| Wheat is an important food crop worldwide. Genetic improvements have contributed much to wheat production since the1960s. Verifying the evolution of agronomic traits and the physiological basis of grain yield will facilitate breeders and agronomists in developing new wheat cultivars, with the aim of stable and high yields. Thirty-three wheat cultivars, bred or widely planted in the Yangtze River Basin from1950to2005, were grown in field experiments under three N rates (0,112.5, and225kg N ha-1) from2007to2011in Nanjing, China. And grain yield, main agronomic traits and physiological basis of nitrogen uptake during the wheat cultivars genetic improvement process were mensurated. Thereafter five represent culitvars were selected to further explore the relationship among root morphology and physiology, plant aging properties, properties of source and sink and nitrogen uptake use relation and the response of nitrogen fertilizer by field and pot experiment. The main results were summarized as follows:1Characteristics of genentic improvements on grain yield and agronomic traits of winter wheat and their physiological basisWheat grain yield increased gradually with the genetic improvement process, the differences of the evolution characteristics on grain yield under different nitrogen rates were significant. Grain yield also increased with increased N fertilization rate; the annual increment was higher at225N (53.8kg ha-1year-1) than at112.5N (43.7kg ha-1year-1) and ON (32.4kg ha-1year-1), indicating that modern improved cultivars were more sensitive to increased N fertilizer application. Kernel number per spike and TKW increased linearly with genetic development of cultivars. Leaf area, LAI, and Pn increased significantly with increasing nitrogen fertilizer. Grain yield, spikes per unit land, and kernels per spike increased significantly with increasing nitrogen fertilizer, but TKW and HI decreased, indicating that nitrogen plays an important role to improve the grain yield and optimize the source-sink relationships of wheat. The grain yields increases were mainly related to the aspects as the follows:(1) The increase of spike yield and harvest index. Spike yield and HI increased linearly with cultivar development from1950s to2000s. Biomass decreased significantly from1950s to1960s but did not changed significantly in cultivars developed since the1960s. This indicates that the improved grain yield of the wheat cultivars was mainly related to improve spike yield and HI, not biomass. The contribution of HI was higher than kernels and TKW to grain yield. The contribution of TKW to grain yield was higher than kernels under high nitrogen rates (N225), but lower at N112.5and NO. In the whole, the kernels and TKW almost had the same contribution to grain yield. The grain number increase was mainly related to the increase of spikelet fertility, but not spikelet number, and the grain weight increase was mainly related to dry matter accumulation and grain filling rate.(2) The improvement of plant type. Cultivar height decreased with cultivar development from the1950s to the1980s, and remained relatively stable in subsequent cultivars. The proportion of the length of the top internode to total plant height increased with cultivar development from the1950s to the1980s and thereafter fell, while the length of the basal internode (BI) maintained a shortening trend. The proportion of the length of the spike to whole plant length and the ratio of neck-panicle node (NPN) length to top internode (T1) length decreased with cultivar development, indicating that the relative height of the spike and flag leaf increased, improving the position and light interception of the spike and flag leaf.(3) The improvement of photo synthetic function. Leaf area per culm, leaf area index (LAI) chlorophyll content, Pn and photosynthetic activity duration (PAD) increased with cultivar development, and the ΦPSII and Fv/Fm declining rate of modern cultivars were slower than earlier cultivars, the results showed that cultivars improvement increased the light energy use ability at after-anthesis and defense photoinhibition ability, delaying the aging of photosynthetic system, is the physiological basis of wheat source enhanced, which was the physiological basis of wheat source imcrease.(4) The improvement of plant antioxidant capacity. SOD, CAT and POD activity of flag leaf were improved significantly with the cultivars genetic progress, and the MDA content decreased significantly, which further showed that cultivars improvement improved the clearance ability of O2-, in order to maintain and extend the LAD and PAD, to establish the physiological basis for high yield.(5) The improvement of the grain filling rate. The grain filling rate of modern wheat cultivars increased significantly, the maximum grain filling period in advance of great significance to improve grain yield and to resist the post-disaster. Which were higher significantly than the early cultivars of the grain filling stage of library source than modern cultivars, nitrogen application significantly reduces the source-sink ratio, indicating that the improved cultivars to reduce the grain filling restrictive, so that a more balanced source-sink relationship.