| The contribution rate of genetic improvement to wheat yield was about 30～50%. In recent years, the yield potential of wheat grain in the world is growing at an average annual rate of 0.6%. Due to the differences in climate and cultivation conditions, the yield potential of wheat varieties developed in various years varied in different growing regions. Yangmai wheat varieties have produced a great impact on the yield improvement of wheat in the middle and lower reaches of Yangtze River since they were released. However, no study has been conducted to systematically investigate the improvement progress of genetic potential of Yangmai varieties. The present study was conducted in the Key Laboratory of Crop Genetics and Physiology, of Jiangsu Province, Yangzhou University during the two wheat growing seasons from 2013 to 2015. The study was arranged in a two-factor split plot experiment design. The two experimental factors were nitrogen level and variety. The nitrogen level, consisting of two levels (0 and 225 kg·hm-2), was used as the main plot factor. The variety was used as the split plot factor. Ten varieties released from the 1970s to the 2010s were used. The objectives of this study was to investigate the evolutional characteristics of yield components, plant morphology, biomass accumulation and translocation capacity, photosynthetic capacity in flag leaves, nitrogen assimilation and translocation, and their relations to grain yield. The main results were as follows:1. The grain yield of Yangmai wheat varieties increased with the progress of variety release. The grain yield of Yangmai wheat varieties released in the 1970s ranged between 5000 and 6000 kg-hm’2. Then the grain yield of the varieties released in the 1980s and 1990s increased to 7000 and 7500 kg·hm-2, respectively. The increase in grain yield was mainly attribute to the increase in both spike number per unit area and 1000-kernel-weight. The grain yield of Yangmai wheat varieties released in the 2000s increased to 7500～8000 kg·hm-2. The increased grain weight mainly resulted from the increase in kernel number per spike and 1000-kernel-weight.2. Grain yield increased as the progress of variety development. However, higher increment was determined under non-nitrogen application conditions. This suggested that newly-released Yangmai varieties were more resistant to low nitrogen level as compared with early-released Yangmai cultivars. Nitrogen application increased the grain yield of all the tested Yangmai wheat varieties released in different years, with a descending order of 1970s> 1980s> 1990s> 2000s. The increment of spike number per unit area became bigger with increased nitrogen level. The increment of kernel number per spike and 1000-kernel-weight became less and less with increased nitrogen amount.3. Compared with Yangmai varieties bred before the 1990s, newly-released Yangmai varieties had higher translocation rate of nitrogen and higher capability of transporting nitrogen from vegetative organs to the grains, but lower post-anthesis nitrogen assimilation capacity. Nitrogen use efficiency for grain yield (NYE) and nitrogen harvest index (HI) showed an increasing tendency with the progress of breeding. Nitrogen physiological efficiency (NPE), nitrogen agronomic efficiency (NAE) and average utilization rate of N fertilizer (NUR) increased for the varieties released from the 1970s to 2005 and decreased from 2005.4. The number of tiller increased for Yangmai varieties released between the 1970s and the 1990s and decreased for the varieties released since 2000. Ear bearing rate increased with the release of Yangmai varieties. The maximum LAI decreased for the varieties that were released before the 1990s and then increased for the varieties released after the 1990s. The decay of LAI at milky stage was slowed with the progress of variety release. The maximum LAI and maximum tiller number of Yangmai varieties released after the 2000s were appropriate and the grain/leaf ratio was maintained at a suitable level, which are in conformity with the indices of high yield population. 5. The plant height of Yangmai varieties evolved from tall to semi-dwarf as the progress of variety release. The plant height of varieties decreased from 100 cm for the varieties released in the 1970s and the 1980s to 80-90 cm for the varieties released after the 1990s. The decrease in plant height was mainly attribute to the shortening of internodes. The leaf type of Yangmai varieties evolved from loose type to semi-compact type and compact type. Before the 1990s, flag leaves were long and had a higher ratio of length and width. After the 1990s, flag leaves were shorter and the length-width ratio decreased. The angles between the stem and the three leaves at the top of wheat plants became smaller, making the plants more compact and leading to higher field transmittance.6. Compared with the varieties released in the 20th century, the peak SPAD readings were higher and declined more stably for newly-released Yangmai varieties released in the 21th century. The net photosynthetic rate of flag leaf during flowering increased with the releasing time, but the difference for the varieties released after the 1990s was small. For newly-developed Yangmai varieties, the net photosynthetic rate, stomatal conductance and transpiration rate declined slowly after flowering. Meanwhile, the concentration of intercellular CO2 and the activity of RuBPase and PEPC of flag leaves were maintained at high levels. For newly-developed Yangmai varieties, the content of soluble protein decreased more slowly, the activity of SOD, POD and CAT was higher after flowering as compared with early released Yangmai varieties. This suggested that newly released varieties were more effective in scavenging superoxide anion radical, slowing the declining speed of leaf senescence, maintaining a longer duration of high photosynthetic rate. All these laid a sold physiological base for increasing the dry matter accumulation after flowering.7. The biomass of Yangmai varieties at mature stage showed an increasing tendency with the time of variety release. There was a significant positive correlation between biomass accumulation and grain yield at ripening stage. Genetic improvement increased the contribution of dry matter accumulation after anthesis to grain yield. In early released Yangmai varieties, the contribution rate of dry matter was 60～70% as compared with 65～75% in newly-developed Yangmai varieties. The capability of transporting photosynthate from stem and sheath to grain was enhanced with the release of variety. In contrast, the capability of transporting photosynthate from leaf to grain yield was weakened. Newly-developed Yangmai varieties reduced the translocation of photosynthate from leaf to grain so that more photosynthate was maintained in leaf, which increased leaf photosynthetic capacity and biomass accumulation after flowering.8. Path analysis showed that the length and area of flag leaf at anthesis were the main factors affecting grain yield, followed by plant height. This indicated that the improvement of plant type was the key factor of increasing grain yield in the progress of Yangmai wheat varieties. Besides the plant type, the net photosynthetic rate of flag leaf at anthesis, LAI at milky stag, soluble protein content at anthesis, and dry matter amount at mature stage were also the important factors affecting grain yield. |
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