Context: Given the significant impact of grain number (GN) on wheat grain yield, it is critical to gain a deeper understanding of the physiological bases of GN determination. The formation of both spike number (SN) and grains per spike (GS) coincides temporally and undergoes a parallel process characterized by the extensive generation of structures (such as tillers or floret primordia), succeeded by substantial degeneration, ultimately leading to a limited number of viable spike-bearing tillers and fertile florets at anthesis. Survival highly depends on resource availability and the existing genetic variability may result in varied SN and GS response to inputs or environmental conditions. Objectives: We examined the dynamics of tillering and floret primordia initiation and death during late reproductive phase aiming to elucidate the mechanisms underlying the differences in SN and GS plasticity in wheat. Methods: The number of living tillers and floret primordia developmental stages were recorded periodically on two contrasting genotypes (Ascott and Sy Moisson) regarding their GN components plasticity in response to two levels of shading, two levels of thinning and an unmanipulated control, over two locations (France and Spain). Treatments started at the onset of stem elongation. Shading finished at anthesis. Results: Survival of tillers and floret primordia rather than their generation was key for the responses to resource availability. Ascott exhibited higher plasticity which was attributed to enhanced tiller and floret primordia survival response, suggesting its superior resource utilization and adaptability to changing conditions. The higher response of florets was observed particularly in the basal spikelets, a portion of the spike that is typically less fertile. Our results emphasize the importance of the final stages of the late reproductive phase to the determination of GN (as during this period the degeneration process occurs) and also corroborates with the evolutionary idea that overproducing primary reproductive structures does not suppose any disadvantage. Conclusions: Plasticity of GN determinants were mainly related to the responsiveness of survival of initiated structures for both SN and GS. Tiller and floret primordia survival strongly responded to the treatments and the genotypic differences in the spike fertility were mainly related to responses in the basal portion of the spike Implications or significance: Overall, this study contributes with insights into the physiological bases of GN determination, offering implications for wheat breeding strategies aimed at improving grain yield and adaptability to diverse environments.