The source–sink ratio during grain filling is a critical factor that affects crop yield in wheat, and the main objective of this study was to determine the source–sink relations at both the canopy scale and the individual culm level under two nitrogen (N) levels at the post-jointing stage. Nine widely-used cultivars were chosen for analyzing source–sink relations in southwestern China; and three typical cultivars of different plant types were subjected to artificial manipulation of the grain-filling source–sink ratio to supplement crop growth measurements. A field experiment was conducted over two consecutive seasons under two N rates (N+, 150 kg ha−1; N–, 60 kg ha−1), and three manipulations were imposed after anthesis: control (Ct), removal of flag and penultimate leaves (Lr) and removal of spikelets on one side of each spike (Sr). The results showed that the single grain weights in the three cultivars were significantly decreased by Lr and increased by Sr, which demonstrated that wheat grain yield potential seems more source-limited than sink-limited during grain filling, but the source–sink balance was obviously changed by climatic variations and N deficient environments. Grain yield was highly associated with sink capacity (SICA), grain number, biomass, SPAD values, and leaf area index during grain filling, indicating a higher degree of source limitation with an increase in sink capacity. Therefore, source limitation should be taken into account by breeders when SICA is increased, especially under non-limiting conditions. Chuanmai 104, a half-compact type with a mid-sized spike and a long narrow upper leaf, showed relatively better performance in source–sink relations. Since this cultivar showed the characteristics of a lower reduction in grain weight after Lr, a larger increase after Sr, and a lower reduction in post-anthesis dry matter accumulation, then the greater current photosynthesis during grain filling contributed to the grain after source and sink manipulation.
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