Effect of N and CO₂ supply on source size per grain at anthesis and its relationship with grain growth in wheat

Rising atmospheric CO2 concentration ([eCO2]) increases the yield of wheat mainly by increasing grain number, but effects on single grain weight are variable. It is discussed whether single grain growth is limited by the sink or the source size under a non-stress environment. This study explores the effect of e[CO2] combined with varying N supply on the source and sink size during grain filling. Source size was defined as the amount of stem reserves per grain (SRG) and the proportion of incident radiation intercepted by the green canopy per grain (fIRG) at anthesis. Data from a 2-year free-air CO2 enrichment experiment with wheat with three N levels (on average 38 [Nd], 190 [Nad] and 320 kg N ha-1 [Nex]) and two CO2 levels (393 and 600 ppm) on SRG, fIRG and grain filling rate (GFR) and duration (GFD) were evaluated. SRG ranged from 2.5 to 12.9 mg and fIRG from 4.0 × 0.001% to 6.8 x 0.001%. Rising N supply or e[CO2] decreased SRG and fIRG via their increases in grain number. Accordingly, there was a negative linear relationship between grain number and SRG (r2 = 0.84) or fIRG (r2 = 0.97). Increasing N supply decreased GFR, but increased GFD, and GFR was increased by e[CO2] under Nad and Nex. For GFR and final grain weight, there was a strong positive (r2 = 0.85), and for GFD, a strong negative linear relationship (r2 = 0.76) with fIRG under Nad and Nex. Under these N levels, fIRG supplied the largest share (>86%) for grain growth and thus single grain growth was possibly source limited under Nad and Nex. Under high grain number such as under Nex and e[CO2], there might be a risk for low final grain weight due to the low SRG that is insufficient for buffering assimilate shortage under unfavourable environmental conditions in early grain filling.