(6) The increase of dry matter accumulation and translocation. Genetic improvement increased the dry matter accumulation and growth rate in the growth stage of emergence to jointing and anthesis to maturity whereas reduced contribution of post-anthesis accumulated dry matter to grains (CPA). Grain yield was significantly positive associated with contribution of pre-anthesis translocation to grains (CPT), dry matter accumulation and growth rate from emergence to jointing and anthesis to maturity, whereas significantly negative with CPA, dry matter accumulation and growth rate from jointing to anthesis. Dry matter production capacity and production efficiency were improved during wheat cultivar improvement, yet dry matter accumulation at different growth stage were coordinated and the contribution of pre-anthesis and post-anthesis accumulation to grains became more balance.In conclusion, breeding for high yield should be related to improvement in kernels per spike and TKW per unit land and increased sink-source ratios with a feasible LAI, and N fertilizer management should be considered during breeding for higher yields. Improving the pre-jointing vegetative growth post-anthesis accumulation and CPT are important physiological basis for wheat grain yield enhancement, which will be a key improvement target for high yield wheat breeding in the future.2Characteristics of genentic improvements on nitrogen uptke and utilization of winter wheat and their physiological basisThe nitrogen uptake and utilization efficiency increased gradually during the cultivars genetic imprvement, and the increase of nitrogen utilization efficiency were mainly due to the improvement of the morphological and physiological process as the following:(1) The improvement of N accumulation and translocation. Soluble protein content, nitrate reductase (NR) and glutamine synthase (GS) activity of wheat flag leaf increased significantly with cultivars genetic progress, indicating that cultivars genetic improvement improved the assimilation nitrogen and ammonium nitrate ability of the plant. The pre-anthesis nitrogen accumulation, nitrogen translocation of each organs increased significantly, nitrogen residual of plant vegetative organs in maturation decrease and grain nitrogen accumulation were improved, which showed that the improvement of nitrogen efficiency were closely related to pre-anthesis nitrogen accumulation, distribution and translocation enhancement. The nitrogen accumulation of different wheat cultivars were jointing to anthesis stage in peak, along with aging propulsion, wheat flowering stage to jointing stage accumulation rate significantly increased. Modern cultivars leaf nitrogen distribution and translocation rate increased significantly. Therefore, increasing the pre-anthesis nitrogen accumulation and improving nitrogen distribution in leaves are the important characteristics of nitrogen efficiency improved, also the important breeding nitrogen efficiency index.(2) The improvement of root type and root physiology. The root dry matter accumulation, growth rate and after-anthesis dry matter translocation from jointing to flowering stage were improved during the cultivars genetic improvement. And the total root length, root surface area and root volume have gradually improvement with cultivar genetic progress, wheat cultivar improvement enhanced distribution proportion about the root weight of0to60cm soil and root density in0-20cm of the soil. Along with the incubation of cultivars genetic progress, wheat root soluble protein content, SOD activity improved significantly and significantly reduce the MDA content, root activity was improved significantly explaining that genetics improvement delayed the root of aging, to improve yield and nitrogen uptake efficiency. Nitrogen efficiency was significantly and positively associated with total root length, root surface area, root volume, NR, GS and SOD activity, was significantly and negatively associated with root MDA content at flowering time, indicating that increasing total root length, and root surface root volume, improving NR, GS and SOD activity, reducing the MDA content played an important role on the nitrogen efficiency.(3) The improvement of photosynthetic area and photosynthetic ability. The leaf area duration per unit nitrogen (LADN) and photosynthetic activity duration per unit nitrogen (PADN) increased linearly with cultivars genetic progress, and there were different expression in different nitrogen treatment under the different leaves, showed that the enhanced source that could pull plant nitrogen-uptake, meanwhile that nitrogen fertilizer plays an important regulated role of the source-sink balance and nitrogen uptake. The maximum effective leaf area was significantly and positively associated with nitrogen grain yield efficiency (NYE), nitrogen uptake efficiency (NUpE), nitrogen physiological efficiency (NPE), the biggest photosynthetic rate was significantly and positively associated with NRE and NPE, PAD was significantly and positively associated with NYE and NPE, yield per-spike was significantly and positively associated with NYE, NAE, NUpE and NPE. LADN and PADN was significantly and positively associated with NFY and NH, while was significantly and negatively associated with NUpE. Indicating that improve LADN and PADN could improve nitrogen translocation from vegetative organs to grain, also explaining LADN and PADN could be used as nitrogen translocation and nitrogen efficiency evaluation index.In conclusion, cultivars improvement increased NR and GS activity of flag leaf at pre-antheshis to improve nitrogen assimilation ability of the plant, thereby increasd the amount of nitrogen accumulation and nitrogen accumulation rato of jointing to anthesis stage; increased the photosynthetic area, photosynthetic capacity, root, total root length, root absorption area, root activity and root nitrogen assimilation improved the nitrogen uptake capacity; increasing the distribution of nitrogen in the leaves, the nitrogen translocation of pre-anthesis. Therefore, the improvement of the NR GS activity and leaf photosynthetic capacity, root absorption area and root activity, to increase the distribution of nitrogen in the leaves are the important physiological basis for wheat N efficiency enhancement, which will be a key improvement target for high N efficiency wheat breeding in the future. |
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