Delayed sowing limits grain number per spike in wheat by restricting young spike differentiation through reduced photothermal resources

Delayed sowing has become increasingly common in wheat production, yet the key developmental processes underlying the reduction in grain number per spike remain poorly understood, particularly the relationships among photothermal resources, young spike differentiation, and spike formation. In this study, a two-year field experiment was conducted with five delayed sowing-date treatments at 15-day intervals, and accumulated temperature, actual sunshine duration, and total solar radiation during young spike differentiation were integrated with the overall morphology of young spikes, spikelet/floret primordium differentiation, mature spike structure, and grain number per spike for analysis. The results showed that, as sowing was delayed, young spike differentiation was generally postponed and its duration was markedly shortened, while accumulated temperature, actual sunshine duration, and total solar radiation continuously declined. These changes were accompanied by restricted development of young spike morphology (length, width, and lateral width), reduced differentiation of spikelet and floret primordia, impaired formation of mature spike structure (length, width, and lateral width), and decreases in grain number per spike, including spikelet number, floret number, and fertility, as well as grain weight per spike. These stress effects became stronger with further delays in sowing. Further integrated analysis showed that the decline in grain number per spike under delayed sowing was consistent with a continuous association pathway of "reduced photothermal resources → restricted young spike development → impaired spike formation → decreased grain number per spike." Among these processes, young spike development was the key link connecting environmental changes with grain number per spike formation, and the decrease in actual sunshine duration showed the strongest statistical association with the reduction in grain number per spike. These findings indicate that the restricted formation of grain number per spike under delayed sowing is closely associated with changes in young spike development and mature spike formation, and they provide a basis for stabilizing wheat yield management under delayed sowing conditions.