Studies On Physiological Mechanism For High Efficient Nitrogen Use In Wheat

The grain yield and quality in wheat are determined by the physiological activities inorgans as roots, stems, leaves and grains, in relation to nitrogen absorbed from the fieldduring the growth and development in wheat. It has been an important task in wheatresearch and production how to produce more grain starch or grain protein with limitednitrogen supply. Therefore, clarifying the mechanism of high nitrogen uptake andutilization in diverse genotypes of wheat has become a key issue to be solved urgently. Inthis paper, according to the order of nitrogen flow from the vegetable organs to the grainswithin the plant, the field and pot experiments were carried out using two types of wheatcultivars with different grain protein contents. The objectives of this study were todetermine the forms and approaches for post-anthesis nitrogen flow from roots to grains,nitrogen assimilation (NA) and nitrogen translocation (NT) in relation to carbohydrateassimilation (CA) and translocation (CT) in wheat plants of two cultivars under defferentnitrogen rates. This would help to elucidate the physiological basis for genetic difference innitrogen nutrition efficiency in wheat. The main results were as follows:1. Analyses on grain yield and nitrogen utilization under different nitrogen rates in twowheat genotypes showed that the yield per plant and yield per hectare were significantlydifferent under varied nitrogen (N) rates. Two cultivars, Yumai 47 with high grain proteinand Yumai 50 with low grain protein, presented distinct difference in yield per plant but nosignificant difference in yield per hetare under lower nitrogen level. On the contrary, bothyield per plant and yield per hetare of Yumai 47 were obviously higher than Yumai 50under higher nitrogen level. The nitrogen translancation of Yumai47 plant aerial part wasobviously higher than that of Yumai 50 after anthesis, but the nitrogen assimilation was nosignificant difference between the cultivars. The biomass transportation of Yumai47 plantabove ground was higher than that of Yumai50. But the biomass assimilation of Yumai47plant above ground was lower than that of Yumai50. The nitrogen use efficiency (NUE)expressed by the ratio of the grain yield or nitrogen in grain to the nitrogen in above-ground plant were significantly different not only between two genotypes, but also among defferentnitrogen rates. NUE in Yumai 47 was higher than that in Yumai 50, implying that ratio ofthe biomass transportation to the biomass assimilation above ground after anthesis ofYumai47 and the capacity in the grains of Yumai 47 for absorbing nitrogen from vegetableorgans were significantly stronger than that of Yumai 50.2. The field experiments were conducted to investigate the dynamic changes of threenitrogen forms as assimilable nitrogen (AN), functional nitrogen (FN) and structuralnitrogen (SN) in wheat plants in responses to varied nitrogen rates, using two wheatcultivars, Yumai 47 and Yumai 50. The results showed that AN, FN and SN were affectedmore significantly by wheat cultivars than by nitrogen rates, especially after anthesis.Assimilable nitrogen content in stem and leaves of low grain protein genotype Yumai 50declined continuouslly after anthesis. In contrast, the high grain protein genotype Yumai 47showed the opposite trend. In grains, AN content in Yumai 50 decreased slowly from1.98～2.35mg.g^-1 to 1.38～1.70mg.g^-1 after flowering. However, in Yumai 47 it declinedsharply from 5.51～5.70 mg.g-1 to 1.15～1.38mg.g^-1 17 days after anthesis, and thenincreased from 1.15～1.38mg.g^-1 to 3.01～3.29mg.g^-1, respectively, for different nitrogenrates at maturity. These results indicated that continuous increase of AN content in the stemand leaves from jointing to filling was closely related to demand of grain protein formationin high protein genotype Yumai 47. No difference was observed in terms of FN contentsbetween two wheat cultivars in relation to different development stages. This implied thatFN content, which participated in absorption and assimilation within leaves, was notsignificantly different in two different genotypes. In addition to a sharper decrease of SNcontent in high grain protein genotype Yumai 47 than low grain protein genotype Yumai 50after anthesis, SN content in stem and leaves showed a similar trend over developmentprogress. They all increased with a peak during flowering period, and then exhibited adecreasing trend. In the grains, dynamics of SN contents in two cultivars showed a similartrend, decreasing from anthesis to maturity, although