| Summary: | In order to provide a theoretical basis and technical approach for the construction and regulation of medium- and high-yield population cultivation practice of wheat after rice, agronomic and physiological characteristics in medium-high yielding populations were investigated by setting different basic seedlings and cutting leaves and ears with isotope tracing method in week-gluten wheat (Ningmai 29). The results showed that the medium-high yield (yield above 7500 kg/km<sup>2</sup>) group could be achieved at medium densities (150 × 10<sup>4</sup>/hm<sup>2</sup> and 225 × 10<sup>4</sup>/hm<sup>2</sup>), whose populations own suitable number of spikes, higher grain number per spike and thousand-grain weight (the larger and stronger ‘sink’). Meanwhile, these two medium-high yielding populations had higher leaf area index and suitable light-transmission rate after anthesis; thus, the leaf net photosynthetic rate after anthesis was higher, and the capacity of carbon assimilates was stronger. From the <sup>15</sup>N test, it can be seen that the relationship between individuals in the medium-high yielding population (medium-density) is more harmonious, and the plant had higher nitrogen utilization efficiency. More nitrogen is concentrated in the spike at maturity. The results of the <sup>13</sup>C pot trials showed that the top-three functional leaves had a higher capacity for source-production, which was also the main source of post-flowering assimilates. Increasing their area to improve the ‘source–sink’ ratio would help coordinate the ‘source–sink’ relationship in the group with a stronger ‘sink’. The main technical approach is to increase the area and duration of the upper-three functional leaves after anthesis on the basis of a larger sink, thus ensuring a higher source–sink ratio and a harmonious ‘source–sink’ relationship after flowering.
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