Pre-anthesis spike growth dynamics and its association to yield components among elite bread wheat cultivars (Triticum aestivum L. spp.) under Mediterranean climate

Context: Wheat (Triticum spp.) grain yield (GY) is highly associated with grain number per unit area (GN m^-2). Biomass accumulation and partitioning are essential to understand pre-anthesis spike growth dynamics which determines spike dry matter at anthesis (SDMa) - a GN determinant. Spike growth takes place during the stem elongation period (SE), from terminal spikelet to anthesis, following leaf and spikelet initiation (LS) from sowing to terminal spikelet. Objective: In this study, bread wheat cultivars were examined under Mediterranean semi-arid conditions to determine (i) the varietal differences in pre-anthesis phase duration, (ii) whether this variability influences biomass partitioning and spike-related traits, and (iii) to what extent, the genotypic variations in pre-anthesis phase duration and spike growth are associated with yield components. Methods: A panel of Israeli commercial bread wheat cultivars were grown in the field during 2016–17 (three environments) and 2017–18 (two environments) and characterized for pre-anthesis phases, floral conditions and spike fertility via histological measurements; spike traits, dry matter accumulation, partitioning at anthesis and maturity and for yield components. Results: Significant variability in the timing of pre-anthesis phases was detected within the tested panel for both LS and SE phases. LS duration was positively associated with growing degree days (GDD) to anthesis across environments (0.72–0.90) while variation in SE was related with differences in GDD to anthesis but to a lesser extent and not in all environments (0.47–0.79). In addition, LS duration, and occasionally SE, were favourably related with a higher dry matter of fertile florets spike^-1 (at anthesis) and SDM (at both anthesis and maturity). Principal component analysis (PCA) clearly separates environments and to a lesser extent cultivars within each environment. Two cultivar pairs 'Zahir-Yuval' and 'Negev-Gedera', which flowered concurrently, showed significant differences in the durations of LS and SE phases across most of the environments. Longer LS, in cultivars Zahir and Negev, exhibited increased spikelets spike^-1, whereas longer SE (e.g., in Yuval and Gedera) enhanced spike fertility through improving the survival rate of floret primordia (FSR%) of central spikelets. However, there was a trade-off for FSR at the proximal and distal spike portions, resulting in lower grain set (%) leading to reduction of final GN (or GY) in cultivars with longer SE. Conclusions: In the tested panel and under our environmental conditions characterized by growing cycle and Mediterranean climate, the duration of both LS and SE contribute to spike fertility, but LS duration seemed a stronger driver than SE for GN and yield enhancement. Implications: Our results highlight the importance of pre-anthesis phases, especially the role of LS in wheat yield increment during the short growing cycle. The varietal combination with variable LS and SE duration could be implemented in breeding pipelines and used as pre-breeding materials for GN improvement. Furthermore, the findings will encourage pre-anthesis traits adoption in Mediterranean bread wheat future breeding programs.