Yumai 47 had a much higher SNcontent at anthesis than that of Yumai 50. The results indicated that nitrogen decompositionfrom SN in leaves and stems was the key source for nitrogen translocation into grain, andhigher SN content at anthesis was favorable for protein formation in grains of Yumai 47with high protein content.3. Pot experiment was conducted using Glufosinate, glutamine synthetase inhibitor, to investigate the relationship between nitrogen and carbon assimilation during grain filling oftwo wheat genotypes under different nitrogen rates. The results showed that the cultivarYumai 47 with high grain protein could directly absorb nitrogen from roots during grainfilling, while cultivar Yumai 50 with low grain protein had to absorb nitrogen by leaves.The relevance between nitrogen assimilation and starch synthesis (C assimilation process)was more significant in the grains of Yumai 50 than in Yumai 47. Similarly, nitrogenassimilation and photosynthesis in the leaves were closely correlated in Yumai 50, but theywere relatively independent in Yumai 47. Therefore, with Yumai 50, the nitrogen fertilizershould be applied earlier, but with Yumai 47, it should be applied somewhat later.4. Pot experiment was conducted using GS inhibitor (Glufosinate, GLF) and ¹⁵N methodto investigate differences in inorganic nitrogen assimilation and NDFF (percentage of organnitrogen from soil nitrogen) between the organs of two wheat cultivars under differentnitrogen rates. The results showed that after leafs GS was restrained by GLF, NDFF inspikes of Yumai 47 was higher than that of control, while it was an opposite pattern inYumai 50. After spike's GS was restrained by GLF, NDFF in leaves of Yumai 47 increased,whereas that of Yumai 50 increased at low nitrogen rates and decreased at high nitrogenrates. The amount of inorganic nitrogen assimilated in Yumai 47 plant was much greaterthan in Yumai 50 after anthesis. The main organs assimilating inorganic nitrogen wereroots and stems for Yumai 47, but were leaves for Yumai 50. The ratio of the inorganicnitrogen assimilation by roots and stems, leaves, and spikes was about 4:1:2 for Yumai 47,and about 1:5:1 for Yumai 50. With increased nitrogen supply, the amount of inorganicnitrogen assimilated in the leaves of Yumai 47 increased gradually, while decreased in theleaves of Yumai 50. And the output of inorganic nitrogen assimilated by the leaves wassignificantly higher than the amount of inorganic nitrogen assimilated by grain inYumai47.In contrast, it was showed the opposite trend in Yumai 50. The results suggested that therewere different pathways for the nitrogen assimilation from roots to grains after anthesis:that in high grain protein genotype Yumai 47 does not need to go through leaves, but that inlow grain protein genotype Yumai 50 has to go through leaves.5. Further studies were undertaken on the ralationships between assimilation andtranslocation of carbon (C) and nitrogen (N) after anthesis in two wheat cultivars underdifferent nitrogen rates. The results indicated that both amount of nitrogen translocation andthat of carbohydrate translocation were higher in Yumai 47 with high grain protein content than in Yumai 50 with low grain protein content, whereas amount of carbon assimilationwas higher in Yumai 50, and there was no significant difference in amount of nitrogenassimilation between Yumai 47 and Yumai 50. The ratio of nitrogen assimilation amount into nitrogen translocation amount was ralatively low, accounting for 10ï¼…to 40ï¼…, whereasthe amount of C assimilation accounted for more in amount of carbohydrate translocation,with 0.5-2.5 times higher than the amount of carbohydrate translocation. With the increasednitrogen rates, the amount of nitrogen assimilation in two genotypes decreased, the amountof nitrogen translocation increased in Yumai 47, but was stable in Yumai 50. However, theamount of carbohydrate translocation showed the trend of first declining and thenincreasing, while apparent declining took place in Yumai 50. The amount of carbonassimilation was higher in Yumai 50 than that in Yumai 47, and the amount of carbonassimilation would improve with the increased nitrogen rates. These results on ratiobetween translocation and assimilation of nitrogen and carbon indicated that nitrogen rateswould have great effects on the proportion of the filling compounds from different originsafter anthesis in wheat, thus influecing the mode of assimilation and translocation of carbonand nitrogen. This would provide a potential capacity and technical approach for artificialregulation of the filling mode in wheat grains.