Delayed sowing increases grain number by enhancing spike competition capacity for assimilates in winter wheat

Wheat yield is linearly related to grain number per unit area, which is greatly influenced by floret fertility. Therefore, a better understanding of the mechanisms controlling floret fertility may be important to further increasing yields. During the 2014–2015 and 2015–2016 growing seasons, Tainong 18, a widely planted winter wheat cultivar (Triticum aestivum L.), was sown in a field on four dates (1, 8, 15, and 22 October). Responses to sowing date were investigated by examining the dynamics of floret generation/degeneration, assimilate partitioning, and endogenous hormone levels in spikes and stems. We found that an increase in grain number per spike (GS) with delayed sowing was mainly due to an increase in the number of fertile florets per spike (FFS). However, the number of grains per unit area remained unchanged due to the constant trade-off between the spikes per unit area and GS. FFS was positively correlated to the proportion of 13CO2 photosynthates distributed to the spikes, the content of water-soluble carbohydrates (WSC), and corresponding ratios between spikes and stems after the onset of floret degeneration. In addition, changes occurring parallel with those in assimilate partitioning-related traits were observed in the content of endogenous hormones, including indole-3-acetic acid (IAA), gibberellic acid (GA3), zeatin riboside (ZR), zeatin, and the ratios of IAA, GA3, and ZR + zeatin to abscisic acid (ABA) in spikes. Furthermore, the ratios of IAA, GA3, and ZR + zeatin content between spikes and stems were positively correlated to assimilate partitioning-related traits. In contrast, ABA content in spikes, and ratios of its content between spikes and stems, displayed a negative relationship with assimilate partitioning-related traits. These results suggest that the increase in GS resulting from higher floret survival rate with delayed sowing was mainly due to enhanced partitioning of assimilates to the spikes through modifying hormone homeostasis between developing spikes and stems.