Effects of genetic improvements on grain yield and agronomic traits of winter wheat in the Yangtze River Basin of China

Wheat is an important food crop worldwide. Genetic improvements have contributed much to wheat production since the 1960s. 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-five wheat cultivars, bred or widely planted in the Yangtze River Basin from 1950 to 2005, were grown in field experiments under three N rates (0, 112.5, and 225kgNha⁻¹) from 2006 to 2009 in Nanjing, China. Wheat grain yield, kernels per spike, 1000-kernel weight (TKW), and harvest index (HI) increased linearly with cultivar development from the 1950s to the 2000s, whereas spikes per unit land decreased significantly with cultivar development during the same period, and stabilized with further genetic improvements in cultivars. Grain yield, kernels per spike, and TKW differed with N rate and with cultivar. Grain yield, spikes per unit land, and kernels per spike increased significantly with increasing N fertilizer, but TKW and HI decreased. Cultivar height decreased with cultivar development from the 1950s to the 1980s, 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 the 1950s to the 1980s and thereafter fell, while the length of the basal internode (BI) maintained a shortening trend. Leaf area per culm, leaf area index (LAI), net photosynthetic rate (Pn), and photosynthetic activity duration (PAD) of the flag leaf increased with cultivar development. Leaf area, LAI, and Pn increased significantly with increasing N fertilizer, while PAD did not. Single spike yield increased linearly with genetic development in cultivars, and these increases mainly resulted from increasing kernel number and weight, which were closely related to source size and cultivar. Grain yield was positively correlated to leaf area, LAI, PnMₐₓ, PAD, and single spike yield; single spike yield was positively correlated to leaf area, LAI, PnMₐₓ, and PAD, suggesting that grain yield improvements were mainly associated with improvements in the source (leaf area, LAI, Pn, PAD, etc.) and sink (single spike yield). Sink–source ratios increased with genetic development of cultivars, suggesting that productivity per leaf improved and that sink–source relationships have reached close to optimum in the Yangtze River Basin. Furthermore, 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.