Large differences exist in the number of grains per spikelet across an individual wheat (Triticum aestivum L.) spike. The central spikelets produce the highest number of grains while apical and basal spikelets are less productive, and most basal spikelets are commonly only formed rudimentary. Basal spikelets are delayed in initiation, yet they continue to develop and produce florets. The precise timings or the cause of their abortion remains largely unknown. Here, we investigated the underlying causes of basal spikelet abortion using shading applications in the field. We found that basal spikelet abortion is likely the consequence of complete floret abortion, as both occur concurrently and have the same response to shading treatments. We detected no differences in assimilate availability across the spike. Instead, we show that the reduced developmental age of basal florets pre-anthesis is strongly associated with their increased abortion. Using the developmental age pre-abortion, we were able to predict final grain set per spikelet across the spike, alongside the characteristic gradient in the number of grains from basal to central spikelets. Future efforts to improve spikelet homogeneity across the spike could thus focus on improving basal spikelet establishment and increasing floret development rates pre-abortion.

Supplemental Dataset S1: All raw field data collected in 2021 and 2022 for mature spikes (post-harvest).

Supplemental Dataset S2: All developmental scores taken for floret development in the basal six and central two spikelets from spike collected in CF 2022.

Supplemental Dataset S3: Raw normalized sugar concentration (µg/mg tissue wt) for samples collected in CF 2021 and 2022.

Supplemental Dataset S4: Number of grains predicted for each spikelet using CDF, with and without optimisation.

All files are in xlsx format and can be opened with Microsoft Excel of Google Sheets.