Transgressive Segregation, Reciprocal Effects and Multi-Trait Selection in an F2 Bread Wheat Diallel Under Speed Breeding.

Speed breeding accelerates wheat line development, yet most reports address protocol optimisation rather than how genetic variation is expressed within the segregating populations grown in the chamber. Because a controlled long-day chamber imposes a uniform, low-stress environment, it minimises the environmental component of variance and lets genetic differences among crosses be observed with little environmental confounding. We characterised an F2 population of 851 single plants derived from a partial ("smart") diallel with reciprocals among three bread wheat cultivars, namely Adana-99 (heat-tolerant), Alada and Lucilla (both heat-sensitive), all four crosses sharing Adana-99 as a common parent. Plants were grown under a protocol-conformant 22 h light/2 h dark photoperiod (light 24.1 °C, dark 17.9 °C) and scored for twelve agro-morphological, phenological and physiological traits. All crosses showed wide phenotypic variation and abundant favourable transgressive segregation; the Adana-99 × Lucilla crosses produced the highest frequencies of transgressive segregants for grain weight per spike, biomass and harvest index, and of early-flowering individuals. Reciprocal differences were significant for biomass, grains per spike and grain weight per spike, in each case favouring Adana-99 as the female parent, indicating a maternal (or cytoplasmic) contribution. After excluding harvest index, which is algebraically derived from grain yield, path analysis identified grain weight per spike and biomass as the direct determinants of single-plant grain yield. Canonical discriminant analysis and a multi-trait genotype-ideotype distance index (MGIDI) were used to describe the multivariate divergence among crosses and to identify segregants combining favourable values across traits. The selected segregants and the Adana-99 × Lucilla combinations are proposed as candidates for multi-environment field validation.