Source-sink relationships during grain filling in wheat in response to various temperature, water deficit and nitrogen deficit regimes

Grain filling is a critical process for improving crop production under adverse conditions caused by climate change. Here, using a quantitative method, we quantified post-anthesis source-sink relationships of a large data set to assess the contribution of remobilized pre-anthesis assimilates to grain growth for both biomass and nitrogen. The data set came from 13 years’ semi-controlled field experimentation, in which six bread wheat genotypes were grown at plot scale under contrasting temperature, water, and nitrogen regimes. On average, grain biomass was ~10% higher than post-anthesis aboveground biomass accumulation across regimes and genotypes. Overall, the estimated relative contribution (%) of remobilized assimilates to grain biomass became increasingly significant with increasing stress intensity, ranging from virtually nil to 100%. This percentage was altered more by water and nitrogen regimes than by temperature, indicating the greater impact of water or nitrogen regimes relative to high temperatures under our experimental conditions. Relationships between grain nitrogen demand and post-anthesis nitrogen uptake were generally insensitive to environmental conditions, as there was always significant remobilization of nitrogen from vegetative organs, which helped to stabilize the amount of grain nitrogen. Moreover, variations in the relative contribution of remobilized assimilates with environmental variables were genotype-dependent. Our analysis provides an overall picture of post-anthesis source-sink relationships and pre-anthesis assimilate contributions to grain filling across (non-)environmental factors, and highlights that designing wheat adaption to climate change should account for complex multi-factor interactions.

Methods
For the processes of data collection: plants were sampled every 2 to 9 days between anthesis and ripeness maturity. Stems, leaf laminae, chaffs, and grains of each subsample were separated, and their dry mass was determined after drying to constant mass in a forced air oven at 80℃. Total nitrogen concentration of oven-dried samples was determined by the Kjeldahl digestion method using a Kjeltec 2300 analyzer (Foss Tecator AB, Hoeganaes, Sweden) between 1991 and 2002, and by the Dumas combustion method using a FlashEA 19 1112 N/Protein analyzer (Thermo Electron Corp., Waltham, MA, USA) in 2007 and 2014.

籽粒灌浆（grain filling）是气候变化引发的逆境条件下提升作物产量的关键过程。本研究采用定量方法，对大型数据集的花后源库关系（post-anthesis source-sink relationships）进行量化分析，以评估花前同化物再转运对生物量与氮素籽粒生长的贡献。该数据集源自13年的半控制田间试验，共包含6个面包小麦基因型（bread wheat genotypes），在小区尺度下设置了不同温度、水分与氮素处理。总体而言，各处理与基因型下的籽粒生物量平均较花后地上生物量积累高出约10%。整体来看，花前同化物再转运对籽粒生物量的相对贡献（%）随胁迫强度升高而愈发显著，范围从近乎0%到100%。相较于温度，水分与氮素处理对该比例的影响更大，表明在本试验条件下，水分或氮素胁迫的影响强于高温胁迫。籽粒氮需求与花后氮吸收的关系通常不受环境条件影响，因为营养器官始终会发生显著的氮素再转运，这有助于维持籽粒氮含量的稳定。此外，花前同化物再转运的相对贡献随环境变量的变化呈基因型依赖性。本研究明确了花后源库关系以及花前同化物对籽粒灌浆的贡献在各类（非）环境因子下的整体特征，并强调，针对小麦的气候变化适应性设计需考虑复杂的多因子互作。

试验方法
数据采集流程如下：在开花期（anthesis）至成熟收获期（ripeness maturity）之间，每2至9天采集一次植株样本。将每个亚样本的茎秆、叶片、颖壳与籽粒分离，置于80℃鼓风烘箱中烘干至恒重后测定干重。1991年至2002年间，采用凯氏定氮法（Kjeldahl digestion method）结合Kjeltec 2300分析仪（Foss Tecator AB公司，瑞典霍加内斯）测定烘干样品的总氮浓度；2007年与2014年，则采用杜马斯燃烧法（Dumas combustion method）结合FlashEA 1112型氮/蛋白质分析仪（美国马萨诸塞州沃尔瑟姆市赛默飞世尔科技公司）完成测定。