. Increasing the amount of nitrogen added decreased the activities of SS, SSS, GBSS in grains during prophase and metaphase of grain filling, and was disadvantage for starch synthesis. With the increasing of nitrogen supplied, starch content of grain decreased, so did the grain weight. 1.3.3 Effect of nitrogen fertilizer rate on nitrogen assimilation and protein degradation in wheat flag leaf Increasing the amount of nitrogen supply from 0 to 240 kgN·hm-2 elevated the activities of nitrate reductase (NR) and glutamine synthetase(GS) in flag leaf, promoted the nitrogen assimilation of flag leaf after anthesis. Applied with excessive nitrogen, activities of NR and GS reduced, nitrogen assimilateion after anthesis decreased, and the activities of proteinase remained low during grain filling, nitrogen remobilization was inhibited. It was the physiological basis that protein content of grain would not increase more with excessivenitrogen fertilizer applied. 1.3.4 Effect of nitrogen fertilizer rate on wheat grain yield and quality With the increasing of nitrogen supply, spike number per hectare and grain number per spike increased, but grain weight decreased; applied with more than 240 kgN·hm-2, spike number per hectare and grain number per spike decreased too. The highest yield acquired when applied nitrogen at the rate of 240 kgN·hm-2, and grain yield decreased when added with more nitrogen. With nitrogen supply increase from 0 to 240 kgN·hm-2, monomeric protein content, soluble glutenin content, insoluble glutenin content in grain increased significantly, the ratio of glutenin to monomeric protein increased, wet gluten content and sedimentation volume elevated, dough development time and dough stability time prolonged, and the quality of wheat improved. Excessive nitrogen supply increased the content of monomeric protein, decreased the ratio of glutenin to monomeric protein dramatically, and the quality became inferior. 2.Effect of shading during grain filling on carbon and nitrogen metabolism and on wheat yield and quality based on different nitrogen fertilizer rates 2.1 Effect of shading on photosynthesis characteristics and NSC metabolism of wheat During the period of shading, the photosynthesis rate, activities of SPS, and the sucrose content of flag leaf decreased significantly. Appropriate use of nitrogen improved photosynthesis rate and sucrose synthesis ability in the flag leaf and increased the sucrose content in it. Excessive nitrogen addition reduced the rate of photosynthesis and ability of sucrose synthesis in flag leaf, the appropriate nitrogen addition for high carbon assimilation of the flag leaf under shading condition was lower than that under natural lighted condition. During the stage of grain filling, shading reduced the content of fructan of DP≥4 and DP3 and total soluble sugar content in stem significantly, and increased the activities of FEH. Shading during the prophase of grain filling affected the NSC content in stem mostly. The sucrose content, fructan content, total soluble sugar content and the activities of FEH at high nitrogen addition level were higher than at low nitrogen addition level. And after shading removed, activities of FEH remained high, so promoted fructan degradation and export from stem, and during anaphase of grain filling activities of FEH reduced.2.2 Effect of shading on nitrogen assimilation and protein degradation in flag leaf of wheat. During grain filling, shading decreased the activities of NR and GS in flag leaf significantly, and suitable use of nitrogen offset the effect of shading on nitrogen assimilation, but excessive nitrogen addition enhanced the effect of shading. Shading reduced the activities of endopeptidases in the flag leaf at low nitrogen addition level, after the shading was removed the activities remained high during anaphase of grain filling. Shading during prophase of grain filling at high nitrogen addition level had no significant Effect on activities of endopeptidases in wheat flag leaf. After the shading removed, activities of endopeptidases in flag leaf enhangced significantly, which induced protein degradation and leaf senescence, and resulted in low enzyme activeities during anaphase of grain filling. Shading during metaphase and anaphase of grain filling under high nitrogen addition level promoted protein degradation significantly. 2.3 Effect of shading on grain yield and quality of wheat Shading during grain filling reduced grain weight and grain number per spike, thus resulted in decreased yield. Shading during prophase of grain filling, grain yield would not increase significantly with the nitrogen supply increasing from 0 to168 kgN·hm-2, and would decrease pronouncedly with nitrogen supply from 168 kgN·hm-2 to more. At low nitrogen addition level, shading during grain filling stage, the soluble and insoluble protein content in grain increased, dough development time and dough stability time prolonged, and the grain quality improved. With no or excessive nitrogen added, shading during prophase or metaphase of grain filling promoted the increase of monomeric protein content, decreased the ratio of glutenin to monomeric protein significantly, shortened the time of dough development and dough stability, deteriorated the quality of grain. Shading during anaphase of grain filling was benefit for the improvement of grain quality. 3.Effect of changing the source —sink ratio on the carbon, nitrogen metabolism and on wheat yield and quality based on different nitrogen fertilizer rates 3.1 Effect of changing the source —sink ratio on photosynthesis characteristics and NSC metabolism of wheat With the increasing of the source —sink ratio, photosynthesis rate and activities of SPSof flag leaf decreased, and the decreasing extent at high nitrogen addition level of 240 kgN·hm-2 and 285 kgN·hm-2 was significantly higher than at low level of 0 and 150 kgN·hm-2. With 50% leaf removed, the activities of SPS increased significantly, but photosynthesis rate reduced for stomatal limitation. Appropriate use of nitrogen was profitable for the increasing of photosynthesis rate and SPS activities of flag leaf, but excessive nitrogen was disadvantage. With the increasing of the source —sink ratio, NSC content of stem increased dramatically, activities of FEH decreased. Increasing nitrogen addition inhibited the activities of FEH, and more NSC retained in stem. With the decreasing of the source —sink ratio, NSC content of the stem decreased dramatically, the activities of FEH increased. Appropriate use of nitrogen offset the effect of deficiency of leaf source on the NSC content in stem, but with excessive nitrogen addition, no NSC content increased in the stem. 3.2 Effect of changing the source —sink ratio on nitrogen assimilation and protein degradation in flag leaf of wheat. With the increasing of the source —sink ratio, the activities of NR and GS of flag leaf increased significantly, and the increasing extent at high nitrogen addition level was little than at low level. With the decreasing of the source —sink ratio, the activities of NR and GS of the flag leaf decreased significantly at low nitrogen addition level during the metaphase and anaphase of grain filling. Suitable use of nitrogen increased nitrogen assimilation ability in flag leaf. But for wheat of lower ratio of source to sink, with excessive nitrogen addition nitrogen assimilation ability did not increased. With the increasing of the source —sink ratio, activities of proteinase of flag leaf decreased. Nitrogen addition decreased the activities of proteinase further during prophaes and metaphase of grain filling, and induced more nitrogen retained in flag leaf. With the decreasing of the source —sink ratio, the activities of proteinase increased. Suitable nitrogen addition delayed the senescence of flag leaf in wheat with low source —sink ratio, and no significant difference was found on the activities of proteinase between treatments with nitrogen supply of 240 kgN·hm-2 and 285 kgN·hm-2.
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