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 (f_IRG) 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, f_IRG and grain filling rate (GFR) and duration (GFD) were evaluated. SRG ranged from 2.5 to 12.9 mg and f_IRG from 4.0 × 0.001% to 6.8 x 0.001%. Rising N supply or e[CO₂] decreased SRG and f_IRG via their increases in grain number. Accordingly, there was a negative linear relationship between grain number and SRG (r² ≥ 0.84) or f_IRG (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 f_IRG under Nad and Nex. Under these N levels, f_IRG 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.

中文翻译：

---

N和CO2供应对小麦开花期单粒源大小的影响及其与籽粒生长的关系

大气CO ₂浓度([eCO ₂ ]) 升高主要通过增加籽粒数来增加小麦产量，但对单粒重量的影响是可变的。讨论了在非应力环境下​​单晶粒生长是否受库或源尺寸的限制。_{本研究探讨了 e[CO 2} ] 结合不同氮供应对籽粒灌浆过程中源汇尺寸的影响。源大小被定义为每颗谷物的茎储备量 (SRG) 和开花时每颗谷物的绿色树冠截获的入射辐射比例 (f _{IR G)。}来自具有三个 N 水平（平均 38 [Nd]、190 [Nad] 和 320 kg N ha）的小麦的为期2 年的自由空气 CO _{2富集实验的数据}^-1 [Nex]) 和两个 CO ₂水平（393 和 600 ppm）对 SRG、f _IR G 和籽粒灌浆率 (GFR) 和持续时间 (GFD) 进行了评估。SRG 范围为 2.5 至 12.9 mg，f _IR G 范围为 4.0 × 0.001% 至 6.8 × 0.001%。增加 N 供应或 e[CO ₂ ]通过增加晶粒数量降低 SRG 和 f _{IR G。}因此，晶粒数与SRG（ r ²  ≥ 0.84）或f _IR G（r ² ≥ 0.97）呈负线性关系 。_{增加 N 供应会降低 GFR，但会增加 GFD，并且在 Nad 和 Nex 下，e[CO 2} ]会增加 GFR 。对于 GFR 和最终粒重，有很强的正向（r ² ≥ 0.85)，对于 GFD，在 Nad 和 Nex 下 与 f _IR G 呈强负线性关系 ( r ^{2 ≥ 0.76)。}在这些 N 水平下，f _IR G 为晶粒生长提供了最大份额 (>86%)，因此在 Nad 和 Nex 下，单个晶粒生长可能受到来源限制。_{在 Nex 和 e[CO 2} ]等高粒数下，由于低 SRG 不足以缓冲灌浆早期不利环境条件下的同化不足，因此可能存在最终粒重低的风险。