Low red/far-red ratio inhibits spike development by impeding photosynthesis and carbohydrate allocation from leaves to stems in wheat

Increasing spike number is essential for achieving high wheat yield under dense planting conditions. However, dense planting reduces the ratio of red to far-red light (R/FR) in the canopy, which inhibits productive tiller formation and spike development. Sufficient assimilate supply is essential for spike differentiation and seed setting, but the mechanism of low R/FR affects spike development remains unclear. In this study, a pot experiment was conducted with supplemental FR light from wheat stem elongation to heading to simulate a low R/FR environment and to analyze its impact on young spike differentiation and the physiology of leaves and stems. The results showed that low R/FR significantly reduced wheat yield, primarily due to a decrease in both spike number and grains per spike. The developmental failure of high-position tillers (tiller III) contributed to over 65% of the total yield loss. Under low R/FR, stem elongation and spike differentiation of tiller III were synchronously arrested, with most tillers aborting during the stamen and pistil primordium stage. Low R/FR treatment significantly reduced the net photosynthetic rate ( P n ) of the fully expanded top leaves of the main stem and tillers, thereby decreasing the overall supply of assimilates. Furthermore, low R/FR inhibited the export of carbohydrates from leaves to stems, leading to a significant increase in soluble sugar content in leaves and a marked decline in stems. The limited assimilate supply intensifies nutrient competition among tillers, causing more carbohydrates to be allocated to low-position tillers and ultimately leading to the developmental failure of tiller III spikes. Hormonal analysis revealed that low R/FR significantly reduced cytokinin (CTK) levels in young spikes of tiller III by inhibiting its synthesis and promoting degradation, while simultaneously inducing abscisic acid (ABA) synthesis, thereby directly inhibiting the developmental progression of young spikes. In summary, low R/FR signaling alters the hormonal balance of CTK and ABA in the young spikes of high-position tillers, coordinately regulating the export and allocation of carbohydrates, ultimately leading to the termination of spike development in high-position tillers.